The Electromagnetic Spectrum

Click or tap any abbreviation in the table — or any card below — to see its explanation. Every term used in the table is listed here alphabetically.

• ACC Adaptive Cruise Control Smart cruise control. Once you set a speed, the car uses a small radar behind the front grille to watch the car ahead and automatically slows down or speeds back up to keep a safe gap. You still steer; the car just handles the gas and brake on the highway. Driver-assistance feature that maintains a set following distance behind the car ahead, modulating throttle + brake automatically. Implemented with a 76–77 GHz long-range radar (LRR), often fused with a forward camera. Foundation feature of every modern ADAS suite and the canonical "Level 1" automation per SAE J3016.
• ADAS Advanced Driver-Assistance Systems The collective name for all the "the car is helping me drive" features on a modern vehicle — adaptive cruise, automatic braking, blind-spot warning, lane-keep, traffic-sign recognition. They're powered by tiny radars and cameras hidden behind the bumpers and windshield. Umbrella for the camera + radar + ultrasound + (sometimes) LiDAR features on modern cars: ACC, AEB, BSD, LCA, lane-keep assist, traffic-sign recognition. Required for top safety ratings (Euro NCAP 5★, US IIHS Top Safety Pick+) since the early 2020s.
• ADC Analog-to-Digital Converter A chip that turns a smoothly-varying voltage (an analog signal, like an antenna picking up a radio wave) into numbers a computer can work with. Faster + higher-resolution ADCs let a radio "see" a wider slice of the spectrum at once with more detail. Converts a continuous-time, continuous-amplitude voltage into a digital sample stream. The ADC sample rate + resolution sets a radio's instantaneous bandwidth and dynamic range — RTL-SDR has 8-bit / 2.4 MS/s, Airspy R2 12-bit / 10 MS/s, USRP X410 14-bit / 491 MS/s. Direct-RF sampling above 1 GHz is now common in modern SDRs.
• ADS Active Denial System A non-lethal "heat ray" weapon used for crowd control. It fires a narrow microwave beam that only penetrates the outer skin and feels like touching a hot stove — people instinctively get out of the way. Strictly military / law-enforcement; civilian use is illegal. Vehicle- or trailer-mounted directed-energy weapon that radiates a focused 95 GHz beam at ~100 kW peak. Energy is absorbed entirely in the outermost ~0.4 mm of skin (penetration depth at λ ≈ 3.16 mm), triggering intolerable heat-pain in 2–3 s without lasting injury at compliant dwell. Restricted military / law-enforcement; civilian construction or use is illegal under ITAR + criminal weapons law in every fielded jurisdiction. See also VMADS.
• ADS-B Automatic Dependent Surveillance-Broadcast What lets websites like Flightradar24 show every plane in the sky in real time. Aircraft constantly shout "I'm here, this altitude, this speed" on a single radio frequency, and a $30 USB stick is enough to pick those signals up at home. Aircraft continuously broadcast position, altitude, and velocity on 1090 MHz so anyone with a receiver can plot live air traffic. A $30 RTL-SDR + dump1090 is the standard hobby setup; flightradar24 and ADS-B Exchange are fed by thousands of these.
• AEB Autonomous Emergency Braking The "the car braked for me" feature. If the front radar + camera see a collision coming and you haven't reacted, the car slams the brakes on by itself. Required on all new EU cars from 2024 and all new US cars by 2029. ADAS feature that applies the brakes automatically when the radar + camera detect an imminent forward collision and the driver isn't reacting. Operates at city speeds (pedestrian / cyclist detection) and highway speeds (vehicle-to-vehicle). Mandatory on new EU passenger cars under GSR2 (2024) and on US new vehicles by 2029 per the NHTSA 2024 final rule.
• AES Advanced Encryption Standard (FIPS 197) The encryption algorithm that scrambles almost everything modern — Wi-Fi passwords, secure websites, Bluetooth pairings, military and police radio. If a radio you can pick up sounds like static noise instead of voice, AES is usually why. It's effectively unbreakable with a strong key. Block cipher (128 / 192 / 256-bit keys) standardised by NIST in 2001 — the dominant symmetric crypto in use today. Wi-Fi WPA2 / WPA3, P25 Phase 2, TETRA TEA7, Bluetooth LE Secure Connections, most TLS suites, and modern public-safety voice all use AES. SDRs see only the encrypted bitstream — they don't break it.
• AGN Active Galactic Nucleus The blazing-bright centre of a galaxy where a supermassive black hole is feeding on gas and stars. It pours out radio waves, X-rays, and gamma rays so brightly that we can detect some across the entire observable universe — they're the brightest single objects in the sky. Supermassive black hole at the centre of a galaxy actively accreting matter and radiating across the spectrum — quasars, blazars, Seyferts, radio galaxies. AGN are the strongest extragalactic radio sources and the canonical targets of VLBI + ALMA; also bright in X-ray and gamma-ray. Used by amateurs as calibration sources for 1.4 GHz dishes.
• AiP Antenna-in-Package A radio chip with the antenna built right inside it, so the whole radio is the size of a fingernail. Standard in car radar chips and millimeter-wave 5G phones — at those frequencies the antenna is so tiny it fits next to the silicon. Antenna integrated inside the same plastic / ceramic package as the radar / mmWave RFIC. Standard on TI IWR / AWR series 60 / 77 GHz radar chips and on 5G FR2 mmWave UE modules (Qualcomm QTM575 / 577). Distinct from AoP (antenna on top of, but not inside, the package).
• AIS Automatic Identification System The ship-tracking equivalent of ADS-B — every commercial vessel broadcasts its name, position, course, and speed on a marine VHF channel. Sites like MarineTraffic show that data live, and an SDR dongle can pull it straight off the air. Maritime equivalent of ADS-B: ships transmit identity, position, course, and speed on VHF marine channels 87B (161.975 MHz) and 88B (162.025 MHz). Decode with an SDR + rtl-ais and plot on OpenCPN or MarineTraffic.
• ALE Automatic Link Establishment A smart shortwave-radio mode that hops around looking for the channel with the best propagation right now, then makes the call. It's how military and emergency-services HF networks stay reliable when the ionosphere keeps changing throughout the day. Adaptive HF tactical mode (MIL-STD-188-141) that scans channels and automatically picks the best path to a destination based on link-quality probing. Backbone of US military, government, and emergency-services HF voice/data networks.
• ALMA Atacama Large Millimeter / submillimeter Array A field of 66 giant radio dishes high in the Chilean desert — so high the air is too thin for most people to walk around. They work together as one telescope to image cold dust, baby stars, and galactic chemistry. All the raw data eventually becomes public. 66-antenna interferometer at 5 km altitude in the Chilean Atacama desert, 31.3–950 GHz, the world's flagship millimetre-wave observatory (operated by ESO + NRAO + NAOJ). All science data is public after a proprietary period — amateurs can download spectra of star-forming regions, AGN, and exoplanet disks from the ALMA Science Archive. See also VLBI, NRAO.
• AM Amplitude Modulation The oldest way of putting voice or music on a radio wave: you make the wave louder and quieter in time with the audio. It's what "AM radio" in your car uses, plus shortwave broadcasters and pilots talking to air traffic control. Modulation that varies the carrier's amplitude with the audio waveform. The original commercial radio mode — still used on the MF broadcast band (530–1700 kHz), shortwave international broadcasting, and aviation VHF voice.
• AMR Automatic Meter Reading The reason a meter-reader doesn't have to come to your door anymore. Modern electricity, water, and gas meters quietly broadcast their readings on short-range radio, and a truck driving down the street picks them all up. Utility meters (electricity, water, gas) that broadcast consumption data over short-range radio so utilities don't have to send a person. Common implementations transmit on 169 MHz (VHF) or 902–928 MHz ISM; rtl_433 decodes most of them.
• AMSAT Radio Amateur Satellite Corporation The volunteer group that designs, funds, and operates satellites just for amateur radio. With a handheld radio and a small antenna pointed at the sky, hams can bounce a voice call off one of these satellites and reach other continents. Volunteer organisation that builds and operates ham-radio satellites. Their LEO birds (AO-7, FO-29, AO-91, SO-50, ESHail-2/QO-100) carry FM or linear transponders accessible with a handheld UHF Yagi.
• ANT+ ANT+ wireless protocol A short-range wireless protocol for fitness gear — heart-rate straps, bike power meters, that sort of thing. It mostly lost out to Bluetooth, but Garmin watches and a lot of cycling computers still support both. Low-power 2.4 GHz ISM protocol used by fitness sensors — heart-rate straps, bike power meters, cadence sensors. Largely overtaken by BLE, but Garmin and Wahoo gear still ship dual ANT+/BLE radios.
• AoP Antenna-on-Package Same idea as AiP but the antenna sits on top of the chip rather than inside it. Lets designers tweak the antenna pattern without changing the silicon — common in 77 GHz car radar. Antenna-array PCB or substrate placed on top of (and electrically bonded to) the RFIC package, usually via solder bumps and a matched dielectric. Common in 60 / 77 GHz automotive radar — gives near-AiP integration with a bit more layout freedom.
• APRS Automatic Packet Reporting System A ham-radio version of "find my friends" + short text messages. Radios beacon their GPS position on a shared frequency, and websites like aprs.fi show the live map. Often used by hikers, balloon-launch enthusiasts, and emergency-response volunteers. Amateur-radio packet network on 144.39 MHz (US) / 144.800 MHz (EU) for live position beacons, weather, and short messages. Most modern handhelds decode it natively; aprs.fi shows the worldwide map.
• ARIB Association of Radio Industries and Businesses (Japan) Japan's version of an ETSI or FCC: the body that writes the technical rulebook for radios sold in Japan. If you buy a Japan-spec car radar or digital TV, ARIB standards are why it's slightly different from the European one. Japanese standardisation body for radio communications. Publishes ARIB STD-T-* series — STD-T48 covers 76–81 GHz automotive radar (Japanese harmonisation with ETSI EN 302 264). Also responsible for Japanese DTV (ISDB-T), B-CAS, and digital-radio standards.
• ARRL American Radio Relay League America's national club for ham radio. They lobby the FCC, organise contests, run the awards programmes, and publish the giant ARRL Handbook — the bible most US hams learn from. The US national amateur-radio association — represents hams to the FCC, runs the incoming/outgoing QSL bureau, and publishes the ARRL Handbook (the canonical hobby reference).
• ASIL Automotive Safety Integrity Level (ISO 26262) The safety rating for parts inside a car, A (least critical) through D (most critical). The auto-brake radar is rated ASIL-D because failing it could kill someone, while the cabin light isn't. It governs how thoroughly the part had to be designed and tested. Functional-safety classification for road vehicles, A < B < C < D by hazard severity, exposure, and controllability. ADAS radar chips ship with ASIL-B/D ratings (TI AWR2944, Infineon BGT60ATR24, NXP TEF82xx); the rating is a contractual + tooling requirement for OEMs. A separate concept from cybersecurity (ISO/SAE 21434).
• ATC Air Traffic Control The voice traffic between pilots and the tower / centre. It lives on aviation VHF frequencies just above the FM broadcast band, and listening (not transmitting) is completely legal — a $100 scanner receiver picks it up clearly. Aviation voice communication on VHF 118–137 MHz (and HF for oceanic flights via HFDL). Standard AM voice with 8.33 kHz channel spacing in Europe; receivers like the Uniden BC125AT scan it legally.
• ATIS Automatic Terminal Information Service A looping radio recording every airport plays for pilots — current wind, weather, runway in use, anything pilots need before they call the tower. You can tune it on a scanner; it's the easiest "live" aviation transmission to find. Continuous looped VHF broadcast (typically 118–136 MHz AM) that gives pilots the airport's current weather, runway in use, and notices. Tune, listen, repeat — also great for SDR scanning practice.
• ATV Amateur Television Hams transmitting actual video over the air — like a personal TV station. Old-school analogue ATV is mostly extinct; modern hams use digital ATV (DATV) and occasionally bounce HD video off the QO-100 geostationary satellite for international video chats. Hams transmitting full-motion video on dedicated bands (70 cm, 23 cm, 13 cm). Analogue ATV is fading; modern setups use DATV with DVB-S2 over UHF/microwave.
• AWOS Automated Weather Observing System A robot weather station at a small airport that reads out current wind, visibility, and pressure on a loop. Pilots tune in before landing; anyone with a scanner can listen too — handy if you want extra-local weather without going through an app. Unattended airport weather stations that broadcast a synthesised voice report on dedicated VHF channels. Pilots tune in for current wind/visibility/altimeter; SDR listeners get free weather coverage of their nearest small airport.
• BeiDou BeiDou Navigation Satellite System (China, BDS) China's version of GPS. It's been fully global since 2020. Modern phones and car satnavs actually listen to GPS, BeiDou, Galileo (EU), and GLONASS (Russia) all at once — more satellites in view means a faster and more accurate fix, especially in cities. Chinese global navigation satellite system, fully operational globally since 2020. Frequencies: B1I 1561.098 MHz, B1C 1575.42 MHz (interoperable with L1), B2a 1176.45 MHz (= L5), B2b 1207.14 MHz, B3I 1268.52 MHz. Multi-constellation receivers combine BeiDou with GPS, Galileo, GLONASS for better sky coverage and faster fixes.
• BLE Bluetooth Low Energy The "sips battery" version of Bluetooth that lets coin-cell-powered things like AirTags, fitness watches, smart bulbs, and door sensors run for months or years on a tiny battery. Your phone speaks BLE all day; it's why nearby trackers can show up in Find My. The low-power flavour of Bluetooth in the 2.4 GHz ISM band. Used by fitness trackers, AirTag-style beacons, smart-home sensors, and almost every coin-cell-powered gadget. Sniffable with an nRF52840 dongle running Wireshark.
• BNetzA Bundesnetzagentur Germany's wireless watchdog. They issue ham licences, set transmit power limits, and chase down people interfering with cell or aviation bands. Same role as the FCC in the US or Ofcom in the UK. Germany's national wireless regulator — the German FCC / Ofcom. Sets local power limits, runs the ham licence exam, and enforces ETSI harmonised standards on consumer gear.
• BPSK Binary Phase-Shift Keying The simplest way to send digital data over radio: flip the wave upside-down to mean "1" and leave it alone for "0". Slow but incredibly robust against noise — it's the bottom rung of every fancier modulation scheme. Simplest digital phase modulation — two phase states (0° / 180°) carry one bit per symbol. Building block of more complex modes like QAM and OFDM subcarriers.
• BSD Blind-Spot Detection The little orange light in your side mirror that warns you about a car in your blind spot. It's powered by tiny radars hidden in the rear bumper that watch the lanes next to you. Short-range 77–81 GHz radar (or rear-corner camera) that warns when a vehicle is in the driver's blind spot. Radar implementation typically uses bumper-mounted SRR modules with ±60–75° field of view. Often paired with LCA + RCTA.
• c Speed of light in vacuum The speed of light — about 300,000 km per second, or roughly 30 cm per nanosecond. All radio waves travel at this speed too. The handy mental shortcut: 1 GHz signal has a wavelength of about 30 cm, and the wavelength shrinks proportionally as the frequency climbs. Universal constant c = 299,792,458 m/s (≈ 3 × 10⁸ m/s — handy for quick frequency-to-λ conversions in your head). Together with frequency ν and Planck's constant h it sits in the two formulas every spectrum reader needs: λ = c/ν and E = hν.
• C4FM 4-level FSK (Yaesu System Fusion) One of three competing digital-voice modes on ham radio (the others being DMR and D-STAR). C4FM is the one Yaesu radios use; calls sound clearer than old-school FM and can route over the internet between repeaters worldwide. Yaesu's digital-voice mode for ham repeaters — 12.5 kHz channels with selectable digital or analogue FM. Competes with DMR and D-STAR on the 2 m and 70 cm bands.
• CAN Controller Area Network (ISO 11898) The internal "nervous system" of a car. Every modern car has dozens of little computers (engine, brakes, doors, radar) that all talk over a CAN bus — a pair of wires running through the chassis. It's how the radar tells the brakes "stop now". The dominant in-vehicle network bus, twisted-pair differential serial, 1 Mbps classic / 5–8 Mbps CAN-FD. Used to glue together body, chassis, powertrain, and ADAS ECUs — including how radar modules report tracked targets to the steering / braking controller. Hobbyists decode junkyard radar with SocketCAN + candump + a USB-CAN dongle (Lawicel CANUSB, Innomaker GS-USB).
• CBRS Citizens Broadband Radio Service A US-only chunk of spectrum where you don't need to buy a licence — you just register and a database tells your gear when it's safe to transmit. Anyone can run a private 4G/5G network on it: warehouses, universities, factories use it all the time. US 3550–3700 MHz shared-spectrum tier with a dynamic database (SAS) coordinating incumbent Navy radar, PAL licensees, and General Authorized Access users. Anyone can deploy a private LTE/5G network in CBRS without bidding for spectrum.
• CDMA Code Division Multiple Access A clever trick that lets lots of users share the exact same radio channel by tagging each one with a different "code" the receiver knows to look for. It was how older Verizon/Sprint cell networks worked, and it's how GPS satellites all broadcast on the same frequency without stepping on each other. Spread-spectrum multiple-access scheme that lets many transmitters share the same band by encoding each with a unique pseudo-random spreading code. Foundation of cdmaOne / CDMA2000 (Verizon legacy, Sprint legacy), GPS C/A and modern GLONASS L1OC, and the W-CDMA layer of UMTS. Largely retired from cellular use in favour of OFDMA.
• CEPT European Conference of Postal and Telecommunications Administrations The club of European telecom regulators that agrees on common rules across the continent. The most visible bit for hobbyists: a CEPT ham licence lets you bring your radio on holiday almost anywhere in Europe and transmit without filing extra paperwork. Pan-European body that harmonises radio rules across 48 member countries. A CEPT-class ham licence (issued under T/R 61-01) lets you operate in most of Europe without applying for individual permits.
