Glossary of scanning terms
Every term used across the Scanning & Monitoring module, 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, roughly in the order the module introduces them.
The hobby
Scanning — Monitoring live radio traffic by rapidly checking channels or following a system, to hear who’s on the air right now. See What radio scanning is
Monitoring — Listening to radio communications you don’t take part in — the receive-only heart of the hobby. See What radio scanning is
Crystal scanner — An early scanner whose channels were fixed by physical crystals, one per frequency — the ancestor of the programmable radio. See A short history of scanners
Programmable scanner — A scanner whose frequencies are entered in software or a keypad rather than set by hardware, the format that opened the hobby up. See A short history of scanners
Conventional system — A system where each channel is a fixed frequency you can park on and listen to directly. See Conventional vs. trunked, in the field
Trunked system — A system that shares a pool of frequencies dynamically, assigning calls to channels on the fly, so you must follow it rather than park on it. See Conventional vs. trunked, in the field
Encryption — Scrambling a transmission so only authorised radios can understand it; to a scanner it decodes as noise and can’t be listened to. See What you can (and can’t) hear today
Clear / analog traffic — Unencrypted transmissions — analog or plain digital — that a scanner can receive and understand, the traffic that’s still open to listen to. See What you can (and can’t) hear today
Legal reception — The principle, varying by country and state, that receiving unencrypted radio is broadly permitted while certain uses are not; the hobby’s ground rules. See Legal & ethical scanning
Station & hardware
Hardware scanner — A dedicated appliance with tuning, trunk-tracking, and recording built in — you switch it on. See Hardware scanners vs. SDR
SDR (software-defined radio) — A receiver that digitises a chunk of spectrum and hands the raw samples to software, which does all the demodulating and decoding. See Hardware scanners vs. SDR
Trunking scanner — A scanner able to follow a trunked system by decoding its control channel, as opposed to a conventional-only receiver. See Choosing a scanner or SDR
Discone — A wideband antenna, shaped like a disc over a cone, that receives well across a huge frequency range — a scanning favourite. See Antennas for scanning
Vertical antenna — An omnidirectional antenna that receives equally from all directions, common for general scanning. See Antennas for scanning
Gain (antenna) — How much an antenna concentrates its reception, trading coverage in some directions for more in others. See Antennas for scanning
Feedline / coax — The cable carrying the signal from antenna to receiver; longer runs and cheaper cable lose more signal. See Feedlines, connectors & grounding
Connector — The fitting that joins feedline to gear (SMA, BNC, N, and others); the wrong or a poor one quietly costs signal. See Feedlines, connectors & grounding
Grounding — Safely bonding your antenna and station to earth to protect against static and lightning. See Feedlines, connectors & grounding
Station — Your assembled setup — receiver, antenna, feedline, and computer — working together as a monitoring position. See Setting up your station
Squelch — A control that mutes the receiver until a signal exceeds a threshold, so you hear calls instead of constant hiss. See Setting up your station
Noise floor — The background level of radio noise below which signals are lost; the lower it is, the weaker the signals you can hear. See Reducing noise & interference
RFI (radio-frequency interference) — Unwanted noise from electronics, chargers, and the like that raises your noise floor and buries signals. See Reducing noise & interference
Filter — Hardware that passes the frequencies you want and rejects the rest, cutting interference and overload. See Reducing noise & interference
Finding systems
RadioReference — The largest community database of radio systems, listing frequencies, control channels, talkgroups, and aliases for tens of thousands of systems. See RadioReference & system databases
Database — A shared, community-maintained record of what’s on the air where, the scanning hobby’s map of the spectrum. See RadioReference & system databases
Band plan — The regulatory floor plan of the spectrum, dividing it into bands for public safety, business, aviation, and the rest, so you know where to look. See Band plans & where services live
Band — A range of frequencies allocated to a class of use (VHF, UHF, 700/800 MHz, and so on). See Band plans & where services live
Search / scan — Sweeping a range of frequencies to find active transmissions the databases don’t list. See Search, scan & discovery
Close call — A technique (and scanner feature) that detects a strong nearby transmitter and jumps to its frequency automatically. See Search, scan & discovery
Waterfall — A scrolling visual of the spectrum over time, where a control channel shows as a solid unbroken column and voice channels flicker. See Identifying an unknown signal
Bandwidth — How wide a signal is in frequency, a key clue to what a mystery transmission is. See Identifying an unknown signal
Modulation — How information is impressed on a carrier (FM, P25 C4FM, DMR, and so on); recognising it helps name a signal. See Identifying an unknown signal
Frequency records — Your personal, organised log of frequencies, talkgroups, and notes that makes your setup smarter over time. See Building your frequency records
Trunking
