Trunked communications systems are designed to "hop" between frequencies during gaps in the conversation in order to minimize the number of frequencies a user group actually needs. These systems allow a large group of 2-way radio users (or even different groups of 2-way radio users) to efficiently use a smaller number of frequencies than would be required with a conventional system. Since each frequency has to be licensed through the FCC, using fewer frequencies was easier on the user as well.
Where a conventional system would require a different frequency for each user or sub-group, a trunking system requires that each user or sub-group have a separate ID. Instead of selecting a specific frequency for a transmission, the user simply selects a talk group. The trunking system automatically transmits the call on the first available frequency, and also sends a code that uniquely identifies that transmission.
The introduction and popularity of trunked systems caused a problem for hobbyists with conventional scanners, however. Since the trunking system might send a call and its response on different frequencies, it is difficult to listen to trunked communications using a regular scanner. The popularity of the scanner hobby pushed manufacturers to develop scanners which could effectively monitor the new systems.
Trunking scanners monitor the data sent with each transmission in order to follow the talk group ID as it moves around. There are several different trunking systems that trunktracking scanners can monitor, including the following:
RadioShack sells a number of analog trunktracking scanners, both on-line and through your local RadioShack store.