• CHANDRA Chandra X-ray Observatory NASA's giant X-ray telescope in orbit. Earth's atmosphere blocks X-rays (good for our health, bad for astronomy), so Chandra had to go to space to see exploding stars, black holes feeding, and galaxy clusters glowing in X-ray. NASA's flagship X-ray space telescope — operating in high Earth orbit since 1999. Imaging targets the keV–10 keV X-ray band invisible from the ground (Earth's atmosphere absorbs all X-rays).
• CMB Cosmic Microwave Background The faint microwave glow that fills the entire sky — leftover heat from the Big Bang, now cooled to just 2.7 degrees above absolute zero. About 1% of the static you used to see on an analog TV tuned between channels was actually the CMB. Telescopes in space have mapped it in extraordinary detail. The 2.725 K thermal afterglow of the Big Bang, peaking at ~160 GHz. Mapped by COBE / WMAP / Planck satellites and now LiteBIRD, CMB-S4, Simons Observatory. Its tiny anisotropies (ΔT / T ≈ 10⁻⁵) encode the geometry, age, and composition of the universe. Strictly a receive-only target and only at high-altitude or space.
• CMOS Complementary Metal-Oxide-Semiconductor The recipe that essentially every modern computer chip is built with — phone processors, image sensors, even car radar chips. Its key superpower is that it sips almost no power when idle, which is why phones can have billions of transistors and still last all day. The dominant integrated-circuit fabrication process — pairs of n-channel + p-channel MOSFETs that together draw nearly zero static current. Modern phones, SDR baseband, image sensors, and even 60 / 77 GHz radar SoCs are CMOS. RF-CMOS at 28 / 22 / 16 / 7 / 5 / 3 nm is what made MMIC-class millimetre-wave radio cheap.
• COSPAS-SARSAT Cospas-Sarsat international satellite Search-and-Rescue The international satellite network that listens for emergency beacons from downed aircraft, sinking ships, and stranded hikers. If you press the SOS button on a personal locator beacon or set off a boat's EPIRB, rescuers can get an exact GPS location within minutes. Credited with saving more than 60,000 lives since 1982. International (40+ countries) satellite system that monitors 406.025–406.100 MHz emergency beacons (ELT aviation, EPIRB maritime, PLB personal). LEOSAR + GEOSAR + MEOSAR (galileo / GPS payloads) deliver alert + cm- class location to rescue coordination centres in seconds. Has saved 60 000+ lives since 1982.
• CSAC Chip-Scale Atomic Clock A real atomic clock the size of a postage stamp. It keeps time so precisely it would only drift by about a second every few thousand years. Used in military radios and self-driving cars to stay accurate when the GPS signal cuts out in a tunnel. Tiny rubidium / caesium-vapour atomic clock the size of a postage stamp (Microsemi SA.45s, ADI MAC-SA5X). Holdover stability ~1 × 10⁻¹¹ over an hour — orders of magnitude better than a TCXO, enough to ride out brief GPS outages on a GPSDO. Used in handheld defence radios, autonomous-vehicle IMU fusion, and undersea / underground deployments where GNSS is unavailable.
• CSI Channel State Information Data your Wi-Fi router already collects to keep your signal clean. Researchers figured out that subtle changes in this data can detect people walking through rooms, hand gestures, and even someone's breathing — without any extra hardware. Wi-Fi has become an accidental motion sensor. Per-subcarrier amplitude and phase data Wi-Fi devices already exchange to equalise the channel. Tools like ESP32-CSI-Tool and Atheros CSI Tool let you reuse it to sense motion, gestures, and even breathing through walls.
• CW Continuous Wave (Morse code) Morse code — beeping a radio carrier on and off. It looks ancient on paper, but Morse signals get through static and weak conditions when voice can't, so plenty of hams still use it every day for long-distance contacts on shortwave. On/off keying of a single carrier — the original digital mode. Still extremely effective for weak signals and used by amateurs daily on the HF bands; CW signals punch through QRM where SSB voice can't.
• D-band D-band waveguide (~110–170 GHz) A very-high-frequency slice of spectrum (well above current 5G) where 6G is being prototyped today. Wavelengths are millimetre-scale, so antennas are tiny and the signal struggles to travel more than a few hundred metres outdoors. Sub-THz waveguide band (WR-6) used for fixed-service backhaul 130–174.8 GHz and current 6G research test-beds. Commercial sources use multiplied 12 / 24 GHz chains (×6 / ×8) into Schottky or GaAs MMICs. Aperture-efficient at moderate distances despite high atmospheric attenuation around water-vapour lines (λ ≈ 1.7 – 2.7 mm).
• D-STAR Digital Smart Technology for Amateur Radio Icom's flavour of digital-voice for ham radio. Same big idea as DMR or C4FM: cleaner audio than analog FM and easy worldwide routing through internet-linked repeaters. ICOM's GMSK digital-voice mode for ham 2 m / 70 cm / 23 cm. Routed via internet-linked reflectors (REF, XRF, XLX); competes with DMR and C4FM in the digital-voice space.
• DAB Digital Audio Broadcasting The European "digital radio" in your car — sharper audio than FM and dozens of stations on the same dial. Replaces analog AM/FM in many countries. A $25 SDR dongle on a laptop can receive it. European digital radio standard on Band III (VHF 174–240 MHz). Carries dozens of stations per multiplex using OFDM; an RTL-SDR + Welle.io decodes it for free.
• DAC Digital-to-Analog Converter The opposite of an ADC — a chip that turns digital numbers back into a smooth voltage that can drive a speaker, a screen, or an antenna. The DAC in your phone is what makes sound come out of the headphone jack; in a software radio, it's what gets the signal back on the air. Inverse of ADC: converts a digital sample stream back into a continuous voltage. The DAC sample rate + resolution sets a transmitter's effective bandwidth + spectral purity. Modern SDRs run DACs from 1.2 GS/s (USRP B210) up to 16 GS/s direct-synthesis (RFSoC) — the upper end can transmit 6 GHz mmWave directly without a mixer.
• DARC Deutscher Amateur-Radio-Club Germany's national ham-radio club — the equivalent of the ARRL in the US or the RSGB in the UK. They run training, contests, and the local "DOK" geographic-code system German hams use to identify their home district. Germany's national amateur-radio association — peer of the ARRL and RSGB. Member of the IARU; runs the German DOK locator system.
• DATV Digital Amateur Television The modern version of amateur television — hams sending live video, sometimes in HD, over their own little "TV stations" on microwave bands. Some use the QO-100 geostationary ham satellite to relay video between continents in real time. Modern ATV using DVB-S2 over the 23 cm, 13 cm, and 3.4 GHz ham bands. Needs a small dish, a Minitiouner-style receiver, and a few watts of power; British BATC operates the QO-100 wideband transponder for satellite DATV.
• dB decibel — logarithmic ratio A compact way to write very large or very small power ratios. The rule of thumb: +3 dB means "about twice as strong", +10 dB means "ten times as strong", and minus numbers mean weaker. So a Wi-Fi signal that drops by 10 dB through a wall is ten times weaker than it was before. A relative power ratio expressed logarithmically: 10·log₁₀(P₁/P₂). +10 dB ≈ 10× more power, +3 dB ≈ 2×, −3 dB ≈ ½. dB on its own is a pure ratio (gain, loss, signal-to-noise); attach a reference and it becomes an absolute level — see dBi (antenna gain), dBm (vs 1 mW), and dBW (vs 1 W).
• dBi decibels relative to isotropic radiator A number that tells you how much an antenna focuses the signal in one direction instead of spraying it everywhere. Higher dBi = narrower, stronger beam, but you have to point it correctly. A rubber-duck antenna is around 2 dBi (barely focused); a satellite dish can be 40+ dBi (very tight pencil beam). Antenna gain expressed against a perfect omnidirectional source. A 1/4-wave whip ≈ 2 dBi, a Yagi 8–14 dBi, a 1 m parabolic at L-band ~22 dBi, a 60 cm Ka-band dish 38–40 dBi. dBi adds directly to TX power in dBm / dBW to give EIRP.
• dBm decibels relative to 1 milliwatt A standard way of writing radio signal strength as a single number. The bigger the negative number, the weaker the signal — −60 dBm is great, −110 dBm is barely there. Phones, Wi-Fi routers, and SDR receivers all report signal levels this way. Positive numbers mean a strong transmitter (your handheld radio is about +30 dBm = 1 watt). Absolute RF power against a 1 mW reference. 0 dBm = 1 mW, 30 dBm = 1 W, 60 dBm = 1 kW; −90 dBm ≈ a typical mobile-phone receive level. Many radios spec output as e.g. "23 dBm conducted" (≈ 200 mW) or "−110 dBm sensitivity" — that 133 dB span is why receivers need so much dynamic range.
• dBW decibels relative to 1 watt Same idea as dBm but the zero point is 1 watt instead of 1 milliwatt. Broadcasters and satellite engineers prefer it because their transmitters output kilowatts. To convert: dBm = dBW + 30. So a 50 dBW FM tower is 80 dBm = 100,000 watts. Absolute power against 1 W. 0 dBW = 1 W = 30 dBm; broadcasters and satellite operators typically quote ERP / EIRP in dBW (an FM tower at 50 dBW = 100 kW, a GPS satellite ≈ 27 dBW = 500 W). Subtract 30 to convert to dBm.
• DCF77 DCF77 (German LF time signal) The German radio station that broadcasts the time to nearly every "radio-controlled" alarm clock in Europe. It's why your kitchen clock automatically updates for summer time without you doing anything — it's quietly listening to a transmitter in Mainflingen, Germany. The 77.5 kHz time-and-date transmission from Mainflingen, Germany — audible across most of Europe. Decoded by virtually every "atomic" radio clock sold on the continent; sits in the LF band where wavelengths are kilometre-scale.
• DFS Dynamic Frequency Selection A rule that makes your Wi-Fi router politely jump to a new channel if it hears weather or military radar nearby. Some of the 5 GHz Wi-Fi channels are shared with radar; this is how Wi-Fi avoids interfering with someone's air-traffic-control screen. Wi-Fi rule on UNII-2A and UNII-2C (5.250–5.350 / 5.470–5.725 GHz): the access point listens for weather and military radar pulses and must vacate the channel within 10 s if it hears one. Required by FCC and ETSI.
• DME Distance Measuring Equipment An aviation gadget that tells the pilot exactly how far they are from a known ground station, by timing how long a radio pulse takes to go there and back. Like an aviation tape measure that works in the air. Aviation navigation aid that measures slant range to a ground beacon by timing a 1 µs UHF pulse round-trip on 962–1213 MHz. Always paired with a VHF VOR or ILS for full position fix.
• DMR Digital Mobile Radio A widely-used digital-voice standard for business radios, security teams, and ham radio. Cheap $60 Chinese DMR handhelds plus a $30 home "hotspot" let hams talk to international friends via internet- linked talkgroups — like Discord voice channels, but on real radios. ETSI TDMA digital-voice standard widely used on commercial and amateur trunked networks (MotoTRBO, BrandMeister, TGIF). Cheap import HTs ($60) put hams onto worldwide talkgroups via internet-linked repeaters.
• DSSS Direct-Sequence Spread Spectrum A way of "smearing" each bit out over a much wider patch of spectrum than it needs, so a noise spike or jammer can only wipe out a small piece of it. GPS uses this trick — that's why those satellite signals can be picked out from beneath the local noise floor. Spreads each data bit across a wide bandwidth using a fixed pseudo-random chip sequence. Foundation of GPS, legacy 802.11b Wi-Fi, and early CDMA cellular.
• DTV Digital Television Modern over-the-air television — sharp HD pictures and many sub- channels packed into the same airwaves that used to carry one blurry analog channel. Each country picked its own flavour (US ATSC, EU DVB-T, Japan ISDB-T, China DTMB) so a TV brought in from abroad usually won't work. Umbrella for digital terrestrial / cable / satellite TV broadcast standards: DVB-T / T2 (Europe), ATSC 1.0 / 3.0 (US), ISDB-T (Japan / Brazil), DTMB (China), DVB-S / S2 / S2X (satellite), DVB-C / C2 (cable). Replaced analogue PAL / NTSC / SECAM in most countries between 2000 and 2020.
• DVB-S Digital Video Broadcasting — Satellite (ETSI EN 300 421) The European standard for satellite TV — the format the data is in when it arrives at your dish. DVB-S2 is the modern version that most of Astra and Eutelsat's channels use today. You can decode it on a PC with a cheap card. Standard for satellite digital TV (10.7–12.75 GHz Ku-band typical), QPSK + Reed-Solomon coding. Successor DVB-S2 (8PSK + LDPC) delivers ~30% more capacity per transponder and is now the workhorse on Astra / Eutelsat / SES. DVB-S2X adds 32APSK and finer modcods. RTL-SDR + Sat>IP / Minitiouner cards demodulate DVB-S / S2 in software for hobbyists.
• DVB-T Digital Video Broadcasting – Terrestrial Europe's free over-the-air digital TV. Funny twist: the cheap USB sticks people bought to watch DVB-T turned out to be hackable into full software-defined radios. That accident — the RTL-SDR — kicked off the whole modern hobbyist SDR scene. European/Asian digital terrestrial TV standard. Same OFDM + COFDM core that powers DAB; the receiver chip from these cheap dongles is the basis of the iconic RTL-SDR.
• E-band E-band (~60–90 GHz, telecom 71–76 / 81–86 GHz) A very-high-frequency slice of spectrum used to connect cell towers to the internet with a small antenna instead of a fibre dig. The "small dish on a rooftop pointing at another rooftop" you see in cities is often an E-band link carrying multi-gigabit traffic. The "light-licensed" telecom backhaul band — paired duplex 71–76 / 81–86 GHz used by carriers for multi-Gb/s point-to-point radio links (Siklu, BridgeWave, Aviat, Ericsson MINI-LINK, Nokia UBT). US registration via FCC 70/80/90 GHz (free, ~10 minutes); EU per-link licensed. Waveguide convention WR-12.
• EAR Export Administration Regulations (US Department of Commerce) The US rules that decide what civilian electronics you're allowed to ship to which countries. Most commercial gear falls under EAR; pure military stuff goes under the stricter ITAR rules. Think of it as "America's export passport check" for tech. Civilian-side US export-control regime (15 CFR Parts 730–774). Covers cryptography, "dual-use" sensors, advanced electronics, and most commercial RF gear. Sister regime to ITAR (which covers explicitly military items). EAR Cat. 3A includes high-end semiconductors and crypto; EAR99 is the catch-all "low-sensitivity" tier.
• EAS Emergency Alert System The "BEEP BEEP BEEP — this is a test of the Emergency Alert System" interruption you hear on US radio and TV. The harsh tones at the start of an alert are actually digital data telling other stations what kind of emergency it is and what area it covers. US national broadcaster alert protocol — those header tones you hear interrupting AM/FM/TV. Carries flood, tornado, and presidential alerts; encoded at the start of the message in audible FSK data.
• EBU European Broadcasting Union The club of Europe's public broadcasters — BBC, ARD, ZDF, RAI and dozens more. They share programmes, set technical standards, and run Eurovision (yes, the song contest). The satellite "bird" that relays the live feed between countries is theirs. Alliance of European public-service broadcasters (BBC, ARD, ZDF, RAI…). Coordinates Eurovision, runs technical standards, and operates the backbone satellite distribution most national broadcasters share.
• EGNOS European Geostationary Navigation Overlay Service Europe's free upgrade for GPS — a couple of satellites that broadcast small correction signals to improve accuracy from "about 5 metres" to "about 1.5 metres" anywhere in Europe. Every modern smartphone uses it automatically. European SBAS correction system, three GEO satellites broadcasting on the same L1 frequency as GPS / Galileo. Improves civilian fix accuracy from ~5 m down to ~1.5 m, with integrity monitoring used for aviation approach. EGNOS-V3 (in deployment) extends to L5.
• EHF Extremely High Frequency (30–300 GHz) The very top end of "radio" — millimetre-wave 5G, automotive radar, satellite downlinks. Antennas are tiny, data rates are huge, but the signal struggles to get through walls or rain. Top of the conventional radio spectrum — 5G FR2 mmWave, automotive 77 GHz radar, satellite downlinks, and military links. Wavelengths under 10 mm; absorbed by oxygen at 60 GHz and water vapor at 183 GHz.
• EIRP Effective Isotropic Radiated Power "Effective" transmit power once you account for the antenna focusing the signal in one direction. A 1 watt walkie-talkie with a normal antenna ≈ 1 W EIRP, but the same watt fed into a satellite dish can behave like thousands of watts in the dish's pointed direction. Legal power limits are almost always written as EIRP. TX power × antenna gain — what your strongest direction actually radiates compared to a perfect omnidirectional antenna. Most legal power limits (FCC Part 15, ETSI EN 300 220) are written in EIRP, not transmitter watts.
• ELF Extremely Low Frequency (3–30 Hz) The deepest, slowest "radio" waves — so long that one full wave is thousands of kilometres across. The only signals that can reach submerged submarines, and what carries Earth's natural electrical "heartbeat" excited by every lightning storm worldwide (the Schumann resonance at 7.8 Hz). The lowest officially recognised ITU radio band. Wavelengths of 10,000–100,000 km penetrate seawater hundreds of metres deep — used for one-way submarine communication (ZEVS) and home to the natural Schumann resonances.
• ELT Emergency Locator Transmitter (aviation) A "black box" SOS beacon built into aircraft. If the plane crashes, impact sensors turn it on and it screams its identity + GPS coordinates up to satellites — search-and-rescue gets a precise location within minutes. Boats and hikers carry similar beacons. Aircraft-mounted distress beacon that transmits on 406 MHz (digital, satellite-detected via COSPAS-SARSAT) and 121.5 MHz (homing for SAR aircraft). Activated by impact-sensor or manual switch in a crash; broadcasts a registered hex ID + GPS position that gets to rescue coordination in minutes.