Control channel — The one frequency a trunked system uses to coordinate everything, continuously sending the signalling that tells radios where calls are; find it and you can follow the whole system. See Programming a trunked system
System parameters — The details identifying a trunked system — type, control channel(s), site and system IDs — that you enter to track it. See Programming a trunked system
Talkgroup — A virtual channel on a trunked system — a group of radios that hear each other — identified by a numeric ID. See Talkgroups, scan lists & priorities
Scan list — A curated set of talkgroups you choose to monitor, so you hear what you care about and skip the rest. See Talkgroups, scan lists & priorities
Lockout — Marking a talkgroup or frequency to be skipped, so noise and uninteresting traffic never interrupt you. See Talkgroups, scan lists & priorities
Priority — A setting that makes an important talkgroup interrupt whatever you’re monitoring the instant it becomes active. See Talkgroups, scan lists & priorities
Grant — A message on the control channel assigning a talkgroup’s call to a specific voice channel, the instruction your setup follows. See Following a call across the system
Voice channel — A frequency temporarily carrying an actual conversation, assigned by a grant and released when the call ends. See Following a call across the system
Multisite — A trunked system built from many sites covering a wide area; you pick the site whose signal you monitor. See Multisite, simulcast & roaming
Simulcast — Several transmitters sending the same signal on the same frequency at once; overlapping coverage can distort reception. See Multisite, simulcast & roaming
Roaming — Radios moving between sites of a system as they travel, which you follow by tracking the right site. See Multisite, simulcast & roaming
Control-channel lock — A clean, sustained decode of the control channel; without it nothing downstream works, so it’s the first thing to check when decoding fails. See When decoding fails
Logging, recording & sharing
Call log — The timestamped text record of which talkgroup or unit was active, when, and for how long — cheap, durable, and useful on its own. See Logging & recording calls
Per-call recording — Saving each call as its own short, tagged audio file, which is what makes an archive searchable instead of an endless tape. See Logging & recording calls
Retention policy — Your rule for how long recordings are kept before old ones are pruned, so a busy system doesn’t fill the disk. See Logging & recording calls
Metadata — The facts captured with each call — timestamp, system, talkgroup, unit, frequency, duration — that make it findable later. See Metadata, tagging & searchability
Alias — A human name mapped to a numeric talkgroup or unit ID (“1201” → “County Fire Dispatch”), the biggest readability upgrade an archive gets. See Metadata, tagging & searchability
Source unit ID — The identifier of the individual radio that keyed up a call, when the system sends it; lets you follow one unit across talkgroups. See Metadata, tagging & searchability
Feed — A live stream of your local scanner audio published to the internet for others to hear. See Audio feeds & streaming
Broadcastify — The largest public directory of live scanner feeds, hosting volunteers’ streams for anyone to listen to. See Audio feeds & streaming
Encoder — Software that compresses your audio into a streaming codec and pushes it to a feed provider’s server. See Audio feeds & streaming
Alert — An action fired when a call matches a rule you set — a talkgroup, a unit, or an activity pattern — so you’re notified when something happens. See Alerting on the calls you care about
Alert fatigue — The failure mode where an alert fires so often you learn to ignore it; the reason to alert narrowly. See Alerting on the calls you care about
Monitoring post — A receiver, antenna, and computer built to run unattended around the clock, capturing traffic whether or not anyone is watching. See Building an always-on monitoring post
Headless — Running a computer with no monitor or keyboard, operated over the network — the normal way to run an always-on post. See Building an always-on monitoring post
Managed service — Software run under a service manager (like systemd) that starts on boot and restarts on failure, so the post heals itself. See Building an always-on monitoring post
Software & GopherTrunk
Trunk-tracker — Software (or a scanner) that reads a system’s control channel and follows grants to voice channels automatically — the top rung of scanning tools. See Scanning with SDR software
Demodulation — Pulling a signal out of the raw SDR samples, the first software step before decoding. See Scanning with SDR software
Decoding — Turning a demodulated digital signal into bits and then into voice or data — where a protocol’s rules are applied. See Scanning with SDR software
GopherTrunk — An open-source, multi-protocol trunk-tracking decoder that turns a bare SDR into a scanner across P25, DMR, NXDN, TETRA, and more. See GopherTrunk as a scanner
Hunt — GopherTrunk’s feature that sweeps a band, detects carriers, identifies the protocol, and reports which one is the control channel. See GopherTrunk as a scanner
Signal path — The order the signal travels — antenna → feedline → SDR → decoder → recording — which is also the order to build and troubleshoot a setup. See A worked end-to-end monitoring setup
IQ capture — A recorded file of raw SDR samples; the single most useful thing you can give a decoder developer, because it reproduces the exact problem. See The community & contributing data
Commons — The shared, community-maintained knowledge — databases, feeds, and open-source decoders — that the hobby runs on and that contributors replenish. See The community & contributing data