Also known as: trunking site, RF site, repeater site
A trunking site is a single repeater location — a tower with its transmitters and receivers — that radiates one control channel and a pool of voice channels covering the area around it.1 It is the physical building block of a trunked system: the antennas, the combiner, and the channel electronics that together define one coverage cell. A radio in that cell registers with the site’s control channel and receives all its channel grants from it.
How it works
Each site runs one dedicated control channel that never carries voice — it streams signalling continuously so idle radios can lock to it, affiliate, and wait. The rest of the site’s licensed frequencies form the voice pool. When a member keys up, the site’s controller picks a free voice channel, transmits a grant on the control channel, and every affiliated radio in the cell retunes to hear the call. A site is identified by a site number that is unique within its parent RF subsystem; larger networks stitch many sites together (see multisite trunking).
A site can be standalone (its own controller makes all decisions locally) or networked (a central controller coordinates it with peers so a talkgroup can be active on several sites at once). Two nearby transmitters carrying the same audio in lock-step is a special case called simulcast, which improves coverage but complicates reception.
In practice
- A site advertises its neighbours on the control channel so radios know where to roam; a monitor reads the same list to map the system (see neighbor site).
- Site parameters — colour/network codes, site number, and the control-channel frequency — are broadcast periodically, letting a scanner identify exactly which cell it is hearing.
- Coverage is a trade-off: high towers reach far but overlap with neighbours, which is why boundary areas hand traffic between sites.
Relevance to SDR
A software receiver sees a trunking site as one control-channel carrier surrounded by a cluster of voice-channel frequencies. GopherTrunk locks a site’s control channel, decodes its identity and neighbour list, and tunes voice grants as they are issued — so from GopherTrunk’s point of view a site is the unit it tracks: one control channel, one set of grants, one coverage cell. Systems such as P25 and DMR Tier III are built from these sites, and a wide-area network is simply many of them linked together.
Sources
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Trunked radio system — Wikipedia, on repeater sites and trunked-system architecture. ↩