• EME Earth-Moon-Earth ('moonbounce') "Moonbounce" — aiming a radio antenna at the Moon and using it as a giant reflector to talk to someone on the other side of the planet. The signal makes a 770,000 km round trip and comes back faint, but modern weak-signal modes can pull a conversation out of the noise. Long-distance amateur technique that aims a high-gain dish at the Moon and bounces signals off the lunar surface back to Earth. Common on 2 m, 70 cm, 23 cm, and microwave ham bands; modes like JT65 and Q65 make it achievable with modest stations.
• EPIRB Emergency Position-Indicating Radio Beacon (maritime) A boat's SOS beacon — the maritime cousin of the aviation ELT. The fancy ones float free if the boat sinks and start broadcasting your GPS position to rescue satellites the moment they hit water. Hikers carry the smaller "PLB" version for the same purpose. Maritime distress beacon required by SOLAS on commercial vessels + recommended on yachts. Same 406 MHz COSPAS-SARSAT + 121.5 MHz homing as ELT; float-free designs detach + activate on submersion. PLB (Personal Locator Beacon) is the hiker / pilot version of the same idea.
• ERP Effective Radiated Power Same idea as EIRP but using a slightly different "perfect antenna" as the reference. ERP is always about 2 dB smaller than EIRP for the same setup. Mostly a quirk of European broadcast rules; numbers converted, the physics is identical. Same idea as EIRP but referenced to a half-wave dipole instead of an isotropic radiator — ERP = EIRP − 2.15 dB. Used by European regulators for broadcast and short-range allocations.
• ESA European Space Agency Europe's NASA — 22 member countries pooling resources for space missions. They run Galileo (Europe's GPS), the Sentinel Earth-observation satellites, and contributed key instruments to big international projects like the JWST. Europe's intergovernmental space agency — runs Galileo (GNSS), the Sentinel Earth-observation constellation, and contributes to the JWST and Hubble.
• ETSI European Telecommunications Standards Institute The body that writes Europe's technical rulebook for radios. When you see "complies with EN 300 220" on the back of a smart-home sensor, that's an ETSI standard. National regulators like BNetzA just enforce what ETSI publishes. Sets the EU's wireless technical standards — the EN 300 220 / EN 301 893 / EN 303 series that hobbyists and product designers follow for license-free transmission. National regulators like BNetzA and Ofcom enforce them.
• EUV Extreme Ultraviolet (~10–124 nm, 10–124 eV) Ultra-short ultraviolet light — between regular UV and X-rays. So energetic that air itself blocks it, so it's used in vacuum chambers. The two famous uses: the special light that prints transistors on every modern chip (ASML's EUV machines), and photographing the Sun's corona from space. The ultraviolet sub-band between vacuum UV and soft X-ray, absorbed by air (must be used in vacuum). Two big real-world uses: 13.5 nm EUV photolithography (ASML TWINSCAN NXE / EXE, powering every leading-edge semiconductor since ~7 nm), and solar / stellar EUV imaging from space (SDO AIA, SOHO EIT, Hinode). Photon energy ~92 eV at 13.5 nm.
• eV electron volt — photon-energy unit A unit for how much energy a single light particle (photon) carries. Higher number = more energetic, more "punch". This is the dividing line between safe everyday light and dangerous ionizing radiation: visible light photons are around 2 eV (safe), X-rays are thousands of eV (need shielding). Energy gained by one electron crossing a 1-volt potential difference (≈ 1.602 × 10⁻¹⁹ J). The natural unit for photon energy from infrared upward: visible light ≈ 1.65–3.26 eV, UV ≈ 3–125 eV, X-rays ≈ 100 eV – 100 keV, gamma > 100 keV. Multiples scale by 1000: μeV (micro) < meV (milli) < eV < keV (kilo) < MeV (mega) < GeV (giga).
• EVN European VLBI Network A team of ~20 European radio telescopes that combine their measurements to act as one huge telescope spanning the continent. The Event Horizon Telescope (the project that produced the first image of a black hole) used EVN dishes alongside global partners. Continental array of ~20 radio telescopes (Effelsberg, Jodrell Bank, Westerbork, Onsala, Medicina, Yebes, …) coordinated for joint VLBI observations. Operates 1.6 / 5 / 22 / 43 GHz primarily; combines with global partners including VLBA for the Event Horizon Telescope. See IVS.
• FAST Five-hundred-meter Aperture Spherical Telescope (China) The world's biggest single radio-telescope dish — half a kilometre across, built into a natural bowl-shaped valley in China. Now the most sensitive instrument for picking up faint pulses from distant pulsars and mysterious fast radio bursts. World's largest single-dish radio telescope, 500 m fixed reflector in a karst depression in Guizhou, China. Operates 70 MHz – 3 GHz with 19-beam receiver. Replaced Arecibo (decommissioned 2020) as the most sensitive metre / decimetre-wave instrument; major contributor to pulsar surveys, fast radio bursts, and HI-line cosmology.
• FCC Federal Communications Commission The US wireless watchdog. They decide what frequencies anyone can use, run the ham-radio exam system, and approve every Bluetooth gadget or Wi-Fi router sold in America (that's what the tiny "FCC ID" on the back of devices means). US wireless regulator. Sets the Part 15 unlicensed limits, Part 97 amateur rules, Part 90 land-mobile rules, and OET-65 RF-exposure guidelines that downstream ICNIRP work into.
• FDD Frequency Division Duplex A cellular setup where your phone listens on one frequency and transmits on a different one at the same time — like having separate "up" and "down" lanes. Most 4G bands work this way. Contrast with TDD, which uses one channel and alternates. Cellular pattern where uplink and downlink use paired but separate frequencies — the default on most LTE bands (B1, B3, B7…). Contrast with TDD where they share one channel and alternate in time.
• FFT Fast Fourier Transform The mathematical trick that turns a chunk of audio or radio samples into "how much of each frequency is in here". Every spectrum display, music EQ, Shazam-style song fingerprint, and SDR waterfall plot is an FFT under the hood. It's probably the most used algorithm of the last 60 years. O(N log N) algorithm (Cooley–Tukey, 1965) for transforming time-domain samples into a frequency spectrum. Every SDR waterfall, every spectrum analyzer, every OFDM modem, every FMCW radar range-FFT — all the same primitive. Hobbyists meet the FFT first as a "sliding window of magnitude bins" in GQRX, SDR# or SDR++.
• FHSS Frequency-Hopping Spread Spectrum A trick where the radio jumps between dozens of frequencies many times per second, with the receiver knowing the secret pattern. If another device interferes on one channel, you only lose a fraction of a millisecond. Old Bluetooth and lots of remote controls work this way — invented for jam-resistant military radio. Transmitter and receiver hop synchronously across many channels per second, dodging interference and resisting jamming. Used by Bluetooth Classic, the FCC Part 15.247 902–928 MHz ISM rules, and many telemetry radios.
• FM Frequency Modulation Higher-fidelity than AM: the audio is encoded by wiggling the frequency rather than the loudness, which is much more resistant to noise. It's what your car's "FM 88–108" dial picks up, and the audio mode on essentially every walkie-talkie made. Modulation that varies the carrier's frequency with audio. The dominant mode on the 88–108 MHz VHF broadcast band, marine VHF, ham 2 m / 70 cm voice, and analogue walkie-talkies (PMR446, FRS, GMRS).
• FMCW Frequency-Modulated Continuous-Wave radar The kind of radar in modern cars. Instead of sending out short pulses like a WWII radar, it transmits a constantly rising-tone "chirp"; the echo's tone tells you the distance and how fast the other car is moving. It's how a car radar can fit on a fingernail. Radar architecture that transmits a continuous chirp swept across a wide bandwidth; the round-trip delay shows up as a beat frequency proportional to range, while Doppler shift between chirps gives velocity. Range resolution Δr = c/(2·BW); a 4 GHz chirp at 77 GHz buys ≈ 4 cm. The dominant architecture for 77 GHz automotive radar — far smaller and cheaper than pulse Doppler.
• FPV First-Person View Flying a drone (or driving an RC car) through goggles that show a live video feed from a camera on the vehicle, as if you were sitting inside. The video link is a small radio transmitter on the drone sending HD video back to the goggles in real time. Live video link from a drone or RC car back to the pilot — typically 5.8 GHz ISM analogue (DJI Avata) or digital (DJI O3, Walksnail, HDZero). Hams license at 1.2 GHz / 2.4 GHz / 5.8 GHz can run higher power under Part 97.
• FR1 5G NR Frequency Range 1 (sub-7.125 GHz) The "regular" 5G — frequencies below about 7 GHz, similar to where 4G lives. This is what your phone is almost always on. Decent range, gets through walls, and faster than 4G but not the headline-grabbing gigabit speeds. The 5G NR low- and mid-band cellular spectrum where most coverage lives. Includes n1, n3, n7, n28, n41, n66, n71, n77, n78, and n79.
• FR2 5G NR Frequency Range 2 (mmWave, 24.25–71 GHz) The "blazing gigabit but only on this street corner" 5G. It uses millimetre-wave frequencies which can carry tons of data but barely get through a window — even your hand can block it. Mostly deployed in stadiums and dense urban hotspots. Millimeter-wave 5G — fast (multi-Gb/s) but very short range and easily blocked by buildings, hands, and rain. Bands n257, n258, n260, n261; sits inside the EHF band.
• FRS Family Radio Service The walkie-talkies you find in the bubble-pack at any US Walmart — no licence, no exam, kids can use them. Range claims on the box ("up to 35 miles!") are wildly optimistic; in practice expect a couple of kilometres outdoors. US license-free 462/467 MHz UHF walkie-talkie service — 22 channels at up to 2 W. Shares hardware with GMRS; in Europe the equivalent is PMR446.
• FSK Frequency-Shift Keying The "two-tone whistle" way of sending data — one frequency means "1", another means "0". Sounds like a fax machine on the radio. Simple, hard to corrupt, and still used by hospital pagers and weak-signal ham modes. Digital modulation that swaps between two (or more) discrete frequencies to send bits — robust, simple, and the foundation of FT8, RTTY, packet, and most paging systems.
• FSO Free-Space Optics A pair of invisible-laser transceivers pointed at each other across open air — basically fibre-optic without the fibre. SpaceX's Starlink satellites use it to relay traffic between each other in orbit, and roof-to-roof setups link buildings without trenching cables. Point-to-point laser link through air for high-bandwidth backhaul. Roof-to-roof FSO competes with mmWave; used inter-satellite by Starlink and inter-data-center for lit-fibre alternatives.
• FSS Fixed-Satellite Service (ITU) The official ITU category for "big dish on the ground talking to a satellite" — corporate VSAT terminals, Starlink ground gateways, Viasat enterprise links. Distinct from the satellite-TV-to-your-home category (BSS) and the handheld satellite phones (MSS). The ITU radio service for fixed-earth-station satellite links — Ku-band (12 / 14 GHz: SES, Eutelsat, Inmarsat-5), Ka-band (20 / 30 GHz: Inmarsat-6, Viasat, Starlink Gateway, OneWeb), and legacy C-band (4 / 6 GHz). Distinct from MSS (mobile-satellite, Iridium / Inmarsat handhelds) and BSS (broadcast-satellite, DVB-S direct-to-home).
• FT4 Fast 4-FSK (WSJT-X) A speed-run version of FT8 — same idea, but each contact takes 7.5 seconds instead of 15. Hams use it in contests when they want to make as many quick contacts per hour as possible. Faster sibling of FT8 — 7.5-second cycles instead of 15. Designed for HF contesting where you want more QSOs per hour at the cost of slightly worse weak-signal performance.
• FT8 Franke-Taylor 8-FSK The most popular ham-radio mode of the last decade. Hams type call sign + signal report into a free program, the computer encodes it into eight musical tones, and the conversation completes automatically in about a minute. It can decode signals so weak you can't hear them — letting tiny QRP rigs work the world. Weak-signal amateur digital mode from the WSJT-X suite. 15-second cycles, ~50 bits per QSO, decodes down to roughly –20 dB SNR — currently the most-popular mode on HF.
• GAGAN GPS Aided GEO Augmented Navigation (India) India's GPS-accuracy booster. A handful of satellites parked over India broadcast small corrections that sharpen GPS fixes from "about 5 metres" to "about 1.5 metres" — critical for aircraft landing approaches into Indian airports. Indian SBAS operated by ISRO + AAI, three GEO satellites (GSAT-8/-10/-15) broadcasting differential GPS corrections on L1. Coverage centred on the Indian sub-continent for civil-aviation use; first SBAS over the equatorial ionosphere, where TEC variations make corrections harder.
• Galileo Galileo (EU GNSS constellation) Europe's version of GPS — fully civilian-run, with somewhat better accuracy than the free GPS signal. Every smartphone made in the last few years uses Galileo and GPS together to nail your location faster, especially in city streets between tall buildings. Europe's GNSS system, operated by ESA and the EU. 30 medium-Earth-orbit satellites; provides civilian PRS service with higher integrity and accuracy than free GPS.
• GBT Green Bank Telescope The largest steerable radio dish on Earth — 100 metres across, parked in a "radio-quiet zone" in rural West Virginia where Wi-Fi and cell phones are banned so the dish can listen for the faintest signals from space. SETI projects use it to scan for alien signals. 100 m fully-steerable single-dish radio telescope in the Green Bank Observatory radio-quiet zone (West Virginia, USA). Operates 100 MHz – 116 GHz; lead instrument of the US NRAO for spectroscopy, pulsar timing, Breakthrough Listen SETI surveys, and HI-line cosmology. Public archive of raw + reduced data.
• GeV giga-electron-volt (10⁹ eV) A billion electron-volts. The energy scale of cosmic-ray particles smashing into the upper atmosphere, the gamma rays from feeding black holes, and the colliding beams at CERN. Far above anything you'd ever encounter in daily life. Photon-energy unit at the very top of the EM spectrum — TeV / PeV gamma rays from blazars, galactic-centre sources, and cosmic-ray air showers tracked by HESS / VERITAS / MAGIC / CTA observatories sit here. 1 GeV = 10⁹ eV; particle-physics colliders measure beam energy in GeV / TeV. See also MeV, keV.
• GLONASS Globalnaya Navigatsionnaya Sputnikovaya Sistema Russia's version of GPS. Smartphones combine its satellites with GPS, Galileo, and BeiDou to get a faster, more accurate lock — especially in northern latitudes where GLONASS satellites stay high in the sky. Russia's GNSS constellation. 24 satellites in three orbital planes; uses FDMA rather than CDMA, so receivers need a different correlator design from GPS-only units.
• GMRS General Mobile Radio Service A step up from FRS walkie-talkies in the US: same channels, but with a cheap $35 family licence (no exam) you can run higher power and use repeaters. Off-roaders, large families, and small businesses use it for longer-range comms without going to ham radio. US 462/467 MHz UHF service — same channels as FRS but licensed (no exam, $35 per family for 10 years), allowing higher power (50 W mobile, repeater operation).
• GMSK Gaussian Minimum-Shift Keying A spectrally-clean variant of FSK — same "two-tone whistle" idea but smoothed so it doesn't bleed into adjacent channels. It's the modulation behind old 2G GSM phone calls and ham D-STAR voice. Continuous-phase FSK variant with a Gaussian pre-filter that eliminates spectral side-lobes. Standard on GSM and D-STAR.
• GNSS Global Navigation Satellite System The umbrella term for all satellite-navigation systems together — GPS (US), GLONASS (Russia), Galileo (EU), and BeiDou (China). Your phone listens to all four at once, which is why it locks on so quickly and works even in city canyons. Umbrella term covering GPS (US), GLONASS (RU), Galileo (EU), and BeiDou (CN). Modern receivers combine all four constellations for faster lock and better urban accuracy.
• GPS Global Positioning System The original US satellite-navigation system. The name is so well known that "GPS" has become slang for any satnav, even when your phone is actually using all four global systems combined. Free, no subscription, accurate to a few metres anywhere on Earth. The original US satellite-navigation constellation — 24+ satellites in medium Earth orbit, civilian L1 carrier on 1575.42 MHz. Now one of four GNSS systems most receivers fuse together.
• GPSDO GPS-Disciplined Oscillator A precision clock that uses GPS satellites to keep itself perfectly accurate. The GPS signal carries an incredibly steady tick from onboard atomic clocks, and this box copies that accuracy into a rock-solid reference frequency on your lab bench. Ham radio operators and SDR experimenters use them to keep frequencies dead accurate. Crystal or rubidium oscillator continuously corrected against the 1 PPS pulse from a GNSS receiver. Output: a 10 MHz reference of long-term stability ≤ 1 × 10⁻¹² (limited by the 1 PPS), short-term phase-noise of the underlying oscillator. The standard frequency reference for SDR labs, ham EME / weak-signal work, and timing distribution. Common units: Leo Bodnar mini- precision GPSDO, Trimble Thunderbolt-E, Bodnar GPSDO.
• GSM Global System for Mobile Communications The original 2G cellular network — the technology behind the first decade of mobile phones. Mostly switched off in modern countries, but still alive in some places for cheap IoT devices and old burner phones. Original 2G European cellular standard on 900 / 1800 MHz, later 850 / 1900 in the US. Still operational in some networks; sniffable with an SDR + gr-gsm if you have your own SIM.
• h Planck constant A tiny universal number that connects "how often a wave wiggles" to "how much energy each photon carries". Multiply Planck's constant by frequency and you get the photon's energy. It's the cornerstone of quantum physics — when it appears in an equation, you've left classical physics behind. Universal constant h ≈ 6.626 × 10⁻³⁴ J·s, defined exactly as 6.62607015 × 10⁻³⁴ J·s since the 2019 SI redefinition. It links photon energy to frequency: E = h·ν = h·c/λ. The constant whose presence distinguishes quantum from classical physics.
