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Trunked Radio Systems:

A trunked radio system cures one problem with the conventional system above. In the conventional system, a separate frequency is needed for each group of users. In a large and dense metropolitan area, however, there may be far more groups of users than available frequencies. Meanwhile, despite this shortage of frequencies, many of the frequencies may not be in use at any given time since most 2-way radio transmissions are not continuous.

A trunked radio system allows a larger number of users/groups to share a smaller number of frequencies. A trunked radio system does this by taking advantage of the fact that not every user will want to transmit at any given time. In fact, very few 2-way radio users transmit continuously. Unlike a conventional system, frequencies/channels are not dedicated to any particular group of users. Instead, a large pool of frequencies is shared among a larger number of users. In a typical trunked system, anywhere from 5 to 40 frequencies may be shared among police and fire departments, utilities, buses, dog catchers, and private businesses. It is very unlikely that all users in each agency will transmit at the same time, so the trunked system can get by with fewer frequencies than users. Users in a trunked radio system are organized into groups known as talk groups. A large agency like a police department may have numerous talk-groups organized similarly to the channels in a conventional system. For example, the Eastside Road Patrol Unit and the Westside Road Patrol Unit each have their own channel in the conventional system above. In a trunked system, each Road Patrol Unit would typically have its own talk group.

Each time a member of a talkgroup starts to transmit, a central computer assigns all members of that talkgroup to an available frequency. All members of this talkgroup will transmit and receive on that frequency for the duration of the conversation. If a member of a second talkgroup starts to transmit at the same time, the central computer will assign all members of that second talkgroup to the next available frequency. All members of that second talkgroup will transmit and receive on that frequency for the duration of the conversation. When a transmission is over, that frequency again becomes available for the central computer to assign to another talkgroup.

When using a conventional scanner, it is difficult to monitor agencies, including many police departments, that use a trunked radio system instead of a conventional system. Users on a trunked radio system are not given dedicated frequencies. Also, a transmission and its reply may be on different frequencies. Imagine your frustration at missing the reply in a police chase and instead hearing the dog catcher!

In order to fix these problems, manufacturers created "Trunk-tracking" or "Trunking" radio scanners. Trunking scanners use special circuitry that monitors the digital control channel signals that tell talkgroup members which frequency to use. A trunking scanner can be set to display the number of the talkgroup at any given time, to follow transmissions and their replies within a talkgroup even as the frequency changes, and to only listen for selected talkgroups if one desires. They have been around since 1997 and  may now be purchased from a variety of vendors or purchased in used condition through services such as eBay.

Common analog trunked radio systems include: Motorola Type I, Motorola Type II, Motorola Type I/II hybrid, General Electric EDACS, and EF Johnson's Logic Trunked Radio (LTR) systems. The Motorola APCO Project 25 system is the only digital system that may be heard on radio scanners, and currently only on a few high-end models (see below for more information on digital radio). Make sure the scanner model you buy can track the type of trunked radio system used in your area. The tables of scanner models in Categories 4 and 5 contain this information for each scanner model.

For analog systems, you should browse through Category 4: Analog Trunking Scanners and Category 5: Digital Trunking Scanners.  If you are only monitoring analog systems, a Category 4 radio will be sufficient and will be significantly less expensive than a Category 5 radio.  If any of the police, fire, or other services you would like to hear in your area use the digital APCO-25 format, regardless of whether the system is in conventional or trunked mode, you will need a police scanner in Category 5.  Please note that all Category 5 scanners can also monitor analog conventional and trunked systems.  If some departments in your area use analog systems and others use digital, a Category 5 scanner will be able to monitor both.

Conventional Systems
Skip Ahead to Part 4: Table of Listening Topics and Scanner Categories
Digital Systems

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