Before this:The control channel: the system's heartbeatIdentifying what you're hearing
Finding the control channel
Key takeaways To follow a trunked system you need exactly one frequency: its control channel. Find it three ways — look it up in a database like RadioReference, recognise its constant data burst on the waterfall (a control channel never goes quiet), or let GopherTrunk’s Hunt feature sweep a band, identify carriers, and report which one is the control channel. Then add the control-channel frequency and system type to your configuration — Hunt can even commit it for you — and GopherTrunk locks on and starts following grants.
You’ve learned to identify what a system is. Now you need the one frequency to point GopherTrunk at. Everything else — voice channels, talkgroups, who’s talking — flows from decoding the control channel, so finding it is the whole job.
Path 1 — look it up in a database
For documented systems, someone has already done the work. RadioReference.com is the largest community database, listing system identities, sites, and — crucially — control-channel frequencies for tens of thousands of systems. Search by county or agency, note the listed control-channel (and alternate control-channel) frequencies and the system type, and you have everything you need to configure GopherTrunk.
This is the easy path, and it’s where you should start. The catch: many systems are undocumented. A new county system, a recently rebanded site, or a private network may not be in any database — which is where the other two paths come in.
Path 2 — recognise it on the air
A control channel has one unmistakable signature: it carries data continuously and never stops. While voice channels light up only when a call is granted and fall silent between calls, the control channel transmits its signalling stream every moment the system is up.
On the waterfall that constant activity stands out as a solid, unbroken column while the voice channels around it flicker. Tune to the suspect carrier, watch its constellation, and if it locks into a steady symbol stream that never pauses, you’ve almost certainly found the control channel. The finding-systems lesson covers the band-scanning side of this in more depth.
Path 3 — let GopherTrunk Hunt for it
When you have no frequencies at all, GopherTrunk’s Hunt feature automates the search. Point it at a band (or feed it IQ captures of a suspected control channel) and it:
- Sweeps the band with an FFT and detects carriers above the noise floor.
- Identifies the protocol of each candidate across all of GopherTrunk’s decoders.
- Decodes the control channel and accumulates what it sees — the system’s identity, the control-channel frequency, and every talkgroup observed in a grant.
- Exports a system map, and can merge the discovery straight into your config so you can start scanning immediately.
Because Hunt doesn’t assume the control channel is the loudest carrier, it works even when the control channel sits off-centre and below a louder voice channel. This is the tool that closes the loop on undocumented systems — and it builds the same map a database lookup would have given you, straight from the air.
Adding the system so GopherTrunk can follow it
Once you know the control-channel frequency and the system type, you add them to
GopherTrunk’s configuration. If you found the system with Hunt, the tool can commit the
discovery into config.yaml for you; if you found it by hand, you enter the frequency
and protocol yourself. Either way, GopherTrunk then locks the control channel and
begins reading grants.
Confirm it worked in the CC Activity panel: within seconds of a good lock you should see affiliations, registrations, and voice grants scrolling past. If the panel stays empty, the control channel either isn’t locked or you’ve got the wrong frequency — a problem we’ll tackle in troubleshooting.
Quick check: on the waterfall, which carrier is most likely the control channel?
Recap
- You only need one frequency — the control channel — to follow a whole system.
- A database like RadioReference lists control channels for documented systems.
- On the air, the control channel is the carrier that never goes quiet — a solid column on the waterfall.
- Hunt sweeps, identifies, and maps undocumented systems automatically.
- Add the control-channel frequency and system type to config, then confirm the lock in CC Activity.
Next, we’ll trace the whole pipeline — from a locked control channel to recorded audio — in following a system end to end.
Frequently asked questions
How do I find a trunked system's control channel?
Three ways, often combined. Look it up in a database like RadioReference, which lists known control-channel frequencies for documented systems. Recognise it on the air by its constant data burst — a control channel never goes quiet. Or let GopherTrunk’s Hunt feature sweep a band, detect candidate carriers, identify the protocol, and report which one is the control channel.
What does a control channel look like on the waterfall?
A control channel carries data continuously, so on the waterfall it shows as a carrier that is always active — a solid, unbroken trace rather than the on-and-off bursts of a voice channel. That constant activity is its single most reliable visual signature. Voice channels light up only when a call is granted; the control channel never stops.
What is GopherTrunk's Hunt feature?
Hunt is GopherTrunk’s tool for discovering systems you don’t have frequencies for. It sweeps a band or processes IQ captures, detects carriers above the noise floor, runs each through the protocol identifier and decoder, and builds a map of the system — identity, control channel, and observed talkgroups — which it can export or merge straight into your config so you can start scanning.
How do I add a system once I know its control channel?
Add the control-channel frequency and system type to GopherTrunk’s configuration. If you discovered it with Hunt, the tool can commit the discovery into config.yaml directly. Either way, once the control channel and protocol are configured, GopherTrunk locks the control channel and begins following grants across the system.