• HAARP High-frequency Active Auroral Research Program A giant Alaskan transmitter that beams 3 megawatts of radio energy up at the ionosphere to study how the upper atmosphere reacts. It spawned more conspiracy theories than research papers, but in reality it's just a very loud antenna for physicists. US ionospheric heater near Gakona, Alaska — 3.6 MW HF transmitter array used for ionosphere physics experiments. Hobbyists can listen for HAARP's test transmissions during scheduled campaigns.
• HackRF HackRF One (open-source SDR) A popular hacker-friendly software radio. Unlike the receive-only RTL-SDR, a HackRF can also transmit, which makes it the go-to tool for wireless security researchers studying how garage doors, car remotes, and IoT devices talk. Open-source half-duplex SDR by Great Scott Gadgets — 1 MHz to 6 GHz, 20 MS/s, transmit and receive. Standard hardware for security research and signal experimentation; fork of the broader USRP and LimeSDR ecosystem.
• HEMT High-Electron-Mobility Transistor A very fancy transistor that's extremely quiet and works at very high frequencies. They're the first thing radio telescopes hear incoming signals through, and they're also what makes modern 5G cell-tower transmitters efficient enough to be practical. Heterojunction (typically GaAs / GaN / InP) field-effect transistor with a quantum-well 2-D electron gas — extreme low-noise + high- frequency performance. Cryogenic HEMT LNAs (≤ 0.3 dB noise figure at 1.4 GHz) are the canonical front-end for radio astronomy, satellite earth stations, and amateur EME / weak-signal work. GaN HEMTs power modern 5G PAs.
• HF High Frequency (3–30 MHz) The "shortwave" band — radio that bounces off the upper atmosphere and can travel thousands of kilometres with just a few watts. It's why hams can talk across continents from a backyard antenna, and why BBC World Service shortwave can reach into countries with no internet. Shortwave — refracts off the ionosphere, so HF signals can travel thousands of kilometres on a few watts. Home of amateur, broadcast, marine, aviation (HFDL), and time-signal services.
• HFDL HF Data Link How airliners send data messages back to dispatch when they're far out over an ocean — too far for normal VHF radio. They use long-range shortwave radio instead, in a digital format you can decode on a PC with an SDR. ARINC-operated worldwide aircraft data network on HF (2.9–22 MHz) — used over the oceans where VHF coverage runs out. Decodable with PC-HFDL or dumphfdl.
• HI Neutral atomic hydrogen (HI line, 21 cm) Hydrogen atoms scattered throughout space hum at a very specific radio frequency (around 1420 MHz). Map that hum and you map where the gas in our galaxy is — it's how astronomers found the spiral arms of the Milky Way. With a 1 m dish and a $25 SDR you can detect this signal yourself from a backyard. Hyperfine spin-flip transition of neutral hydrogen at 1420.4057517 MHz (λ ≈ 21.106 cm). The most abundant tracer in the universe and the cornerstone of galactic radio astronomy — measure galactic rotation curves with a 1 m dish + 1420 MHz LNA + RTL-SDR in 30 minutes. Photon energy ≈ 5.87 μeV — see ν·h.
• HPUE High-Power User Equipment A cell phone or modem that's allowed to transmit at roughly twice the normal power. Mostly seen in fixed home 5G modems and the FirstNet emergency-services network — the extra power helps reach a cell tower from farther away. A power-class-2 cellular phone or modem allowed to transmit at 26 dBm (≈400 mW) instead of the usual 23 dBm (200 mW). Enabled on n14 FirstNet and n41 / n78 / n79 mid-band 5G to extend uplink range.
• HT Handheld Transceiver Any handheld walkie-talkie-style two-way radio. Hams usually mean Baofengs and similar; public-safety pros mean Motorolas. Modern handhelds support both old-school FM voice and digital modes that sound much cleaner. Walkie-talkie-style portable two-way radio, 1–8 W typically. The workhorse form-factor of amateur radio (Baofeng UV-5R, Yaesu FT-65, Kenwood TH-D75), public-safety (Motorola APX, Hytera PD7), and PMR446 / FRS / GMRS. Modern HTs run digital modes — DMR / D-STAR / C4FM / P25 — alongside FM.
• IARU International Amateur Radio Union The international club-of-clubs that speaks for ham radio at UN spectrum meetings. When commercial wireless companies push to claim a chunk of spectrum, IARU shows up to argue for keeping it available to hams. Worldwide federation of national ham societies (ARRL, RSGB, DARC, etc.). Represents amateurs at ITU World Radiocommunication Conferences.
• ICAO International Civil Aviation Organization The UN agency that writes the rulebook for civil aviation worldwide — what pilots say on the radio, how aircraft are tracked, what altitudes routes use. Every plane has a unique ICAO hex code visible to anyone tracking ADS-B. UN agency that sets international aviation rules — radio call-sign schemes, ATC phraseology, ADS-B mandates, and the 24-bit aircraft hex addresses you decode on 1090 MHz.
• ICNIRP International Commission on Non-Ionizing Radiation Protection The independent scientific body that sets safe exposure limits for radio waves and lasers — the limits behind "your phone is safe" and "stay this many metres from this antenna". National regulators around the world just copy ICNIRP's numbers into law. Independent body that publishes the global RF and optical exposure guidelines national regulators (FCC OET-65, BNetzA, Ofcom) base their safe-distance rules on.
• IEC International Electrotechnical Commission The international standards body for "anything electrical" — plugs, batteries, USB-C, laser-safety classes, even the rules for testing a kitchen blender. Most of the technical promises printed on consumer-electronics boxes trace back to an IEC standard. Geneva-based standards body for electrical and electronic technology — safety, EMC, batteries, optical fibres. Co-publishes ISO/IEC standards and overlaps with the IEEE on radio topics.
• IEEE Institute of Electrical and Electronics Engineers The world's biggest engineering club — and the standards body behind Wi-Fi (IEEE 802.11), Ethernet (802.3), and Zigbee (802.15.4). When you read about "Wi-Fi 7" or "10 Gigabit Ethernet", the actual technical spec is an IEEE document. Professional society that authors the 802.x family (Wi-Fi 802.11, Zigbee 802.15.4, Ethernet 802.3, etc.) and many of the spectrum-relevant standards referenced in this table.
• IF Intermediate Frequency A "waystation" frequency inside a radio. The antenna picks up a signal at one frequency, and the radio mixes it down to a fixed "IF" before doing all the hard filtering and decoding work. It's how every radio from a 1930s tube set to a satellite TV box worked — until software radio collapsed it all into a chip. Frequency to which an incoming RF signal is mixed for filtering + amplification before final demodulation, in a superheterodyne receiver. Choice of IF determines image-rejection and selectivity: classical AM broadcast 455 kHz, FM broadcast 10.7 MHz, FM HT 21.4 MHz / 455 kHz, satellite L-band 950–2150 MHz from a Ku-band LNB. SDRs have largely collapsed the IF chain into direct sampling.
• IFR Instrument Flight Rules "I'm flying on instruments, I can't see outside" — the rules pilots follow when they're in cloud or at night and have to trust their cockpit displays + ATC instead of their eyes. Drives the whole radio-navigation infrastructure at airports. Aviation flight regime in which the pilot relies on instruments rather than visual reference — typical in cloud or at night. Drives demand for ILS, DME, and the ATC radio infrastructure.
• IGS International GNSS Service A free volunteer network of GPS stations around the world that publishes very precise tracking data. Surveyors, scientists, and serious hobbyists download IGS data, combine it with raw recordings from a $200 GPS module, and get centimetre-level positions hours later — for free. Voluntary federation of ~500 worldwide tracking stations producing high-precision GNSS products: precise satellite orbits + clocks (mm-level), Earth- orientation parameters, ionosphere TEC maps, troposphere zenith delays. All data and products are public; combined with RINEX raw observations from a hobby u-blox receiver, IGS lets you do cm-level post-processed positioning anywhere on Earth.
• IIHS Insurance Institute for Highway Safety (USA) A US safety lab paid for by car insurers — they crash-test new cars and hand out the "Top Safety Pick" awards car ads brag about. Their increasingly strict tests are what pushed automakers to ship auto-braking and lane-keep radar on every new model. US non-profit funded by auto insurers that crash-tests new vehicles and rates ADAS performance — the "Top Safety Pick" and "Top Safety Pick+" awards. Their AEB + headlight + side-impact tests have driven much of the radar / camera adoption in US-market vehicles over the past decade.
• ILS Instrument Landing System The "invisible glide path" airliners follow down to the runway in bad weather. Two ground-based transmitters send radio beams — one for the left-right alignment, one for the up-down descent — and the autopilot rides them all the way to landing. Precision aircraft landing aid — a 108–112 MHz VHF localizer carrier (lateral guidance) plus a 329–335 MHz UHF glideslope (vertical). Tunable on a regular SDR.
• IMU Inertial Measurement Unit A chip that knows which way is up and how fast you're rotating — the thing inside your phone that turns the screen when you tilt it. In a self-driving car or drone it fills in the gaps when GPS cuts out (in tunnels, under bridges) by tracking how the vehicle has moved. Bundle of accelerometers + gyroscopes (and usually magnetometers) that estimates orientation and short-term motion in 6 or 9 degrees of freedom. Fused with GNSS in autonomous vehicles, drones, and smartphones, an IMU lets the system "coast" through GPS dropouts (urban canyons, tunnels) by integrating sensed motion. Common parts: Bosch BMI270, ICM-42688-P, MPU-6500, ADIS16505.
• IR Infrared (~700 nm – 1 mm, ~1.65 eV – 1.24 meV) The "heat" light — just below visible red. Your skin gives off infrared (thermal cameras see it), your TV remote uses it, and fibre-optic cables carry data through it. You can't see it, but a lot of the modern world runs on it. Spectrum band between visible red and microwave, conventionally split into NIR (0.78–3 μm), MWIR (3–8 μm), LWIR (8–14 μm — the thermal-imaging band), and far-IR / sub-mm (15 μm – 1 mm). Hosts thermal imaging, fibre-optic telecoms (1310 / 1550 nm), free-space optical links, IR remote controls, and CO₂-laser industrial cutting (10.6 μm).
• IRAM Institut de Radioastronomie Millimétrique A European millimetre-wave observatory with a 30 m dish in the Spanish mountains and a smaller array in the French Alps. It was one of the dishes that contributed to the famous first image of a black hole's shadow. Franco-German-Spanish millimetre-wave observatory operating the 30 m IRAM-30m single dish (Pico Veleta, Spain) and the NOEMA interferometer (Plateau de Bure, France) at 70–375 GHz. Major partner in VLBI networks and the Event Horizon Telescope; archive is publicly accessible.
• IRNSS Indian Regional Navigation Satellite System / NavIC India's regional version of GPS — branded "NavIC". Only a handful of satellites, parked to cover India and surrounding countries. Newer phones sold in India support it alongside GPS, Galileo, and BeiDou. Indian regional GNSS, 7 satellites + augmentation, with primary coverage over the Indian subcontinent. Civilian signals on L5 1176.45 MHz and S-band 2492.028 MHz; the S-band coupling makes it unique among GNSSes. Multi-constellation receivers (u-blox NEO-M9N, ZED-F9P, Quectel LC79H) include NavIC support for users in the region.
• ISM Industrial, Scientific, Medical The "license-free" radio bands where anyone can transmit at low power — 2.4 GHz (Wi-Fi, Bluetooth, microwave ovens), 5.8 GHz (Wi-Fi, drones), 868/915 MHz (smart-home sensors). It's why your Wi-Fi router doesn't need a permit, but also why these bands are incredibly crowded. Worldwide license-free spectrum bands (433 MHz EU, 902–928 MHz US, 2.4 GHz, 5.8 GHz, 24 GHz) shared by Wi-Fi, Bluetooth, microwave ovens, and a long tail of consumer gadgets — anyone can transmit at low power without a license, subject to EIRP limits.
• ITAR International Traffic in Arms Regulations (US Department of State) The strict US rules for exporting genuine military hardware — weapons, missiles, military radar, defence satellites. Crossing ITAR is a serious federal offence; it's the legal reason a US aerospace company can't just ship a missile-guidance chip overseas. US export-control regime for defence articles (22 CFR Parts 120–130). Cat. IV "Launch Vehicles, Missiles, Rockets, Torpedoes, Bombs, & Mines" covers directed-energy weapons like ADS; Cat. XI covers military electronics and radar; Cat. XV covers spacecraft. Sister regime to EAR (commercial / dual-use).
• ITU International Telecommunication Union The UN body that decides, at the global level, who gets to use which radio frequencies for what. They split the world into three "Regions" (Europe/Africa, Americas, Asia/Pacific) so that adjacent countries don't accidentally interfere with each other. UN agency that allocates global spectrum and divides the world into Regions 1 (EU/Africa/RU), 2 (Americas), and 3 (Asia/Oceania). Every national regulator (FCC, BNetzA, etc.) operates downstream of ITU.
• IVS International VLBI Service for Geodesy and Astrometry The international group that coordinates the radio telescopes used to measure exactly how Earth wobbles and rotates. Their data is what keeps GPS and every other navigation system aligned to the actual sky — Earth's tilt and spin drift slightly, and IVS measures it. Global coordination body for VLBI Earth-orientation, geodesy, and astrometry. Defines the standard frequencies (S/X/K/Q-band — 2.3 / 8.4 / 22 / 43 GHz), session schedules, and data-correlation pipelines. Continuously refines the celestial reference frame (ICRF) used by every GNSS and deep-space mission.
• JJY JJY (Japanese LF time signal) Japan's official time-signal radio station — the one Japanese "radio-controlled" alarm clocks and watches sync to. Equivalent to DCF77 in Germany or WWVB in America. Reaches most of East Asia. Japan's LF time-and-date transmitter — 40 kHz from Otakadoyayama and 60 kHz from Hagane. Receivable across East Asia; the Japanese counterpart of DCF77 and WWVB.
• JS8 JS8Call A ham-radio mode that's basically FT8 stretched into a real chat program — type messages, leave voicemails on the air, talk in groups. Hugely popular for emergency communications because it works on tiny battery-powered shortwave rigs. Real-time keyboard-to-keyboard mode based on FT8 tones — adds free-text messaging, store-and-forward relay, and group calls. Popular for ham HF chatting and emergency comms.
• JT65 JT65 (Joe Taylor 65-FSK) A predecessor of FT8 — same general idea but slower (one-minute transmissions). Hams who bounce signals off the Moon or off meteor trails still use it because the longer transmission gives the decoder more to work with. Earlier weak-signal mode from WSJT-X — 60-second cycles, 65 tones. Largely supplanted by FT8 on HF but still common on EME and meteor scatter.
• K-band K-band (~18–26.5 GHz) The microwave band best known for police speed-radar guns ("K-band radar"). Older cars used 24 GHz K-band radar for blind-spot detection until they were phased out for the modern 77 GHz radar. Microwave waveguide band (WR-42) — sandwiched between Ku-band (below) and Ka-band (above). Used by police radar (24.125 GHz "K-band radar guns"), legacy automotive 24 GHz SRR (now phased out), VLBI standard at 22 GHz, and the H₂O maser radio-astronomy line at 22.235 GHz.
• Ka-band Ka-band (~26.5–40 GHz) The microwave band used by Starlink, Viasat, and modern satellite internet. Smaller dishes than older satellite TV used to need, but rain attenuates the signal noticeably — heavy rain can briefly drop your Starlink to a crawl. Microwave band (WR-28) for satellite uplink (FSS 27.5–30 GHz — Inmarsat, Viasat, SES, Starlink Gateway), satellite downlink (17.7–21.2 GHz — note this is Ku/Ka boundary terminology), cellular 5G FR2 (n257 / n258 / n261 mmWave), and microwave backhaul. Heavy rain-fade, line-of-sight only.
• keV kilo-electron-volt (10³ eV) Thousand-electron-volt photons — the energy range of X-rays. The "thousands of eV" is why X-rays can knock electrons off atoms and damage cells, which is why dental X-ray machines need lead shielding and you only get a few per year. Photon-energy unit for soft to hard X-rays: medical CT 80–140 keV, diagnostic X-ray 30–80 keV, soft X-ray astronomy 0.1–10 keV, hard X-ray 10–100 keV. Above ~100 keV photons cross the conventional "X-ray to gamma-ray" boundary. See eV, MeV.
• KNX-RF KNX-RF (wireless KNX) The wireless version of KNX, the European pro standard for smart-building wiring (lights, blinds, heating). Used when adding KNX to an older building where you can't rip the walls open to pull new cables. Wireless variant of the KNX building-automation standard — runs at 868 MHz ISM in Europe. Used for retrofitting KNX into buildings without pulling new wires.
• Ku-band Ku-band (~12–18 GHz) The microwave band most satellite TV uses — the dishes bolted to European balconies and American rooftops to receive Astra, SES, DirecTV, etc. Ham radio operators repurpose old satellite dishes to listen to hydrogen gas in the Milky Way. The dominant satellite-TV downlink band (10.7–12.75 GHz, e.g. SES / Eutelsat / Astra) plus VSAT terminal uplink (14 GHz). Smaller dishes than C-band (60–90 cm typical), some rain-fade, widely deployed worldwide. Hobbyists repurpose offset Ku dishes as 1.4 GHz HI-line radio telescopes (the dish geometry is the same; only the feed changes).
• L1 GNSS L1 band (1575.42 MHz) The main GPS frequency — what every phone, watch, and car satnav listens on. The original civilian satellite-navigation signal that became free for the world in the 1980s. Wavelength is about 19 cm, so a phone-sized GPS antenna can pick it up. The legacy civilian GNSS frequency, carrying GPS C/A + P(Y) + M-code, Galileo E1, BeiDou B1C, and QZSS L1. The single-frequency band almost every smartphone, fitness watch, and car nav uses. λ ≈ 19.04 cm, photon energy ≈ 6.5 μeV. See also L2 / L5.
