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operation. When a user transmits (on an input frequency), the
signal is picked up by the repeater and retransmitted (on an
output frequency). The user’s radios always listen for activity
on the output frequency and transmit on the input frequency.
Since the repeater is located very high, there is a very large
line of sight. Typical repeater systems provide coverage out to
about a 25 mile radius from the repeater location.
Trunked Scanning
While conventional scanning worked great while there were
only a few groups wanting to use the frequencies, with the
advent of smaller, lower-cost radios more and more agencies
and businesses wanted to take advantage of the utility of 2-
way radio. As a result, the bands that were used most
became full, so new users were not able to take advantage of
the technology as quickly as they wanted.
Trunking solved this frequency shortage by allowing multiple
groups to use the same set of frequencies in a very efficient
way. While each type of trunking system operates a little
differently (see the next few sections), they all work on the
same basic premise: even in a system with a lot of users, only
a few users are ever transmitting at any one time.
Instead of being assigned a frequency, as with conventional
systems, each group is assigned a Talkgroup ID. A central
computer controls the frequency each group operates
on...and this frequency selection is made each time a user
transmits. So, while on a conventional system queries, replies,
and follow-ups are all on a single frequency, they could each
be on completely different frequencies on a trunked system.
This semi-random frequency assignment made monitoring
such a system impossible prior to Uniden’s invention of the
Trunktracking scanner.
Motorola Trunking
While there are 4 different types of Motorola trunking systems,
they all use the same basic trunking method. The system
consists of one control channel plus one or more voice
channels (typically 10, 20, or 30 total channels). When a user
presses Push To Talk (PTT) to transmit, their radio first sends
their talkgroup information to the control channel. The
computer then assigns that talkgroup to a specific voice
channel and transmits that data over the control channel. All
radios in that talkgroup switch over to the assigned voice
channel and the user can begin speaking. This all typically
takes place in about a second...the person transmitting hears
a beep from their radio when the channel is assigned and it is
OK to start talking.
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