Trunked Radio Overview

A brief non-technical explanation of 
trunked radio for the scanner enthusiast 

Many of the questions I receive on my website are related to “trunked” radio systems.   A bit of background knowledge of how these systems work is essential to understanding how to programming a trunk-tracking scanner.  So in this article, I’ll attempt to demystify “trunking”, and hopefully, add some clarity to the subject.

First, let me add a few caveats.  The intended audience for this article is the casual scanner enthusiast.  So for the sake of clarity, I’ll not be delving too deeply into the technical aspects of trunked radio systems.  Also, since the vast majority of public safety systems in the United States are based on Motorola Type II trunking, this article will be based primarily on the operation of those systems.

In order to understand trunked radio, a bit of discussion about conventional non-trunked systems is in order.  For example, let’s discuss the Houston Police Department (HPD) radio system. 

The range from mobile-to-mobile is relatively limited, so like most dispatch systems, HPD uses a series of “repeaters.”   So when a police officer on the west side of town needs to communicate with either dispatch or another officer, he sets his radio to the “West” channel.  This caused his radio receiver to be tuned to a frequency of 460.150 MHz.  When he transmits, the signal is sent on a frequency of 465.150 MHz.  (This scheme is known as a 5 MHz offset, and is relatively common for UHF frequencies.)   The repeater then receives on this frequency, and re-transmits the signal with greater power and at a higher elevation on 460.150 MHz.  When the officer completes his transmission, the radio reverts back to receiving at 460.150 MHz.  When any other officer or the dispatcher transmits, the operation is the same: transmitting on one frequency to the repeater, which retransmits the signal on a second frequency for all other units to hear.  The net effect is that an officer at ground level can communicate with dispatch and other officers at a much greater range.

The problem with the dedicated-channel method of communications is the uneven utilization of frequencies.  One agency, or one segment of the agency, always has exclusive use of a given frequency.  While the East side frequency may be only utilized 20%-30% of the time, the West side may be nearly saturated with radio traffic.  Additional UHF frequencies in large cities are usually very scarce and may be expensive to obtain.  In order to load balance the frequencies in a system, a complicated and expensive reprogramming of radios and/or re-mapping of districts may be required. 

One of the main advantages of trunked radio systems is efficient use of the radio spectrum.  Where the dedicated channel system of HPD use more than 20 frequencies to serve one agency, a trunked radio system can serve numerous agencies and thousands of users with less than 30 frequencies. 

The trunked radio system achieves this spectrum efficiency by allowing the radios on the system to use any available frequency, while switching all other radios on the same channel (known as a talkgroup) to the same receive frequency.  For example, Harris Country Precinct 4 Constable dispatch uses a talkgroup of 11376.  So all radios in this talkgroup constantly follow the conversations, even though they may be on any frequency within the trunked system each time they transmit. 

How is this accomplished?  Within each radio, and at the repeater, is a microprocessor that controls the radio.  All radios in the system monitor the “control channel” that is constantly sending out data on one of the system frequencies.  When a radio on a given talkgroup begins transmitting, a signal is sent to the controller, which locates an open frequency and gives the command to switch all radios on that talkgroup to the same receive frequency.  On the transmitting radio, an acknowledgement tone (such as two quick beeps) is heard, letting the user know that he may begin talking.  When the user releases the microphone transmit switch, his radio, and all others on the talk group, return to the control channel.  Then, when someone (such as the dispatcher) replies, the whole process is repeated.  The following graphically depicts a typical scenario in a trunked system.

So unlike a conventional system, where one agency or subset of an agency uses a dedicated frequency, the trunked system rotates throughout a set of assigned frequencies on the system.  The frequency may change (or may stay the same, depending on system configuration and availability of frequencies) each time a member of a specific talkgroup transmits.

On a conventional system, if a police officer in the West district want to talk to someone in the Central district, she may switch her radio to the Central channel.  When this occurs, the frequency of the radio is changed to Central, and she may proceed with the conversation.  On a trunked system, instead of changing frequencies, the officer changes talkgroups.  In either case, the officer only sees the alphanumeric display on the radio—which indicates the agency or district—and the workings of the system are transparent to her.

What does this mean to someone using a scanner to monitor a trunked system?  First of all, a conventional scanner that isn’t equipped to follow trunked systems is useless.  All you will hear is a series of fragmented conversations as the system switches conversation from one frequency to another.  So to monitor a trunked radio system, you will need a scanner designed specifically for this purpose.  Uniden manufactures and sells a line of scanners designated as “TrunkTracker”.  GRE sells another line of scanners through Radio Shack, designated as “trunking” scanners.  Prices vary from the low $200 range to nearly $600 for scanners capable of monitoring digital systems (more on digital systems in another article). 

Once you obtain a scanner capable of receiving trunked systems, the real fun begins.  The scanners typically work very well, but the instruction manuals are usually woefully inadequate at explaining the programming process.  Even after using scanners since the 70’s, I still have fits programming a scanner I’ve not used before. 

The best way to start is to obtain all the frequencies and talkgroup information for the system you will be monitoring.  Two great sources are this website (in the Harris Country Trunking section below) and the definitive frequency and talkgroup list at http://www.radioreference.com.

Begin by entering the frequencies into the scanner, and be sure to follow the instructions for loading frequencies of a trunked system.  This is very important!  If you don’t designate the frequencies as a part of a trunked system, the radio won’t follow the conversations correctly.

Additionally, some radios require you to enter all frequencies for the system, where others only require you to enter the trunking control channels.  On a twenty-eight-frequency system, there will usually be three or four designated control channels.  Since the control channel transmits continuously, the control channel frequency may change daily to reduce the wear and tear on the control channel transmitters.  If you are using a scanner that only requires control channels, be sure to designate this in your setup routine.  For example, on the Uniden BC250D, this is accomplished with the CNTRL CH ONLY option.

One thing to watch out for is systems that have multiple repeaters on the trunking system.  For example, the Southeast Texas system has interconnects between eleven repeaters http://www.star-net.org.  On this system (as of this writing) only the main repeater (downtown) simulcasts the traffic for all outlying areas.  All remote repeaters carry only the traffic for the region served by the repeater.

After entering the frequencies, you will need to enter the talkgroups, and if applicable, the associated alphanumeric tags.  The alpha tags are tedious to enter, but well worth the effort.  Instead of just seeing the numeric talkgroup displayed (11376), you can get a meaningful designator, such as PD DISPATCH.  Since talkgroups can be entered into separate banks like frequencies in a conventional scanner, you may wish to take some time to carefully organize them before you begin the entry process.  I always set mine up in an Excel spreadsheet the way I want them, which makes entry much more systematic.  As an added bonus, you’ll have a good backup of the data if you ever drop your scanner in the toilet!

If your scanner has the option of a computer interface and software, I strongly recommend going that route.  The amount of time and headache this will save is well worth the cost.  One of my favorite scanner programming software packages is from BuTel software.  They provide a full line of easy-to-use programs for many trunking scanners at http://www.butel.nl/.

I hope this information is helpful in gaining some understanding of how trunked radio systems work.  Keep in mind that there are at least three more major trunked systems besides the Motorola Type II system.  And there are ever several varieties of the Motorola system!  Each one operates differently, and presents its own unique scanner programming challenges.  However, once you get past the initial programming hurdle, changes and maintenance is usually quite easy.  And given the quantity and quality of traffic on most of these systems, they are a treasure trove for the scanner listener!

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