• L2 GNSS L2 band (1227.60 MHz) The second GPS frequency. Receivers that can hear both L1 and L2 at once can cancel out the distortion the upper atmosphere adds to the signal — the trick behind survey-grade "centimetre-accurate" GPS. Second GPS frequency, carrying P(Y) + L2C + M-code; also legacy GLONASS L2 + BeiDou B3I 1268.52 MHz. Pairing L1+L2 lets dual-frequency receivers cancel ionospheric delay directly (it scales as 1/ν²) — the basis of survey- grade RTK positioning. λ ≈ 24.42 cm.
• L5 GNSS L5 band (1176.45 MHz) The newest, beefiest GPS frequency — wider bandwidth and stronger signal, designed for aviation safety. Modern iPhones and Pixels listen on L5 too; if your phone's GPS just got noticeably better around 2019–2020, this is why. Modernised GPS "safety-of-life" frequency, also Galileo E5a, BeiDou B2a, IRNSS / NavIC, and QZSS L5. Wider bandwidth + higher TX power + sits in the protected aeronautical radio-navigation band — best multipath rejection of any GNSS signal, and the path to triple-frequency RTK. λ ≈ 25.48 cm.
• LCA Lane-Change Assist A driving-aid that warns you not to change lanes when there's a car coming up fast in your blind spot. Same hardware as blind-spot detection, just a smarter alert — it watches further behind you when you signal a lane change. Short-range radar feature that warns of approaching vehicles in the adjacent lane during a lane-change manoeuvre — typically paired with BSD and turn-signal logic. Uses bumper-corner 77–81 GHz SRR modules.
• LCD Liquid Crystal Display The most common type of screen — a backlight shining through tiny "shutter" cells that twist to let colours through. Calculators, older monitors, and the screens on most radios still use LCDs. OLED is the newer competing tech where each pixel makes its own light. Display technology using polarised light + voltage-addressable liquid-crystal cells in front of a backlight. Dominant on radios, SDR front-panels, and most consumer devices since the 1990s. Distinct from emissive technologies (LED / OLED) where pixels make their own light.
• LED Light-Emitting Diode A semiconductor that turns electricity directly into light. Long life, very efficient, and the colour is set by the material — that's why LED lights now come in everything from infrared (TV remotes) to UV (germicidal lamps) and every visible colour in between. Semiconductor diode that emits light when forward-biased — a radiative band-gap recombination. Wavelength is set by material (GaAs ≈ 870 nm IR, InGaN 450 nm blue, AlGaInP 630 nm red, LightCorps UV-C ~270 nm). Backbone of indicators, displays (OLED), automotive lights, free-space optical links, and even tunable-spectrum grow-lamps.
• LF Low Frequency (30–300 kHz) "Long-wave" radio — wavelengths kilometres long, signals that follow the curve of the Earth instead of going straight. It's where the DCF77/WWVB time signals live, and where some European countries still broadcast long-wave AM news. Long-wave — kilometre-scale wavelengths that hug the ground for stable long-distance propagation. Hosts DCF77, WWVB, JJY time signals, aeronautical NDBs, and the 2200 m amateur band.
• LiDAR Light Detection And Ranging Radar that uses laser pulses instead of radio waves. It builds a 3D point cloud of everything around — that's how self-driving cars "see", and how the iPhone Pro measures rooms. Much higher resolution than radar, but rain and fog dim it; that's why cars use both. Optical equivalent of radar — a pulsed or amplitude-modulated laser (905 / 1550 nm typically) measures range to objects via time-of-flight or coherent FMCW. Used in self-driving cars (Velodyne, Innoviz, Luminar Iris), aerial topographic surveys, archaeology, ARKit / depth sensing on phones, and autonomous- drone obstacle avoidance. Complementary to 77 GHz LRR radar — LiDAR is higher resolution, radar penetrates fog / rain better.
• LimeSDR LimeSDR (open-source SDR) Another open-hardware software-defined radio — a competitor to HackRF. Can transmit and receive at the same time, which makes it handy for experiments where you want to relay a signal or run a private cellular base station. Lime Microsystems' open-hardware SDR — full-duplex, 100 kHz–3.8 GHz on the Mini, 30 MHz–3.8 GHz on the USB. Competes with HackRF and USRP.
• LNA Low-Noise Amplifier A small amplifier mounted right at the antenna that boosts a faint signal without adding noise. Putting an LNA on a satellite-TV dish is the difference between watching TV and watching static — every weak-signal receiver setup needs one. First-stage amplifier mounted close to (ideally at) the antenna, designed to add as little noise as possible. The single biggest factor in a receiver's weak-signal sensitivity, especially above VHF.
• LNB Low-Noise Block downconverter The chunky bit at the end of a satellite dish's arm. It takes the very-high-frequency signal from the satellite, amplifies it, and converts it down to a lower frequency that ordinary coax cable and TV tuners can handle. Satellite-dish front-end that combines an LNA with a frequency downconverter — Ku-band in (10–12 GHz) becomes L-band out (950–2150 MHz) on a coax that an ordinary tuner or SDR can read.
• LO Local Oscillator The "tuning knob" inside every radio — a clean, adjustable tone that the radio mixes with the incoming signal to slide a chosen station into its filters. If the LO drifts, the radio's tuning drifts; the better the LO, the cleaner the radio. The clean, tunable carrier that's mixed against an incoming (or outgoing) RF signal in a superheterodyne radio. LO phase- noise sets the receiver's adjacent-channel rejection floor, and LO drift causes the "frequency wandering" you see on SDRs that aren't GPSDO- locked. Modern radios use fractional-N PLLs to synthesise LOs over wide ranges.
• LoRa Long Range (chirp-spread-spectrum) A clever radio modulation that lets a battery-powered sensor talk to a gateway many kilometres away, at the cost of being painfully slow. The radio behind Meshtastic, smart-city sensors, and a lot of cheap GPS tracker tags. Semtech's low-power, low-data-rate radio modulation. Lets a 25 mW transmitter reach 5–15 km outdoors at the cost of just a few hundred bits per second. Foundation of LoRaWAN, Helium, and Meshtastic.
• LoRaWAN Long Range Wide-Area Network The networking layer that turns plain LoRa radio chips into a real network. Think of it as Wi-Fi for tiny battery sensors — gateways on rooftops collect packets and forward them to the internet. Open MAC-layer protocol on top of LoRa — defines gateways, network servers, and end-device classes A/B/C. TheThingsNetwork runs the largest community deployment.
• LRR Long-Range Radar (76–77 GHz automotive) The forward-looking radar behind a car's front grille — the one that sees vehicles 200 m up the road for adaptive cruise control and automatic braking. Narrow beam, long range. Front-mounted radar covering 200–250 m at ±10° field of view. Provides the range + velocity input for ACC and forward AEB. Distinct band slice (76–77 GHz, ITU footnote 5.559) from the wider 77–81 GHz UWB short-range allocation.
• LTE Long-Term Evolution (4G cellular) "4G" — the cellular generation between old 3G and modern 5G. Still the workhorse of most networks: 5G coverage often falls back to LTE when there's no strong 5G nearby. Your phone routinely jumps between them without telling you. 3GPP cellular standard now sitting alongside 5G NR. Its bands keep being numbered (B1, B3, B7…) and many were directly re-used as the corresponding 5G n-bands (n1, n3, n7…). Uses OFDMA downlink with SC-FDMA uplink, both FDD and TDD.
• M-Bus Meter-Bus (EN 13757) The European standard most utility companies use to read electricity, water, and gas meters automatically. The wireless flavour (wM-Bus) means a van driving down your street can collect a whole neighbourhood's readings in seconds. European utility-meter standard with a wireless variant (wM-Bus) at 169 MHz VHF or 868 MHz ISM. The protocol used behind much European AMR infrastructure.
• MEMS Micro-Electro-Mechanical Systems Tiny mechanical parts built right into silicon chips — moving levers and gears the size of a hair's width. They're how phones know when you've rotated the screen, how AirPods sense taps, and how cars' airbags trigger on impact. Micron-scale moving silicon structures: accelerometers, gyroscopes, microphones, pressure sensors, RF resonators, and optical mirrors. Every smartphone has a 6-axis MEMS IMU; MEMS RF resonators + filters now compete with quartz crystals + SAW filters in modern radios.
• MeV mega-electron-volt (10⁶ eV) Million-electron-volt photons — the energy of gamma rays from radioactive decay and the energy released when matter meets antimatter. This is the energy scale of nuclear medicine and the fingerprint emissions of radioactive isotopes. Photon-energy unit for nuclear and gamma-ray spectra: ⁶⁰Co decay emits 1.17 / 1.33 MeV gammas, ¹³¹I emits 364 keV (just below the MeV range), pair-production threshold ≈ 1.022 MeV. PET imaging uses 511 keV annihilation pairs. See eV, GeV.
• meV milli-electron-volt (10⁻³ eV) Thousandth-of-an-electron-volt photons — the energy scale of millimetre-wave and far-infrared radiation. Way below the energy of ordinary heat, which is why RF can only warm things, never knock electrons off atoms (the way X-rays can). Photon-energy unit for the millimetre / sub-THz / far-infrared range. A 60 GHz mmWave photon ≈ 0.25 meV; thermal energy at room temperature kT ≈ 26 meV (so RF photons are sub-thermal — they don't ionise anything, they only heat). See μeV below and eV above.
• MF Medium Frequency (300 kHz – 3 MHz) The home of AM broadcast radio. Daytime, signals only travel a few hundred kilometres; at night they bounce off the ionosphere and a cheap pocket radio can pick up stations from another continent — sometimes still in the dial today. Hosts the AM broadcast band (530–1700 kHz), the 630 m and 160 m amateur bands, and marine/aeronautical voice. Strong night-time skywave DX — you can hear stations a continent away with a ferrite-rod radio.
• MIMO Multiple-Input Multiple-Output The "lots of antennas at once" trick — a Wi-Fi router with multiple sticks isn't doing it for looks. By using several antennas simultaneously, the radio can send several streams of data in parallel, dramatically boosting speed. Modern 5G towers can have 64+ antenna elements. Multi-antenna technique that boosts capacity and range by sending parallel data streams or steering beams. Standard on Wi-Fi 5/6/7, LTE, and 5G NR; massive-MIMO base stations have 64+ elements.
• MMIC Monolithic Microwave Integrated Circuit A microwave radio shrunk down onto a single chip. They're why a 77 GHz car radar can fit on a fingernail and a Wi-Fi router costs $30. Twenty years ago, the same circuit took up a desktop full of metal-cased modules. Single-chip RF / microwave circuit on a III-V (GaAs, GaN, InP) or SiGe / CMOS substrate, integrating amplifiers, mixers, oscillators, and switches above ~1 GHz. Modern automotive 77 GHz radar SoCs (TI AWR / IWR, Infineon BGT, NXP TEF82xx) are CMOS MMICs; cryogenic LNAs use GaAs / InP HEMT MMICs. The reason a commercial 77 GHz radar fits on a fingernail.
• mmWave Millimeter-wave (≈30–300 GHz) "Millimetre wave" — radio with wavelengths just a few millimetres long. Lots of data, but barely punches through a wall (or even a rainstorm). Used by car radars, the fast 5G mmWave at airports, and the body scanners at airport security. Radio above ~30 GHz with wavelengths under 10 mm — the EHF band. Used by 5G FR2, automotive 77 GHz radar, body scanners, and emerging point-to-point backhaul.
• MPE Maximum Permissible Exposure (RF / laser safety) The "how close can you safely stand to this antenna or laser" number, set by law. Bigger transmitters like FM towers, cellular base stations, and lasers all have fenced-off MPE zones around them. For typical consumer gear, you're nowhere near the limit. Regulatory upper limit on RF or laser power density a person can be exposed to without harm. RF MPE is set by ICNIRP 2020 / FCC OET-65 (e.g. 10 W/m² above 30 GHz general public). Laser MPE is set by IEC 60825 and varies by wavelength + pulse-width. Sets the standoff distances every TX antenna and laser scanner has to honour.
• MQTT Message Queuing Telemetry Transport A tiny, efficient messaging protocol that smart-home and IoT devices use to talk to each other. Home Assistant, ESP32 sensors, and most Matter / Zigbee bridges speak MQTT. Think of it as "Twitter for sensors, but lightweight enough to run on a battery". Lightweight publish/subscribe protocol designed for IoT — small headers, retained messages, last-will. The de facto messaging layer over LoRaWAN, Wi-Fi, and cellular IoT.
• MRR Mid-Range Radar (77–81 GHz automotive) Mid-range car radar — wider field of view than the long-range cruise-control radar, longer reach than the corner short-range radars. Used for traffic-jam assist and lane-change features. Intermediate-range automotive radar, 50–100 m at ±30–45° field of view — fills the gap between front LRR and corner SRR. Used for traffic-jam assist and Level 2+ highway pilot features.
• MSAS MTSAT Satellite Augmentation System (Japan) Japan's GPS-accuracy booster, similar in role to America's WAAS or Europe's EGNOS. Aimed at making aircraft landings into Japanese airports safer. Being merged into the QZSS satellite system. Japanese SBAS operated by JCAB on MTSAT-1R / -2 (now QZSS-augmented). Broadcasts differential corrections on L1 for civil aviation in Japan + parts of East Asia. Being superseded as QZSS rolls out full SBAS-equivalent payloads.
• MSF MSF (UK LF time signal) The British radio-time-signal station — the equivalent of DCF77 in Germany. UK "radio-controlled clocks" sync to it. Reaches most of north-west Europe. UK 60 kHz time signal from Anthorn — the British counterpart of DCF77 and WWVB. Receivable across most of north-west Europe.
• MSK Minimum-Shift Keying A tidy variant of FSK where the two tones are as close together as possible without interfering. Mostly relevant as the building block for GMSK — the modulation in old GSM phones and ham D-STAR. Continuous-phase FSK with the smallest possible frequency separation that keeps the two tones orthogonal. Spectrally efficient; the basis of GMSK.
• MURS Multi-Use Radio Service A US license-free walkie-talkie service on five VHF channels — used by retail stores ("clean up on aisle 4"), construction crews, and farmers. VHF travels through buildings a bit better than the UHF FRS/GMRS bands. US license-free VHF walkie-talkie service — five 151–154 MHz channels at up to 2 W. Often used for retail and short-range business comms; better in-building propagation than FRS.
• n78 5G NR Band n78 — global C-band TDD The 5G channel most of the world uses for mid-band 5G — the "C-band" around 3.5 GHz. When your phone says "5G" in Europe, India, or much of Asia, this is what it's connected to. Sweet spot between coverage and speed. TDD 3300–3800 MHz — the canonical European, UK, German, Japanese, and Indian 5G C-band, plus large parts of Asia. The most-deployed 5G mid-band in the world; lives inside FR1.
• NAA NAA (US Navy VLF transmitter, Cutler ME) A giant 1.8-megawatt US Navy radio station on the coast of Maine that transmits to submerged submarines worldwide. The signal is at such a low frequency that the antenna farm covers several square kilometres of wire. 24 kHz, 1.8 MW US Navy submarine-comms transmitter on the coast of Maine — one of the most powerful radio transmitters ever built. Audible worldwide on a VLF long-wire receiver.
• NB-IoT Narrowband IoT (3GPP) A cellular flavour optimised for tiny battery-powered sensors — think a smart-water-meter that needs to phone home once a day for ten years. Runs over the existing 4G/5G network on a tiny sliver of spectrum. Cellular standard for very-low-power, low-data-rate sensors. Runs in 200 kHz slices of LTE / 5G spectrum and is operated by every major carrier — competing with LoRaWAN and Sigfox.
• NCAP New Car Assessment Programme The "stars" you see in car safety ratings ("Euro NCAP 5-star"). An independent organisation crash-tests new cars and rates how well the safety systems work. Their tests are increasingly strict — that's why every new car ships with auto-braking and lane assist. Family of independent crash- and ADAS-test programmes — Euro NCAP (Europe), ASEAN NCAP, ANCAP (Australasia), Latin NCAP, KNCAP (Korea), JNCAP (Japan), C-NCAP (China). 5★ ratings effectively require functional AEB + lane-keep + automatic-emergency-call in Europe since 2018 — the regulatory engine behind universal ADAS adoption.
• NDB Non-Directional Beacon An old-school radio beacon at an airport that just sits there transmitting its Morse-code identifier. Pilots used to use a needle on the cockpit to point toward it. Mostly replaced by GPS, but many beacons are still on the air as a backup. Aviation/marine radio beacon transmitting an LF / MF Morse-keyed identifier (190–1750 kHz) so an aircraft's ADF needle can point home. Largely superseded by GPS but many remain on the air.
• NEXRAD Next-Generation Radar (US weather radar) The US national weather-radar network — 159 huge Doppler radars scanning the sky for rain, hail, and tornadoes. That swirling precipitation map on your weather app is built from NEXRAD data, which is streamed freely in real time. The 159-site WSR-88D Doppler radar network operated by NOAA and the FAA on 2.7–3.0 GHz SHF. Source of the colourful precipitation maps on weather apps; raw Level-II data is publicly streamed.
• NFC Near-Field Communication The "tap-to-pay" radio in your phone, your transit card, and your hotel keycard. Works only over a few centimetres on purpose — both for security and to let many cards live in a wallet without confusing the reader. 13.56 MHz inductive link with a few-cm range — tap-to-pay, transit cards, hotel keys, access badges, and the NFC reader built into every modern phone. Subset of RFID.
