(7/5/2001) by Ben Saladino KC5IRJ
This explanation would be impossible without first mentioning conventional radio systems. Simplified, on conventional two-way radio systems each channel has a dedicated frequency. For example, the Hurst Police Department has three channels F1 453.975, F2 460.05, and F3 460.15. When an officer switches channels, he is tuning the radio to a different frequency or channel. On a trunked radio system a channel can be on virtually any frequency in the system. A trunked radio system usually consists of 5 or more frequencies, and normally not more than 30. One of the frequencies is used as a control channel and is often switched automatically every twenty-four hours. Each radio in the trunked system monitors the control channel to determine what frequency is currently assigned for the active channel on the radio. That frequency may be any frequency in the trunked radio system, except the control channel, so in a 8 frequency trunked system, there will be 7 frequencies worth scanning.
Spectrum efficiency is the short answer, but it's not the only reason. Since channels on a trunked system are "virtual" compared to a conventional system, there can be more and flexible channels. For example, the city of Bedford has a trunked system that has 8 frequencies, but dozens, if not hundreds, of possible channels. This particular system has at least 1 channel for each of the following users: Bedford Police and Fire, Colleyville Police, Fire, and Public Works, Euless Police and Fire, and Keller Police, Fire and Public Works. That's at least 10 channels right there, and most of those users have more than one channel assigned. How is that possible with only eight frequencies? Well, it's not, at least not all at the same time, but that's how virtual channels provide spectrum efficiency. It's unlikely that all 8 frequencies will be in use at the same time. When that happens, lower priority users get a busy signal. An interesting note about the Bedford system is that it started out as a five frequency system before Euless joined. When Euless joined, they relinquished their 3 conventional frequencies for use in the trunked system, so now everyone on the system benefits from the extra frequencies.
Other benefits of trunked systems include the ability to disable individual radios in the system that may have been lost or stolen. Another aspect of individual radio ids is the ability for dispatchers to track whose radio is transmitting. That's a big safety feature, especially when an officer may not be able to talk on the radio. There is also an emergency button on most public safety trunked radios. On shared trunked systems like the one in Bedford, it's easy for different departments within a city and even between different cities to share a channel. With conventional radios that was often a problem because different users might have been on different frequencies bands which would have required a more expensive or different radio. There are other benefits, but those are some of the most important.
Trunked radio systems started out on 800 Mhz frequencies and are now showing up in the 900 Mhz range, and the federal government is even going trunked in their 400Mhz band for some departments.
Almost everyone. Public safety, business, transportation, federal government, public works, the list continues. City and state government systems are usually on separate trunked systems, but sharing among local departments as in the Bedford example from above is very common. Locally, Fort Worth is another trunked system with several different cities on board including the Tarrant County Sheriff. Smaller businesses typically do not have their own trunked system, but are members of a trunked system with many other businesses. Some reports even claim that the federal government is buying time on business trunked radio systems.
The big players in trunked radio systems are Motorola, Ericsson (formerly GE), Uniden, and Johnson LTR. The last two I'm not certain about. The Ericsson systems are the worst for us, because each transmission ends with a data bursts that make scanning very difficult. In the Dallas/Fort Worth Area, the D/FW Airport, Irving, and Richardson use Ericsson (formerly GE) Ericsson's US headquarters are in Richardson. I wonder what kind of deal the city got for that.) . Almost all public safety trunked systems are one of several Motorola types.
Sample WAV files of Motorola and Ericsson control channels.
Remember virtual channels from above? Each time someone uses a radio it may be on a different frequency, even though the user has not changed channels on the radio. Channel does not equal frequency in a trunked system.
