Glossary of RF & SDR terms
Every term used across the learning path, defined in plain language and linked to the lesson where it’s explained in full. Skim it as a refresher, or use your browser’s find (Ctrl/Cmd-F) to jump to a word. Terms are grouped by theme and ordered roughly from fundamentals to systems.
Want the long version? Many of these terms have a full, in-depth article in the GopherTrunk Field Guide — and the first mention of a covered term on any page links straight to it.
Radio wave fundamentals
Amplitude — The strength or “height” of a radio wave; more amplitude means a stronger signal. Your SDR reports it as a power level. See What is a radio wave?
Carrier — A plain, steady radio wave with no information on it yet. Modulation changes the carrier to carry a message. See What is a radio wave?
Electromagnetic spectrum — The full range of electromagnetic waves, from radio through light to X-rays. Radio occupies roughly 3 kHz–300 GHz. See What is a radio wave?
Frequency — How many times per second a wave cycles, measured in hertz (Hz). Higher frequency = shorter wavelength. See What is a radio wave?
Hertz (Hz) — The unit of frequency: one cycle per second. Common multiples are kHz (thousand), MHz (million), and GHz (billion). See Frequency, bands & the spectrum
Modulation — Deliberately varying a carrier’s amplitude, frequency, or phase to encode information. See Digital modulation & constellations
Wavelength (λ) — The physical length of one wave cycle, in metres. Approximated for radio by λ ≈ 300 ÷ frequency in MHz. See What is a radio wave?
Band — A defined, contiguous range of frequencies assigned to a use (e.g. the FM broadcast band). See Frequency, bands & the spectrum
HF / VHF / UHF — Conventional names for frequency ranges: High Frequency (3–30 MHz), Very High Frequency (30–300 MHz), Ultra High Frequency (300 MHz–3 GHz). See Frequency, bands & the spectrum
Power, noise & decibels
dB (decibel) — A logarithmic ratio between two powers. +10 dB = ×10 power; +3 dB ≈ ×2. Used for gain and loss. See Decibels & signal power
dBm — Absolute power referenced to 1 milliwatt. Received signals are negative numbers; closer to zero is stronger. See Decibels & signal power
dBFS — Decibels relative to digital full scale, used inside the SDR. 0 dBFS is the ADC’s ceiling; real samples sit below it. See Decibels & signal power
Gain — Amplification, expressed in dB. Also the SDR setting that controls how strong the signal is at the ADC. See Gain, AGC & avoiding overload
Noise floor — The ever-present background level of receiver and environmental noise, in dBm. A signal must rise above it to be usable. See Decibels & signal power
Path loss — How much a signal weakens (in dB) as it travels from transmitter to receiver. See Decibels & signal power
SNR (signal-to-noise ratio) — The gap in dB between a signal and the noise floor. Digital modes need a minimum SNR to decode. See Decibels & signal power
Signals & modulation
Baud (symbol rate) — Symbols transmitted per second. Different from bit rate when each symbol carries multiple bits. See Symbols, baud & bitrate
Bit rate — Bits per second = symbol rate × bits per symbol. See Symbols, baud & bitrate
Symbol — One transmitted state of the carrier, carrying one or more bits. See Digital modulation & constellations
AM / FM / SSB — Analog modulations that vary amplitude (AM), frequency (FM), or send a single sideband (SSB). See Analog modulation
FSK (frequency-shift keying) — Digital modulation that switches the carrier between set frequencies. 4FSK (four levels) underlies P25 C4FM and DMR. See Digital modulation & constellations
PSK (phase-shift keying) — Digital modulation that switches the carrier’s phase; QPSK uses four phases. See Digital modulation & constellations
QAM — Modulation varying both phase and amplitude to pack many bits per symbol; needs higher SNR. See Digital modulation & constellations
C4FM — The four-level FSK used by P25 Phase 1. See Digital modulation & constellations
Constellation diagram — A plot of symbols in the IQ plane; tight clusters mean a clean signal. See Digital modulation & constellations
Eye diagram — Overlaid symbol waveforms vs. time; a wide-open “eye” means good decoding margin. See Digital modulation & constellations
Software-defined radio
SDR (software-defined radio) — A radio whose tuning, filtering, and demodulation happen in software. See What is software-defined radio?
ADC (analog-to-digital converter) — The chip that samples the analog signal into digital numbers. Clipping it causes distortion. See What is SDR?
IQ samples — Pairs of numbers (In-phase and Quadrature) that capture a signal’s amplitude and phase together. The SDR’s output. See IQ data & complex signals
Sample rate — How many samples per second the SDR produces; sets how much spectrum (bandwidth) you can see at once. See Sample rate, bandwidth & Nyquist
Nyquist limit — The rule that you must sample at least twice the bandwidth you want to capture. See Sample rate, bandwidth & Nyquist
Bandwidth — The width of spectrum, in Hz, occupied by a signal or captured by a receiver. See Sample rate, bandwidth & Nyquist
AGC (automatic gain control) — Circuitry or software that adjusts gain on its own. See Gain, AGC & avoiding overload
PPM (parts per million) — A measure of an SDR’s frequency error; correcting it aligns the radio to true frequency. See Calibration & troubleshooting
RTL-SDR / HackRF / Airspy — Common SDR hardware. RTL-SDR is the cheap entry point; HackRF and Airspy add bandwidth, sensitivity, or transmit. See SDR hardware and the Hardware guide
DSP
DSP (digital signal processing) — Manipulating sampled signals with math: filtering, tuning, demodulating. See The demodulation pipeline
FFT (Fast Fourier Transform) — The algorithm that turns IQ samples into a spectrum view; the basis of the waterfall. See The FFT & reading a waterfall
Waterfall — A scrolling spectrum display showing signal strength across frequency over time. See The FFT & reading a waterfall
Filtering — Keeping a chosen slice of spectrum and discarding the rest. See Filtering & decimation
Decimation — Reducing the sample rate after filtering to save CPU. See Filtering & decimation
Demodulation — Recovering the original message from a modulated signal. See The demodulation pipeline
Clock recovery — Finding the timing of a digital signal so each symbol is sampled at the right instant. See Clock recovery & symbol timing
Digital voice & trunked radio
Vocoder — A speech codec that compresses voice into tiny data frames; IMBE (P25) and AMBE+2 (DMR) are common. See Vocoders and the Vocoders reference
Trunked radio — A system that shares a pool of frequencies, assigning a channel per call. See What is trunked radio?
Conventional radio — Non-trunked radio where each group uses a fixed frequency. See What is trunked radio?
Control channel — The data-only frequency that coordinates a trunked system and announces channel assignments. GopherTrunk decodes it first. See What is trunked radio?
Voice channel — A frequency temporarily assigned to carry a call. See What is trunked radio?
Talkgroup — A virtual channel identifying a group of users; you follow talkgroups, not frequencies. See What is trunked radio?
Affiliation — A radio registering with the trunked system over the control channel. See What is trunked radio?
Radio ID — The unique identifier of an individual radio transmitting on a system. See the Radio IDs panel
P25 / DMR / NXDN / TETRA — Digital land-mobile-radio standards GopherTrunk decodes. See The digital protocol landscape
Encryption — Scrambling that prevents voice from being decoded; encrypted talkgroups stay silent. See Encryption & what you can decode
APRS / AIS / ADS-B — Non-trunked data signals (amateur position, ship tracking, aircraft tracking) GopherTrunk can also decode. See Other signals you’ll meet
Didn’t find a term? It’s probably explained in one of the learning-path lessons — or open an issue on GitHub and we’ll add it.