• NHTSA National Highway Traffic Safety Administration (USA) The US car-safety regulator. They set seatbelt standards, run the recall system, and recently mandated that every new car sold in America from 2029 onward must have automatic emergency braking built in. US Department of Transportation regulator for vehicle safety. Sets Federal Motor Vehicle Safety Standards (FMVSS), runs the US NCAP star-rating programme, and recalls non-compliant vehicles. The 2024 final rule mandating AEB on all light vehicles by 2029 is the most consequential US regulatory ADAS push to date.
• NIST National Institute of Standards and Technology America's official "what time is it" agency. They run atomic clocks in Boulder, Colorado, broadcast the time on shortwave (WWV) and long-wave (WWVB), and host the internet time servers most computers sync to. US federal lab that maintains the national time standard — operates WWVB (60 kHz LF time signal), WWV/WWVH (HF time signals), and the NTP/PTP infrastructure most computers sync to.
• NLK NLK (US Navy VLF transmitter, Jim Creek WA) A 250-kilowatt US Navy submarine-comms transmitter near Seattle. One of three sister stations (with NAA in Maine and NPM in Hawaii) broadcasting on very low frequencies to subs anywhere in the western hemisphere. 24.8 kHz, 250 kW US Navy submarine-comms transmitter near Seattle — sister station to NAA and NPM. Receivable across most of the western hemisphere on VLF.
• NOAA National Oceanic and Atmospheric Administration The US weather agency. They operate the weather satellites, the national radar network, NOAA Weather Radio (an emergency broadcast on dedicated frequencies), and the Sun-watching alerts about solar storms that disrupt radio. US science agency that operates weather satellites (NOAA-15/18/19, GOES-East/West), NEXRAD radar, and NOAA Weather Radio (162.4–162.55 MHz). The Sun-watching SWPC issues space-weather alerts that affect HF propagation.
• NPM NPM (US Navy VLF transmitter, Lualualei HI) The Hawaiian sister station of NAA and NLK — a 425-kilowatt VLF Navy transmitter on Oahu. The "loudest" signal you'll pick up across the Pacific on a VLF receiver. 21.4 kHz, 425 kW US Navy submarine-comms transmitter on Oahu — one of the dominant VLF signals audible across the Pacific. Sister station to NAA and NLK.
• NPRM Notice of Proposed Rulemaking (US regulatory) The "we're thinking of making this rule, what do you think?" step in US federal regulation. The FCC or NHTSA publishes a draft and invites public comments before finalising. Each major spectrum rule change started life as an NPRM. Stage of US federal rulemaking where an agency (e.g. FCC, NHTSA) publishes proposed regulations + invites public comment before finalising. The Spectrum Horizons NPRM (95–275 GHz) and the AEB-mandate NPRM are recent examples that shaped the spectrum table here.
• NR 5G New Radio The official name for the 5G radio technology. "5G NR" is just the formal way to say "the new radio behind 5G", as opposed to the older LTE radio behind 4G. The 5G physical layer — successor to LTE. Adds flexible numerology, mmWave operation, and massive MIMO; spectrum is split into FR1 and FR2.
• NRAO National Radio Astronomy Observatory (USA) America's official radio-astronomy lab — they operate famous arrays like the Very Large Array in New Mexico (the dishes from the film Contact) and the cross-country VLBA. They also publish their data freely, so anyone can play with it. US national radio-astronomy facility (NSF + Associated Universities) operating the VLBA, Karl G. Jansky Very Large Array (VLA), and partner-share in ALMA. Hosts public data archives, summer student programmes, and education kits used by amateur radio astronomers (SARA, OSRT).
• NTP Network Time Protocol The internet protocol that keeps every computer in the world roughly time-synced. Your laptop quietly chats with internet time servers a few times an hour to keep its clock accurate. Without NTP, file timestamps and security certificates would be chaos. Internet protocol for synchronising clocks (RFC 5905), in continuous use since 1985. A stratum-1 NTP server is one with a directly- attached primary reference clock — typically GPS + PPS, occasionally a CSAC or rubidium. A Raspberry Pi + u-blox + PPS-pinned chrony gives sub-microsecond accuracy for a few hundred dollars — the canonical hobbyist timing project.
• NTRIP Networked Transport of RTCM via Internet Protocol A way of streaming GPS-precision corrections over the internet. Farms, surveyors, and self-driving cars subscribe to an NTRIP feed so their receivers can lock in centimetre-accurate positions. HTTP-based protocol (RFC-style spec by BKG) for streaming RTK base-station corrections over the public internet. NTRIP casters serve thousands of base streams; clients subscribe with credentials and receive RTCM 3.x messages in real time. Backbone of national CORS networks, agricultural fleet correction services, and DIY RTK setups.
• NXDN Next-Generation Digital Narrowband Yet another digital two-way-radio standard, common with railways and utility companies. Same general idea as DMR or P25 — cleaner digital voice and easier networking than old analog FM. Open digital-voice + data standard developed by Icom + Kenwood, competing with DMR and P25. Uses FDMA in 6.25 / 12.5 kHz channels — popular for utility, public- safety, and rail communication, especially in the Americas + Asia. Decodable with DSDPlus + an SDR (unencrypted traffic only).
• Ofcom Office of Communications The UK's wireless watchdog. They auction off spectrum to mobile carriers, regulate TV and radio broadcasting, and run the ham-radio exam system. Same role as the FCC in the US. The UK's wireless regulator — UK equivalent of the FCC and BNetzA. Licenses spectrum, runs the ham exam (with the RSGB), and enforces ETSI rules.
• OFDM Orthogonal Frequency-Division Multiplexing The trick behind almost every fast modern radio: instead of sending one wide signal, send hundreds of narrow ones in parallel. It's much more resilient to multi-path echoes inside buildings. Wi-Fi, 4G, 5G, DAB radio, and digital TV all use it. Modulation scheme that spreads data across many narrow subcarriers carried in parallel. The backbone of Wi-Fi, LTE, 5G NR, DVB-T, DAB, and powerline networking.
• OFDMA Orthogonal Frequency-Division Multiple Access OFDM where the subcarriers are divided up among different users at the same time. It's what lets a single 4G cell tower or Wi-Fi 6 router serve hundreds of devices smoothly without stuttering. OFDM with subcarrier groups assigned per-user instead of all-to-one — lets a base station serve many low-rate clients simultaneously. Standard downlink in LTE, 5G, and Wi-Fi 6.
• OSRT Open Source Radio Telescope A DIY radio-telescope project — small dish + cheap amplifier + USB SDR + free software — that detects the hum of hydrogen atoms in the Milky Way for under $300. A great rainy-weekend science project. Community-driven hardware / software project for amateur radio astronomy at 21 cm and beyond — typically a 1.0–1.5 m offset parabolic dish, low-noise amplifier, RTL-SDR, and GNU-Radio H-line scripts. Educational kits cost ≤ $300 and detect the galactic HI line in ≈ 30 minutes integration.
• OTH Over-The-Horizon radar Radar that bounces its pulses off the upper atmosphere to look thousands of kilometres past the horizon — used for early warning of ships and aircraft far out at sea. Soviet OTH radar was once such a nuisance that hams nicknamed it "the woodpecker". HF radar that bounces signals off the ionosphere to detect targets thousands of kilometres beyond the line-of-sight horizon. Russia's old "woodpecker" and Australia's JORN are the canonical examples; SDR users hear them as pulsing buzzes across the HF bands.
• P25 Project 25 (APCO P25) The North-American digital-radio standard for police, fire, and emergency services. Modern dispatch systems use it almost universally. Listening to unencrypted P25 with an SDR is legal in most US states; sensitive talk-groups are encrypted. North American digital land-mobile radio standard for police, fire, and EMS. Phase 1 is FDMA, Phase 2 is two-slot TDMA — both compete with DMR and TETRA. Decodable with OP25 and an SDR.
• PA Power Amplifier The "loud" part of any transmitter — the amplifier that boosts a signal up to its broadcast power. In your phone it's a tiny chip, in an FM tower it's a refrigerator-sized cabinet. Efficiency matters: a wasteful PA means worse battery life or more heat. The final stage of any transmitter — boosts modulated RF up to legal output power. Topology choice dominates radio design: Class A (linear, inefficient — broadcast TX), Class AB (most ham HF), Class C (FM), Class D / E / F (high-efficiency switching), Doherty / envelope-tracking (modern cellular). Modern PAs increasingly use GaN HEMTs.
• PAL Priority Access License (CBRS) In the US CBRS shared-spectrum scheme, "Priority Access" is the middle tier: companies who paid at auction get reserved access, while everyone else gets only what's left over. Used mostly by private cellular networks for large businesses. Middle tier of CBRS — auctioned 10-year licences to specific 10 MHz blocks in 3550–3700 MHz. Sits between protected incumbent radar and General Authorized Access (GAA) hobbyist/private use.
• Pc Pc1–Pc5 (Continuous magnetospheric pulsations) Slow electromagnetic "heartbeats" of the Earth's magnetic field, driven by the solar wind. Periods range from a fraction of a second to ten minutes — so slow you can't hear them, but a DIY magnetic coil + audio recorder can capture them as data files. ULF/ELF natural geomagnetic pulsations classified by period: Pc5 (150–600 s), Pc4 (45–150 s), Pc3 (10–45 s), Pc2 (5–10 s), Pc1 (0.2–5 s). Driven by solar wind and magnetospheric resonances; receivable with a DIY induction-coil magnetometer.
• PCB Printed Circuit Board The flat board with traces and components inside every modern electronic device. Hobbyists can design one for free in KiCad, send the files off to a $10-per-board fab in China, and have a real PCB in their hands a week later. The democratisation of electronics. Substrate (FR-4, RT/duroid, Rogers, ceramic) onto which components are soldered + traces are etched / printed. RF designers care about substrate Dk + Df (loss tangent), copper weight, and stackup — at 77 GHz, sub-mm trace widths and Rogers RO3003 / RT/duroid 5880 are mandatory; FR-4 is too lossy. Hobbyist tools: KiCad, EasyEDA, JLCPCB / OSHPark fab.
• Photon Energy Energy carried by a single photon (E = hν = hc/λ) How much "punch" a single particle of light packs. It's tiny for radio waves, slightly bigger for visible light, much bigger for X-rays, and huge for gamma rays. Above a certain threshold (~10 eV), each photon carries enough energy to knock electrons off atoms — that's the line between safe and "needs shielding". A photon's energy is set entirely by its frequency: E = h·ν = h·c/λ. Useful hobbyist short-form: E[eV] ≈ 1240 / λ[nm] — so visible 380 nm ≈ 3.26 eV, 700 nm ≈ 1.77 eV. RF photons sit at sub-thermal μeV / meV levels (no chemistry); X-ray keV and gamma MeV photons are ionising — that boundary is why this column matters for safety.
• PHY Physical layer (OSI Layer 1) The bottom layer of any networking stack — the actual modulation that turns bits into wiggles on a radio carrier. When engineers say "the 5G PHY" or "the Wi-Fi PHY" they mean the lowest layer that deals with antennas and waveforms directly. The bottom layer of a radio stack — modulation, coding, framing, timing, synchronisation, channel estimation. The PHY is what an SDR actually decodes: LTE PHY, 5G NR PHY, Wi-Fi 802.11 PHY, DVB-T2 PHY, etc. Higher layers (MAC, RLC, PDCP, IP) ride on top and are usually encrypted at the PDCP-equivalent layer in modern systems.
• PLC Power-Line Communication / PowerLAN Networking that sends data over the electricity wires already in your house — the "powerline adapter" boxes you plug into wall sockets. Convenient, but the wires leak radio waves everywhere and can wreck shortwave reception across the whole neighbourhood. Data carried over the electrical wiring already in your walls (2–30 MHz narrowband, standardised by IEEE 1901, HomePlug AV2, and ITU G.hn). Convenient for whole-home networking, but radiates as a major HF noise source and ruins reception on shortwave.
• PLL Phase-Locked Loop A control circuit that locks one oscillator's frequency to follow another reference. It's how almost every modern radio's "tuning" works internally — and how FM demodulation, clock recovery in your computer, and many other timing tricks happen. Feedback control loop that locks an oscillator's phase + frequency to a reference. The basic building block of every modern synthesiser, FM demodulator, clock-recovery circuit, and frequency multiplier. Fractional-N PLLs synthesise arbitrary frequencies from a single 10 MHz reference; the modern radio "tune" knob is a PLL register write.
• PMR446 Private Mobile Radio at 446 MHz Europe's license-free family walkie-talkies — the ones you find in pairs at outdoor shops. No exam, no licence, can be used in any EU country. The European cousin of the US FRS service. Range is a couple of kilometres in real-world conditions. License-free EU walkie-talkie service — 16 analogue FM channels and 32 digital channels on 446.0–446.2 MHz. 500 mW ERP, integrated antenna only. Equivalent to US FRS.
• POCSAG Post Office Code Standardisation Advisory Group The protocol behind the pager beeping in a hospital doctor's pocket. Decades old, but reliable and dirt-cheap. Still used widely by hospitals and (in Germany) the volunteer fire service. SDR hobbyists decode it because the protocol is unencrypted. 1980s paging protocol still in widespread use for hospital pagers, German fire-service alerts (DAPNET), and amateur DX-cluster paging. FSK at 512 / 1200 / 2400 bps; trivially decoded with multimon-ng + an SDR.
• POTA Parks On The Air A wildly popular ham-radio game: pack a portable radio, drive or hike to a registered park, set up a wire antenna in a tree, and try to make as many contacts with other hams as possible. Combines two great hobbies — radio and being outdoors. Amateur-radio operating award programme — activate or chase contacts from registered parks (national parks, state parks, historic sites) using portable HF / VHF gear. Hugely popular since 2017. Sister programme to SOTA; together they make portable operation the most active corner of modern ham radio.
• PPS Pulse Per Second A clean once-per-second tick output from a GPS receiver, aligned to the actual second boundary as kept by satellite atomic clocks. Building your own super-accurate computer clock or precision lab setup almost always starts with this signal. A 1 Hz logic-level pulse aligned to UTC seconds, output by GNSS receivers and atomic clocks. Used to discipline oscillators (GPSDO), trigger SDR sample-aligned captures, and pin NTP servers to sub-microsecond accuracy. The 1 PPS edge sets the timing precision floor (~10 ns at the receiver).
• PSK Phase-Shift Keying A family of digital modulations that encode bits by shifting the radio wave's phase — like nudging the wave forward or backward by a fraction of a wavelength. More phase positions = more bits per transmission. Used inside almost every modern wireless link. Family of modulations that encode bits as phase shifts of the carrier. BPSK, QPSK, 8-PSK progressively pack more bits per symbol; Wi-Fi and LTE OFDM subcarriers use them as building blocks.
• PSK31 PSK31 A typing-based ham-radio mode from the late 1990s — open the program, tune in, type, and chat. It only needs a tiny slice of spectrum, so several conversations can fit in the audio bandwidth of one SSB voice channel. Mostly replaced by FT8 today. Narrowband (~31 Hz) keyboard-to-keyboard ham digital mode by Peter Martinez (G3PLX) — sounds like a warbling tone in SSB audio. Was the workhorse HF chat mode before FT8 took over.
• PVT Position, Velocity, Time The three things every GPS chip outputs: where you are, how fast you're moving, and what time it is. Sat navs, planes, drones, self-driving cars — they all live and die on a good PVT solution several times per second. The three quantities a GNSS receiver computes from pseudo-range + Doppler measurements to ≥ 4 satellites: 3-D position, 3-D velocity, and a clock offset (corrected to sub-100 ns). Every consumer "GPS chip" produces a PVT solution once per second; survey-grade and self-driving fusion runs PVT at 10–20 Hz with RTK + IMU coupling.
• QAM Quadrature Amplitude Modulation A modulation that packs lots of bits into each "symbol" by using many distinct amplitude + phase combinations. Wi-Fi 7 uses 4096-QAM — twelve bits per symbol — which is why it's so fast at short range. The grid-of-dots "constellation" diagrams in radio textbooks are QAM. Modulation that combines amplitude and phase changes for very high bit density — 16-QAM (4 bits/symbol), 64-QAM (6), 256-QAM (8), up to 4096-QAM in Wi-Fi 7. The constellation diagrams you see in textbooks.
• QFH Quadrifilar Helix antenna A bird-cage-shaped antenna that listens equally well across the whole sky above it. Perfect for receiving low-orbit weather satellites that pass overhead, since you don't have to track them. Easy to make from PVC pipe and copper wire. Four-helix antenna with a near-isotropic upper hemisphere pattern — excellent for receiving LEO weather satellites (NOAA APT, Meteor MN-2) on 137 MHz VHF. Easily home-built from PVC pipe and copper wire.
• QRM Q-code: man-made interference Ham-radio slang for "interference from other electronics" — your neighbour's LED lights, switch-mode chargers, plasma TVs, that sort of thing. The opposite of QRN (natural noise like lightning static). QRM has gotten dramatically worse as cheap electronics have multiplied. Ham-radio shorthand for interference from other operators, consumer electronics, switch-mode power supplies, plasma TVs, LED lighting, etc. Distinct from QRN (atmospheric / natural noise). The two main villains of HF reception in suburban backyards.
• QRP Q-code: low transmit power Ham-radio slang for "running very low transmit power" — typically 5 watts or less. There's a whole subculture of hams who pride themselves on talking across continents with the power of a USB charger and a wire antenna in a tree. Operating below 5 W CW / digital, 10 W SSB — by choice, often portable, often homebrew. Works global distances under good propagation thanks to coherent digital modes (FT8, WSPR); celebrated as a craft within ham radio. Opposite: QRO (high power).
• QSO Q-code: a two-way radio contact Ham-radio slang for "a single radio conversation between two operators". Could be a 30-second call or a 30-minute chat. Hams keep logbooks of every QSO they make — that's the foundation of the various award programmes. The basic unit of amateur-radio activity: a single conversation between two stations, exchanged in any mode. Logged in the operator's ARRL LoTW or QRZ logbook. "Working DX" means making QSOs with distant stations; "5×9 73" is the canonical short QSO sign-off.