After determining what frequencies a trunked radio system includes, program them into their own bank. For example, I have Bedford's frequencies programmed into bank 7 on all of my scanners, Fort Worth in bank 9, D/FW Airport in bank 3, Arlington in bank 10, and so on. Once you've programmed all of the frequencies, make sure they are all unlocked. Hit the scan button until it stops on a frequency that has a continuos data sound. Lock out that channel in the scanner, and you're ready to go. Normally, you will have to lockout a new control channel everyday. DO NOT FORGET TO UNLOCKOUT THE PREVIOUS CONTROL CHANNEL! Note: There are lots of other data channels floating around on some trunked systems, so make sure you only lock out the frequency with the continuous data, most other data sounds are short bursts, and will alternate among the different frequencies. Now it's just a matter of trying to follow a conversation. For instance, an officer calls the dispatcher.
|Transmission||Channel On Police Radio||Frequency Used in Trunked System|
|Officer: "325 Bedford"||BPatrol1||854.9875|
|Dispatcher: "Bedford 325 Go ahead"||BPatrol1||855.2375|
|Officer: "Request 46"||BPatrol1||866.0375|
|Dispatcher: "Bedford 325 Clear to 46"||BPatrol1||854.9875|
Notice the channel never changes for the dispatcher and officer, but the frequency, which they are not concerned with, does change. All of the radios set to channel BPatrol1 switched to the current frequency for BPatrol1 because the radio monitors the data channel the entire time checking to see what frequency is currently assigned to the channel that is set on the radio.
So following the conversation above you would have had to hit scan a few times to tune to the correct frequency, and you probably would have stopped on other frequencies that were being used by other channels on the trunked system. You have to listen more closely to follow the conversation, otherwise you'll be listening to several different conversations mixed together. No, it's not as easy to follow as staying tuned to a single frequency, but it can be done for most trunked systems, provided you know how units are numbered and can recognize voices and topics of conversations.
Other scanning publications may offer more advanced ways of scanning trunked systems by programming your scanner in certain ways, but I have not explored those methods to make a recommendation. I normally leave delays turned off in my trunked system banks, because it's unlikely that when a transmission ends, the reply to it will be on the same frequency, so continuing to scan immediately seems more logical to me than waiting a second or two, but that's debatable too.
Scanning GE/Ericcson systems is even more difficult, because each transmission ends with a data burst that makes scanning a real pain. Although I haven't heard it locally, there have been many reports from around the country that the GE systems actually play the "we bring good things to life" jingle after each transmission.
One last suggestion that I have for scanning trunked systems is to scan only the repeater input frequencies for the system (in some cases). This is handy if you are close to the portables and handhelds that are transmitting. For example, an airshow, festival, etc. It's also helpful if you live in a city that is part of a trunked system, but you are somewhat isolated from most other users on the system. For example, North Richland Hills, is part of the Fort Worth trunked system. By listening to the input frequencies, you will only hear activity from radios that are fairly close to North Richland Hills. OK that's a great tip, but what the heck is an input frequency? Well, that's the actual frequency that a handheld or portable transmits on. It's 45 Mhz lower than the output frequency of the trunked radio system repeater, which the handheld and portables (and usually scanners) receive on. A repeater receives the frequency from the handheld, say on 809.9875, and repeats it but with more wattage on 854.9875, so that it can be heard for greater distances. I hope that makes sense, because it really can help filter out conversations that are not interesting due to their distance.
Until recently there has not been a way for the average scanner user to really scan trunked radio system channels like conventional systems. Finally, Uniden has released their new line of Trunk Tracker scanners that will scan Motorola (the most popularly used for public safety), GE/Ericsson EDACS, and LTR trunked systems. This is a HUGE step for the scanner hobbiest! Check in here for the trunked system channel ids for use with the Uniden TrunkTracker® Radios.
TrunkTracker Tips and Frequently Asked Questions
If you have lots of money and really good connections, you may be able to buy a trunked radio and have it programmed with the users you want to hear (receive only, no transmitting). For the average scanner hobbiest, don't get your hopes up. This is how the media does it though, and I suspect that there are some individuals out there who have trunked radios that technically they should not have. Motorola has been to court a time or two against people who make this possible.
Motorola Trunking Training
Return to Scanning the Dallas/Fort Worth Area