• QZSS Quasi-Zenith Satellite System (Japan) Japan's clever regional satnav system. Instead of orbiting low over the equator like GPS, QZSS satellites trace a figure-eight so at least one is always nearly straight overhead Japan — handy for getting a clean signal between Tokyo's skyscrapers. Japanese regional GNSS, four satellites in inclined geo- synchronous orbits that always have one near zenith over Japan / Oceania. Broadcasts L1 / L2 / L5 / L6 — interoperable with GPS for added capacity in urban canyons. L6 carries cm-level CLAS corrections (a regional RTK alternative).
• RAS Radio Astronomy Service (ITU-defined) The official "we're listening to space, please nobody transmit here" reserved spectrum. ITU sets aside specific frequency windows (like 1420 MHz for hydrogen-line astronomy) and bans terrestrial transmitters from them so radio telescopes can hear faint signals from the cosmos. The ITU radiocom- munication service for passive observation of natural radio emissions. Allocated primary at protected windows (1400–1427 MHz HI, 22.21–22.5 GHz H₂O, plus mm-wave / sub-THz protected slots); interference thresholds set extraordinarily strict by ITU-R RA.769 (e.g. −255 dBW/m²/Hz at 1.4 GHz). National compliance enforced by FCC / BNetzA / Ofcom.
• RCTA Rear Cross-Traffic Alert The feature that pings + flashes a warning when you're backing out of a parking space and another car is approaching from the side. Same little corner radars as the blind-spot system, just looking backward. Bumper-corner short-range radar feature that warns of vehicles approaching from the side while reversing out of a parking spot. Same 77–81 GHz SRR hardware as BSD / LCA, just a different sensor-fusion mode and FOV.
• RDS Radio Data System The reason your car radio shows the station name and current song title on the dash. FM stations sneak a tiny digital data stream into the audio that radios can decode — also used for traffic alerts and keeping the dashboard clock accurate. Inaudible 57 kHz subcarrier on FM broadcast that carries station ID, song title, traffic alerts, and clock time. Decoded by every car radio and easy to dump with rds-tools + an RTL-SDR.
• RF Radio Frequency The catch-all term for "radio waves" — anything an antenna can receive, roughly from a few kilohertz up to a few hundred gigahertz. Everything in this spectrum table from VLF through mmWave 5G is RF. Above ~300 GHz the same waves get called terahertz, infrared, light, or X-rays instead. Electromagnetic energy in the ~3 kHz – 300 GHz range — everything in this table from ELF through EHF mmWave. Above 300 GHz it's sub-millimetre / terahertz / IR / visible / UV / X-ray / gamma — all the same EM wave, just at higher frequencies + higher photon energy.
• RFID Radio-Frequency Identification The general technology behind contactless tags — your transit card, the security tag in clothing, the chip in your dog's neck, the toll transponder on your windshield. Some only work at 1 cm (NFC), some work at metres (UHF tags scanned in warehouses). Wireless tag-and-reader system — LF (125 kHz), HF (13.56 MHz, includes NFC), and UHF (860–960 MHz) variants cover everything from livestock chips and inventory tags to passport pages and toll transponders.
• RHCP Right-Hand Circular Polarisation A radio wave whose orientation spins clockwise as it travels. Satellites use circular polarisation because they tumble in orbit — if they used a fixed direction, your antenna would lose signal every few minutes. That's why GPS antennas are little discs and not whip antennas. EM-wave polarisation that rotates clockwise as seen from behind the wave. The standard for GPS + most GNSS, weather-satellite APT / LRPT, and AMSAT LEO satellites. A linearly-polarised antenna takes a 3 dB hit against an RHCP source — the reason GNSS antennas use patches / helices. Opposite hand: LHCP (left-hand), used on some satellite services + downlink reuse pairs.
• RINEX Receiver Independent Exchange Format A standard plain-text file format for raw GPS measurements. Every receiver brand can export it, every surveying program can read it. Saves you from being locked into one vendor's tools. Plain-text format (versions 2.x / 3.x / 4.x) for raw GNSS observation data: pseudo-ranges, carrier-phase, signal-strength per satellite per epoch. The interchange standard for IGS, post-processed surveying, and academic research. Save RINEX from your u-blox ZED-F9P, post-process with RTKLIB or PRIDE-PPPAR, get cm-level positions days later — without needing an RTK base.
• RNSS Radio-Navigation Satellite Service The official ITU category for GPS-style navigation signals. These frequencies are protected internationally — jamming or spoofing them (which terrorists and criminals occasionally try) is a serious crime almost everywhere. The ITU radiocom- munication service for GNSS signals. Allocated primary at 1164–1300 MHz (covers L2, L5, Galileo E5/E6, BeiDou B2/B3) and 1559–1610 MHz (L1, Galileo E1, BeiDou B1, GLONASS L1). Protected internationally; jamming or spoofing is a criminal offence in nearly every jurisdiction.
• ROSAT Röntgen Satellite (X-ray) A German X-ray space telescope from the 1990s that produced the first detailed all-sky X-ray map. Its catalogue of X-ray sources is still used today as a starting point for follow-up by newer telescopes. German-led X-ray space telescope (1990–1999) that produced the first all-sky survey at 0.1–2.5 keV. Catalogue still used for target-of-opportunity follow-up by CHANDRA and XRISM.
• RSGB Radio Society of Great Britain The UK's national ham-radio society — equivalent of the ARRL in the US or DARC in Germany. They write the syllabus for the three UK licence levels (Foundation, Intermediate, Full) and represent UK hams to Ofcom. UK's national amateur-radio society — peer of the ARRL and DARC. Member of the IARU; runs the Foundation / Intermediate / Full UK ham licence syllabus with Ofcom.
• RTK Real-Time Kinematic positioning A trick that boosts GPS accuracy from "a few metres" to "a centimetre or two" by using a nearby fixed base station as a reference. Used by surveyors, agricultural autopilots, and self-driving cars. Cheap consumer kits now bring it within hobbyist reach. GNSS technique that uses carrier-phase measurements (not just pseudo-ranges) plus a nearby base station of known coordinates to resolve ambiguities and reach cm-level fix accuracy in real time. Modern multi-band (L1+L2+L5) receivers like the u-blox ZED-F9P deliver this for ≤ $200 — survey-grade accuracy is now a hobby purchase. See NTRIP for streaming corrections.
• RTL-SDR RTL2832U USB SDR dongle The famous $25 USB stick that turned millions of people into software-radio hobbyists. Designed as a cheap European TV tuner; someone figured out you could repurpose it to listen to almost any radio signal between 24 MHz and 1.7 GHz. Aviation, weather sats, pagers — it's the gateway drug. The $25 USB stick that started the SDR-for-the-masses revolution — repurposed Realtek DVB-T tuner chip running in raw I/Q mode. Covers 24 MHz – 1.7 GHz; an entire generation of ADS-B, POCSAG, and weather-sat projects run on these.
• RTTY Radioteletype The granddaddy of digital ham-radio modes — basically a typewriter talking to another typewriter over the air. Glacially slow by modern standards but still pops up in contests because of its distinctive sound. Classic mechanical teleprinter mode using FSK — 45.45 baud, 170 Hz shift, 5-bit Baudot. Predates packet by decades and still used in HF contests.
• SAR Specific Absorption Rate The "how much radio energy your body absorbs" number printed in phone safety sheets. Every phone sold has been tested to keep this well below the legal limit; comparing SAR between phones is mostly marketing noise since they're all far under the safety threshold. RF-exposure metric — watts of RF energy absorbed per kg of body tissue. ICNIRP and FCC set the limits every phone, ham radio, and Wi-Fi router must comply with.
• SARA Society of Amateur Radio Astronomers The community group for amateur radio astronomers — people who point dishes at the sky from their backyards. They share plans, publish a journal, and help newcomers detect their first 21 cm hydrogen signal or solar burst. Membership organisation for amateur and educational radio astronomy. Quarterly journal, observation database, mentoring on backyard 1–4 m dishes for 21 cm HI mapping, solar / Jupiter monitoring, and pulsar searches. Friendly entry point next to OSRT.
• SAS Spectrum Access System (CBRS) The cloud database behind US CBRS shared spectrum. Devices have to ask the SAS "what channel can I use here right now?" before transmitting, so they don't step on a Navy radar nearby. It's spectrum sharing run by software instead of fixed licences. Cloud database that coordinates the three CBRS tiers — incumbent Navy radar, PAL licensees, and General Authorized Access. Devices ask the SAS for a channel before transmitting.
• SAW Surface Acoustic Wave (filter / resonator) A tiny filter chip that uses sound waves rippling across the surface of a crystal to filter radio signals. Every smartphone has dozens of them — one for each cellular band it supports. Without these, your phone couldn't separate Wi-Fi from cellular from GPS. Filter built on a piezoelectric substrate (LiTaO₃, LiNbO₃) where interdigital transducers launch acoustic waves across the surface and reconvert them at the output. Compact, inexpensive, and ubiquitous: every cellular phone has dozens of SAW (and BAW / FBAR) filters defining its band-pass behaviour.
• SBAS Satellite-Based Augmentation System The umbrella name for free GPS-correction services broadcast from satellites — WAAS in the US, EGNOS in Europe, MSAS in Japan, GAGAN in India. They sharpen GPS accuracy from ~5 m to ~1.5 m, and your phone uses them automatically. Family of geostationary-satellite overlays that broadcast differential GNSS corrections + integrity messages on the L1 frequency. Regional flavours: WAAS (US), EGNOS (EU), MSAS (Japan), GAGAN (India). Improves civilian fix accuracy from ~5 m to ~1–1.5 m; the integrity bound is what makes SBAS aviation-approach-rated.
• Schumann Schumann resonances A natural electromagnetic "hum" of the Earth — the planet between surface and ionosphere acts like a giant resonating chamber, with a 7.83 Hz fundamental tone constantly excited by lightning around the world. You can record it with a coil of wire and a sound card. Standing electromagnetic waves trapped between Earth's surface and the ionosphere — fundamental at 7.83 Hz, then ~14 / 20 / 26 / 33 Hz harmonics. Continuously excited by global lightning; receivable in the ELF band with a sensitive coil.
• SDK Software Development Kit A bundle of libraries and example code from a chip maker that lets you write your own apps on their hardware. Think "Here's how you program our radar chip" or "Here's how to talk to our LTE modem". A vendor-supplied bundle of libraries, headers, examples, and docs you build your application against. RF examples seen often in the table: TI mmWave Radar SDK, Nordic Semiconductor nRF Connect SDK, Espressif ESP-IDF, Quectel QMI / MBIM, Qualcomm QXDM. "Open" SDKs (e.g. SrsRAN, OpenAirInterface) drove much of the PHY-research community.
• SDR Software-Defined Radio A radio where the demodulation happens in software on your computer, instead of in dedicated chips. One $25 SDR dongle can be a police scanner, plane tracker, weather-satellite receiver, and spectrum analyzer all in the same evening — just change the software. Receiver or transceiver where the demodulator runs in software, not hardware. Cheap RTL-SDR dongles ($25) cover 24 MHz – 1.7 GHz; HackRF, LimeSDR, and PlutoSDR add transmit; USRP sits at the high end.
• SETI Search for Extra-Terrestrial Intelligence The decades-long project of pointing radio telescopes at the sky in case anyone else is broadcasting. So far: no confirmed detections. Anyone can download archived SETI data and run their own searches on a laptop. Long-running research programme listening for narrowband artificial radio signals from other civilisations. Modern public data: the Breakthrough Listen archive (Green Bank + Parkes raw voltages), SETI@home retrospective, Allen Telescope Array surveys. The "water hole" 1.4–1.7 GHz between HI and OH lines is the canonical hunting band. Hobbyists analyse archived data with open-source pipelines.
• SHF Super High Frequency (3–30 GHz) The microwave band — 5 GHz / 6 GHz Wi-Fi, satellite TV downlinks, weather radar, and most point-to-point links between buildings. Atmosphere doesn't absorb much in this range, so signals can carry a lot of data over a clear line of sight. Microwave band — 2.4 GHz / 5 GHz / 6 GHz Wi-Fi, satellite TV Ku-band, weather radar, and the 13 cm / 9 cm / 6 cm / 3 cm amateur bands. Atmospheric absorption is low across most of the band — ideal for high-bandwidth point-to-point.
• Sigfox Sigfox LPWAN A French rival to LoRaWAN — same idea ("battery sensor talks home"), different patented modulation. Subscription-based and capped to 140 tiny messages a day. Sells to pallet trackers, smart bins, and similar low-data devices. French LPWAN provider — ultra-narrowband (100 Hz) BPSK on 868 MHz EU / 902 MHz US. 12-byte payloads, 140 messages/day. Competes with LoRaWAN and NB-IoT.
• SKAO Square Kilometre Array Observatory A new radio telescope under construction across Australia and South Africa, built by 16 countries together. When finished it will be by far the largest radio telescope ever — capable of spotting an airport radar on a planet many light-years away. Inter-governmental observatory (~16 member states) building the Square Kilometre Array — SKA-Low (50–350 MHz, 131 000 dipoles, Murchison, Australia) and SKA-Mid (350 MHz – 15.4 GHz, 197 dishes, Karoo, South Africa). When complete it will be the world's largest radio telescope and produce ~700 PB of science data per year.
• SOTA Summits On The Air Ham radio combined with hiking. Operators climb to designated mountain summits with a portable radio in their pack and try to make a few contacts from the top. Sister to POTA (parks); both have become the most active corners of modern ham radio. Amateur-radio award programme for activating + chasing contacts from designated mountain summits worldwide. Pioneered modern portable HF / VHF backpacking ham activity. Sister programme to POTA; both have free online logbooks (sotawatch.sota.org.uk, pota.app) that show live spots in real time.
• SRR Short-Range Radar (77–81 GHz automotive UWB) The short-range automotive radars that live in a car's bumper corners. Wide field of view, short range — designed to watch pedestrians, blind spots, and parking obstacles right next to the car. Most modern cars have four to six of them. Bumper-mounted automotive radar covering 30–60 m at ±60° or wider, used for blind-spot detection (BSD), LCA, RCTA, parking, and pedestrian / cyclist detection. UWB chirps in 77–81 GHz (per ETSI EN 302 264 / FCC Part 95.4002) replaced the legacy 24 GHz K-band SRR phased out 2018–2022.
• SSB Single Sideband A more efficient version of AM voice — saves bandwidth and power by sending only one "side" of the AM signal. It's the standard voice mode on shortwave: ham operators, sailors, and pilots over oceans all use it. Sounds like Donald Duck until the receiver is tuned right. AM variant that transmits only the upper or lower sideband for ~3× the efficiency. The workhorse voice mode of HF amateur, marine, and aviation; the audio sounds robotic until your receiver's BFO is tuned correctly.
• Starlink Starlink (SpaceX LEO satellite internet) SpaceX's satellite internet service. Instead of one big satellite far away, thousands of small ones orbit low and close, which is why latency is much lower than older satellite internet. Your "Dishy" antenna automatically tracks satellites overhead. SpaceX's low-Earth-orbit broadband constellation — 5500+ satellites cross-linked with laser (FSO) backhaul. User terminals communicate at Ku/Ka-band (12–18 / 27–40 GHz, in the SHF band).
• TDD Time Division Duplex A cellular setup where the phone and tower share one frequency, rapidly taking turns sending and receiving. Better for asymmetric use (you download more than you upload) — that's why mid-band 5G almost always uses TDD. Uplink and downlink share the same frequency channel and alternate in short time slots — efficient for asymmetric traffic and standard on 5G NR mid-band (n40, n41, n78, n79). Contrast with FDD where UL and DL use paired separate frequencies.
• TDMA Time Division Multiple Access "Take turns" multiple access — several users share a channel by being given alternating time slots. The trick old 2G phones used to fit lots of callers into one channel, and still common in business digital radios. Multiple-access scheme where users share a frequency by being assigned distinct time slots. Foundation of GSM (8 slots / 200 kHz), DMR Tier II (2 slots / 12.5 kHz), TETRA (4 slots / 25 kHz), and P25 Phase 2. Largely replaced in cellular by OFDMA, but still dominant in two-way radio.
• TEC Total Electron Content (ionosphere) A measurement of how thick the ionosphere's electric "blanket" is. GPS signals slow down through this blanket, so a thicker ionosphere means a small position error. After solar storms TEC spikes, and consumer GPS accuracy gets temporarily worse. Integrated electron column-density along a ray-path through the ionosphere, in TECU = 10¹⁶ electrons/m². TEC sets ionospheric delay on GNSS — scales as 1/ν², which is why dual-frequency receivers (L1+L2) cancel it directly. Maps published by IGS, NASA JPL, NOAA SWPC every 15 min; correlates strongly with solar activity, geomagnetic storms, and the equatorial anomaly.
• TETRA Terrestrial Trunked Radio The European digital-radio standard for police, fire, ambulance, and other emergency services. The European cousin of America's P25. Voice and data are encrypted, so you can hear it on a scanner but not understand it. European digital trunked-radio standard for police, fire, and other emergency services on 380–400 MHz UHF. Competing standards include P25 (US) and DMR.
• TheThingsNetwork The Things Network A free, community-built LoRaWAN network spanning 150 countries. Anyone can plug in a $100 gateway, contribute coverage, and use the network for their own IoT sensors at no cost. It's basically "Wikipedia for IoT radio coverage". Community-operated free LoRaWAN network — 20 000+ gateways in ~150 countries. Anyone can contribute a gateway and use the global free-data plan for non-commercial IoT.
• Thread Thread (IPv6 mesh networking) The smart-home mesh protocol underneath Matter — uses the same low-power radio as Zigbee but works like proper internet networking. Apple, Google, and Amazon hubs all speak Thread, which is why a $30 sensor can talk directly to your HomePod. Low-power IPv6-over-802.15.4 mesh — same physical layer as Zigbee but with a real IP stack. Foundation of Apple HomeKit / Google Home / Matter Thread border routers.
• UE User Equipment (3GPP cellular) The cellular industry's term for "the device on your end" — your phone, modem, hotspot, smartwatch, or IoT module. Carriers and chip makers use UE everywhere instead of saying "the phone". The 3GPP / cellular term for the device side of a connection — phones, modems, IoT modules, fixed-wireless CPEs. Categorised by UE Category (Cat 1 – Cat 22 in LTE, Cat 1 / 4 / NB1 / NB2 / M1 / M2 in IoT, multiple at NR). Specifies max throughput, MIMO order, modulation, and supported bands.
• UHF Ultra High Frequency (300 MHz – 3 GHz) The most economically valuable slice of radio spectrum. Mobile phones, GPS, Wi-Fi 2.4 GHz, Bluetooth, walkie-talkies, garage-door remotes, smart-home sensors, and most digital TV all live here. If you bought wireless gear in the last 20 years, it almost certainly uses UHF. Covers cellular, Wi-Fi 2.4 GHz, GPS L1, ISM 433/868/915 MHz, Bluetooth, the 70 cm and 23 cm amateur bands, and most TV broadcast — the densest, most-utilised slice of spectrum.
• ULF Ultra Low Frequency (300 Hz – 3 kHz) The band between extremely-low (ELF) and very-low (VLF). Mostly used by specialised submarine comms and earth-science geophysics. A wire loop and a sound card is enough to listen and look for the natural noises of the magnetic field. Above ELF and below VLF. Used for advanced submarine comms and audio-magnetotelluric geophysics. Receive-only for hobbyists with a coil and sound-card.
• UNII Unlicensed National Information Infrastructure The official FCC name for the 5 GHz and 6 GHz Wi-Fi bands — the chunks where Wi-Fi 5, 6, 6E, and 7 live. The 6 GHz UNII band was opened up in 2020 and is why your latest Wi-Fi router is so much faster than the old one. FCC umbrella term for the 5 GHz and 6 GHz Wi-Fi spectrum tiers — UNII-1, UNII-2A, UNII-2C (DFS required), UNII-3, plus the new 6 GHz UNII-5/6/7/8 opened in 2020 for Wi-Fi 6E and Wi-Fi 7.
• USRP Universal Software Radio Peripheral The professional, expensive end of software-defined radio. Where HackRF and RTL-SDR are hobbyist tools, USRPs are what universities and defence labs use for serious research — multiple GHz of bandwidth, full duplex, very expensive. Ettus Research's high-end SDR family — full-duplex MIMO, 6 GHz+ tuning, 200+ MS/s. The reference platform for academic and military signal-processing research; runs on GNU Radio.
• UV Ultraviolet (~10–400 nm, ~3.1–124 eV) The light just past violet that you can't see — but your skin and eyes definitely can. Above a certain energy, UV starts breaking chemical bonds, which is why too much causes sunburn and skin cancer. The shorter the wavelength, the more dangerous. EM band immediately above visible violet, conventionally split UV-A (315–400 nm — black-light, tanning), UV-B (280–315 nm — Vitamin D + sunburn), UV-C (100–280 nm — germicidal, ozone- blocked at sea level), VUV (10–200 nm — vacuum-only), and EUV (10–124 nm, overlap with VUV). Photon energies cross the ionising threshold (~120 nm / ~10 eV).
• UWB Ultra-WideBand A wideband radio that fires extremely short pulses, letting it measure distance with cm-level precision. It's what makes an AirTag's "point exactly to it" arrow work, and how newer car keys let you unlock your car as you walk up. Radio that uses ≥ 500 MHz instantaneous bandwidth (or ≥ 20% fractional bandwidth) — typically pulsed 3.1–10.6 GHz or chirped 77–81 GHz. Three big classes today: indoor positioning & proximity (Apple U1/U2 / NXP Trimension / IEEE 802.15.4z, used in AirTags, Find My, car-key access — UWB digital key), 77–81 GHz automotive SRR, and ground-penetrating radar.
• VCSEL Vertical-Cavity Surface-Emitting Laser A tiny laser that fires straight up out of a chip — mass-produced on silicon wafers like ordinary semiconductors. They light up fibre-optic cables in data centres, and your phone uses arrays of them for Face ID structured-light scanning. Semiconductor laser that emits perpendicular to the chip surface (rather than from a cleaved edge like a Fabry-Pérot). 850 / 940 / 1310 nm typical wavelengths. Underpins datacenter optics, mouse sensors, Face ID structured-light, and high-speed multimode- fibre transceivers. Cheap, mass-producible, eye-safe.
• VHF Very High Frequency (30–300 MHz) The band that hosts FM radio, aviation voice, marine VHF, and NOAA weather. Signals usually only travel as far as you can see from the antenna, but the atmosphere occasionally bounces them over surprising distances ("sporadic E"). FM broadcast (88–108), aviation voice (118–137), marine VHF (156–162), the 6 m / 2 m amateur bands, NOAA weather, and a long tail of land-mobile services. Mostly line-of-sight with occasional sporadic-E openings.
• VLBA Very Long Baseline Array Ten US radio dishes scattered from Hawaii to the Virgin Islands, combined by computer to act as one giant telescope the width of the country. The wide separation gives extraordinarily sharp views of distant cosmic objects. Dedicated 10-antenna VLBI network operated by NRAO across the US (Mauna Kea HI through St. Croix VI — ~8000 km baselines). Operates 1.4 / 2.3 / 5 / 8.4 / 15 / 22 / 43 / 86 GHz for astrometry, AGN structure, and galactic / extragalactic surveys. See VLBI, EVN.
• VLBI Very Long Baseline Interferometry A technique that combines radio telescopes on different continents into one virtual telescope as wide as the planet. That's how the Event Horizon Telescope team produced the famous "black hole shadow" image in 2019 — by syncing eight telescopes across the globe to atomic-clock precision. Technique that combines signals recorded simultaneously by widely- separated radio telescopes (often inter-continental, sometimes with a satellite element) to synthesise an aperture as large as the baseline between them. Angular resolution ≈ λ/B → micro-arcseconds at cm wavelengths. Each station records onto a hydrogen-maser-locked clock; correlators (DiFX, SFXC) align the bitstreams later. Used by VLBA, EVN, the Event Horizon Telescope, and Earth-orientation geodesy via IVS.
• VLF Very Low Frequency (3–30 kHz) Very-low-frequency radio — wavelengths tens of kilometres long. Used by navy submarine transmitters because the signals can penetrate seawater. A wire antenna plus a PC sound card is enough to listen, and you'll also hear "whistlers" — natural radio chirps triggered by lightning. Submarine and time-signal transmitters (NAA, NLK, NPM) operating at megawatt power levels. Receivable with a long-wire antenna into a PC soundcard — popular for low-budget propagation experiments and recording natural whistlers.
• VMADS Vehicle-Mounted Active Denial System The vehicle-mounted version of the ADS "heat ray" — a big dish on a truck. Same idea as ADS: aim, fire a microwave beam, everyone in the target area instinctively gets out of the way. Strictly military / law-enforcement. Truck-mounted variant of the 95 GHz ADS — Raytheon ADS Mod 1, with ~2 m parabolic reflector and 100 kW gyrotron source. "Silent Guardian" is a smaller ~30 kW palletised version; Solid-State ADS is a GaN-based generation under DoD R&D. Same ITAR / weapons-law restrictions apply.
• VNA Vector Network Analyzer A radio engineer's "tape measure for antennas and filters". It sweeps signals through a device and tells you exactly what gets reflected, absorbed, or passed through. Used to be a $10,000 lab instrument; the NanoVNA brought it to $60 and put one in every ham's toolbox. Test instrument that measures complex S-parameters (magnitude + phase) of an RF network across frequency. Used to characterise antennas, filters, cables, and amplifiers. Hobbyist game-changer: the NanoVNA (50 kHz – 1.5 GHz, $60) and NanoVNA-V2 / SAA-2N (3 GHz, $100) made VNAs household items in ham shacks.
• W-band W-band (~75–110 GHz) A very high microwave band that hosts car radar (77 GHz) and airport body scanners (94 GHz). Sits in a "clear window" between two frequencies where water vapour eats radio signals. Waveguide band (WR-10) covering automotive 76–81 GHz radar, 94 GHz airport / military mmWave imaging, the 95 GHz ADS directed- energy band, and 81–86 GHz E-band backhaul (sometimes called upper E or low W). Atmospheric window between water-vapour absorption peaks at 60 / 119 / 183 GHz.
• WAAS Wide Area Augmentation System (USA) The US version of EGNOS — a free service that boosts civilian GPS accuracy from a few metres to about 1–2 metres. Run by the FAA originally for aircraft landing approaches; your car satnav, boat plotter, and phone get the benefit for free. US SBAS operated by the FAA, with three geostationary satellites broadcasting differential GPS corrections + integrity on the L1 frequency. Provides aviation LPV-200 approaches (~3 m vertical accuracy) and lifts general civilian accuracy below 2 m. Free and unlicensed — every consumer GPS chip can use it with a firmware tick-box.
• Wavelength λ — distance between successive wave peaks The physical length of one ripple of a radio or light wave. Antennas have to be roughly the same size as the wavelength, which is why long-wave (LF) antennas are kilometres of wire while a 77 GHz radar antenna fits on a fingernail. The spatial period of an EM wave: λ = c/ν. Antenna sizes (1/4-wave whip = λ/4, dipole = λ/2), patch-array pitch (λ/2), Fresnel zones, and beam diffraction all scale with λ. This column shows the canonical λ for each band so you can sanity-check antenna dimensions and aperture-vs-frequency intuition at a glance.
• Wi-Fi Wi-Fi (IEEE 802.11) The wireless network in your house. Each new "Wi-Fi N" version adds tricks for more speed and better handling of crowded networks. Wi-Fi 7 (the newest) can theoretically push tens of gigabits per second, though real-world speeds depend heavily on walls, distance, and the slowest device on the network. The dominant wireless LAN family — 2.4 / 5 / 6 GHz, multiple PHYs. Wi-Fi 4 (n) added MIMO, Wi-Fi 5 (ac) added MU-MIMO, Wi-Fi 6 (ax) added OFDMA, Wi-Fi 6E added the UNII-5/6/7/8 6 GHz tiers, and Wi-Fi 7 (be) adds 320 MHz channels and 4096-QAM.
• WiGig Wireless Gigabit (60 GHz / 802.11ad / ay) Very-short-range, very-fast Wi-Fi at 60 GHz. Tens of gigabits per second across a room, but blocked by a sheet of paper. Found in wireless VR headsets and some specialised point-to-point links between buildings. Worldwide-unlicensed 57–66 GHz Wi-Fi at multi-Gb/s rates over short range, four channels of 2.16 GHz. Original IEEE 802.11ad (WiGig 1.0); 802.11ay extends to ~40 Gb/s with channel bonding + MIMO. Commercial gear: TP-Link Talon AD7200 (discontinued, secondhand), Mikrotik wAP 60G / cnWave V3000 (license-free P2P & P2MP), some VR streaming kits.
• WSJT-X WSJT-X (weak-signal modes software) The free PC program ham operators run for weak-signal modes like FT8, JT65, and WSPR. Plug a radio's audio into your laptop, point WSJT-X at it, and it'll automatically decode signals so faint you can't hear them by ear. Joe Taylor's free software suite for ham weak-signal modes — FT8, FT4, JT65, Q65, MSK144, WSPR. Audio in, audio out, deep error-correction.
• WSPR Weak Signal Propagation Reporter A ham-radio beacon mode that lets a milliwatt-class transmitter reach the other side of the world. Pico-balloons launched from classrooms with a $5 transmitter routinely circle the Earth using WSPR; you can watch live spots on wsprnet.org. Amateur beacon mode (in WSJT-X) that broadcasts 50 bits over 110 s at –30 dB SNR. Pico-balloons, QRP beacons, and home stations use it to map worldwide ionospheric propagation in near-real-time on wsprnet.org.
• WWV WWV — NIST radio time signal (USA) NIST's iconic shortwave time station — the calm voice that says "At the tone, the time will be…" once a minute, with a tick every second. Operating since 1923; useful for setting clocks, but also a reliable test signal to check shortwave reception and antennas. NIST shortwave time-and-frequency station from Fort Collins, Colorado. Broadcasts continuously on 2.5 / 5 / 10 / 15 / 20 MHz with voice + tick + IRIG-B + WWVB-style digital data. Used for clock synchronisation, propagation comparison, and as a canonical ham-band frequency reference. Sister station WWVH from Hawaii on the same frequencies.
• WWVB WWVB (NIST 60 kHz time signal) The radio station every American "atomic" wall clock is silently listening to. Broadcasts the time from Colorado on long-wave; your clock catches it overnight and sets itself, which is how it automatically handles daylight-saving time changes. The US LF time-and-date transmitter from Fort Collins, Colorado. Broadcasts on 60 kHz at 70 kW — received by virtually every "atomic" alarm clock sold in North America. Operated by NIST; counterpart to DCF77 (EU) and JJY (JP).
• WWVH WWVH — NIST radio time signal (Hawaii) The Hawaiian counterpart of WWV, on the same shortwave frequencies but with a female voice instead of a male one. Together they cover most of the Pacific and Asia. If you hear both at once on a receiver, propagation is unusually good. Pacific complement to WWV, broadcasting from Kauai on 2.5 / 5 / 10 / 15 MHz. Female voice (vs. WWV's male) is the easiest cue to tell them apart on the air. Together they give time-signal coverage for North America, the Pacific, and much of Asia.
• XRISM X-Ray Imaging and Spectroscopy Mission A Japanese-American X-ray space telescope launched in 2023, built to capture extremely detailed X-ray "fingerprints" of galaxy clusters and feeding black holes. The successor to its short-lived predecessor Hitomi. JAXA/NASA X-ray space telescope launched 2023 — successor to Hitomi. Carries the Resolve micro-calorimeter spectrometer for high-resolution X-ray spectroscopy of clusters and AGN. Continues the lineage of ROSAT and CHANDRA.
• Z-Wave Z-Wave sub-GHz home-automation mesh A smart-home protocol like Zigbee, but on a less crowded radio band. The trade-off: gear is usually more expensive, but works farther through walls and doesn't fight Wi-Fi for airtime. Common in smart locks and security sensors. Low-power 868 MHz EU / 908 MHz US protocol used by smart locks, sensors, dimmers, and switches. Mesh of up to 232 nodes; simpler and less crowded than 2.4 GHz alternatives like Zigbee.
• ZEVS ZEVS (Russian ELF transmitter) A Russian Navy transmitter so low-frequency (82 Hz!) that it can reach submerged submarines anywhere on Earth. Its antennas are enormous wire grids spanning kilometres. One of the only purpose-built ELF transmitters that exist. Russian Navy 82 Hz ELF submarine-comms transmitter on the Kola Peninsula ('Zevs' = Zeus). One of the very few intentional ELF emitters worldwide; signals propagate globally through the Earth-ionosphere waveguide.
• Zigbee Zigbee (IEEE 802.15.4 mesh) The radio protocol behind Philips Hue bulbs, IKEA Tradfri, Aqara sensors, and lots of cheap smart-home stuff. Mesh-based: each device relays packets for its neighbours, so the network gets better the more devices you add. Low-power 2.4 GHz mesh protocol used by Philips Hue, IKEA Tradfri, Aqara, and the Thread / Matter ecosystem. Up to ~65k nodes per network with very long battery life.
• λ wavelength symbol (Greek lower-case lambda) The Greek letter "lambda", used everywhere in physics to mean "wavelength". The handy mental shortcut: at 1 GHz the wavelength is about 30 cm; double the frequency and the wavelength halves. The physical distance between two adjacent wave peaks. Linked to frequency by λ = c/ν: 1 GHz → 30 cm, 100 MHz → 3 m, 100 THz → 3 μm, 500 nm = green light. Antenna dimensions, diffraction limits, and rain-fade onset all scale with λ — it's the ruler the whole spectrum is measured with. See also Wavelength.
• μ micro / Greek lower-case mu (SI prefix 10⁻⁶) The Greek letter "mu" — the scientific prefix for "one millionth". A micrometre (μm) is a thousandth of a millimetre. Visible light wavelengths are a fraction of a μm, mid-infrared a few μm, thermal imaging 8–14 μm. As an SI prefix it means one-millionth: μm = micrometre, μs = microsecond, μW = microwatt, μeV = micro-electron-volt. Visible light wavelengths are 0.38–0.78 μm, mid-IR 3–8 μm, thermal-imaging long-wave infrared 8–14 μm.
• μeV micro-electron-volt (10⁻⁶ eV) Microelectronvolt — one millionth of an electron-volt. The energy of a single radio-wave photon. Vastly weaker than visible light's photons, which is why radio waves only warm things and never ionise them — they simply don't carry enough punch to break a chemical bond. Photon-energy unit at radio / microwave frequencies. The HI 21 cm line (1420 MHz) photon carries ≈ 5.87 μeV — a hyperfine spin-flip transition, not an electronic one. Even lower: 1 kHz audio-band RF ≈ 4 × 10⁻¹² eV. See meV, eV.
• ν frequency symbol (Greek lower-case nu) The Greek letter "nu". Physicists use it for frequency where engineers tend to write f. Either way, it just means "how many times per second the wave wiggles". The physics symbol for frequency, used wherever electrical engineers would write f. ν is in hertz (cycles per second); multiplied by Planck's constant h it gives photon energy E = hν, and it pairs with wavelength via λ = c/ν. Used as the column header on narrow screens because "Frequency Range" doesn't fit.