This article first appeared in the December 2000 issue of Monitoring Times.
First-time scanner listeners are often confused about where to find trunked radio frequencies. Internet newsgroups and websites are often good sources of information, but what if you're interested in a system that no one else is talking about?
This month we'll go over where to look for frequency information and how trunking frequencies are assigned. We'll also take a look at some new frequencies that are on the horizon.
In the United States, the Federal Communications Commission (FCC) is the governmental authority that assigns specific radio frequencies to individual users. These assignments are made in accordance with a set of rules, also issued by the FCC, that are published in the Code of Federal Regulations. The CFR is divided into sections called Titles, and Title 47 (Telecommunication) is of interest to scanner listeners.
Within Title 47 are a number of Parts. Most people who read the label on the back of electronic devices may have read something like
Part 15 is subtitled "Radio Frequency Devices" and contains a lot of rules regarding how an electronic device should (and should not) operate. This also happens to be the section that prohibits the manufacture of scanners capable of receiving cellular telephone frequencies, and requires that scanners have a permanent label reading:
Land Mobile Radio
Part 90 of Title 47, subtitled "Private Land Mobile Radio Services", lists the rules pertaining to the operation of, you guessed it, land mobile radio. In this Part you'll find the various band plans and allowed modes of operation for both conventional and trunked radio systems, buried within the typical regulatory jargon.
At present there are three common areas where trunked radio operates, namely VHF High Band, UHF, and 800 MHz.
VHF stands for Very High Frequency and refers to a portion of the radio spectrum between 30 MHz and 300 MHz. It is divided into "bands," named low, mid, and high. VHF low band covers the range of 30 MHz to 50 MHz. VHF mid-band runs from 72 MHz to 76 MHz, and VHF high band is from 108 MHz to 174 MHz. Most trunking systems in VHF operate at the top of the high band between 150 MHz and 174 MHz.
UHF stands for Ultra High Frequency and refers to a portion of the radio spectrum between 300 MHz and 3,000 MHz (3,000 MHz is the same as 3 GHz). Most trunking systems that are called UHF operate between 450 MHz and 512 MHz. Confusingly, 800 MHz falls within the UHF portion of the spectrum, but most people prefer to talk about 800 MHz as separate from UHF.
FCC defines Trunking
Although there are many technical differences between trunking systems, the FCC has historically recognized only two types. Centralized trunked systems use a control channel to transmit channel information to mobile units. Motorola and EDACS are both centralized systems. Radios in a decentralized trunked system listen to each channel in order to find one that is available for use. Logic Trunked Radio (LTR) is a type of decentralized system. In the view of the FCC, centralized systems run the risk of causing "harmful interference" because the radio doesn't listen to a voice channel before transmitting. The control channel tells the radio which frequency to use without regard for any other users that might be nearby. Since decentralized systems search for a quiet channel before transmitting, the risk of interfering with another user or system is minimized. Because of this risk, the FCC has not usually authorized centralized trunked systems in shared frequency bands below 800 MHz.
150 to 174 MHz
Public safety agencies in VHF typically operate between 150 MHz and 174 MHz. These are usually older, non-trunked (so-called conventional) systems that are often very congested. The radio propagation characteristics of VHF allow good coverage with relatively few towers, so it's an economical solution.
This is a very crowded area of the spectrum, with a wide variety of government and commercial users. You may find a limited amount of trunking activity here, but most of the radio systems are conventional.
For some uses in this band, frequencies may be assigned in pairs with a separation of 5.26 MHz. In this band channels are typically assigned every 15 kHz, so try searching in 5 kHz steps with a bandwidth of either 12.5 kHz or 25 kHz.
450 - 470 MHz
This UHF segment is also very crowded, with a lot of land mobile activity. UHF tends to have somewhat less range than VHF, but works better in urban areas. Along with a large number of older public safety trunked systems, you'll find municipal and utility repair crews and even a dogcatcher ("animal control") or two.
In this band frequencies are generally assigned as a pair, with the base station transmitting 5 MHz lower than the mobile unit. For example, an electric repair crew using mobile radios transmitting on 456.025 MHz would have a corresponding base station radio transmitting on 451.025 MHz.
Between 460 MHz and 470 MHz in particular you should find police and fire activity. Mobile units transmit between 465 MHz and 470 MHz in 25 kHz steps, while the corresponding base transmits between 460 MHz and 465 MHz. Low power, 2-watt handheld radios may operate in 12.5 kHz steps.
Scanner listeners should use a step size of 12.5 kHz and a bandwidth of 25 kHz.
470 to 512 MHz
The frequency spectrum between 470 MHz and 512 MHz was originally assigned to television stations operating on channels 14 through 20. New frequencies in this range are only available in 11 cities (Boston, Chicago, Dallas, Houston, Los Angeles, Miami, New York, Philadelphia, Pittsburgh, San Francisco, and Washington, DC). In Los Angeles, these frequencies are limited for use only by Public Safety agencies.
In this band frequencies are usually assigned in pairs, with the mobile unit transmitting 3 MHz higher than the base station. An example would be a mobile transmitting on 485.00625 MHz and the corresponding base station transmitting on 482.00625 MHz.
Assignable frequencies occur in 6.25 kHz increments with a bandwidth of 25 kHz.
The 800 MHz contains the majority of trunked radio frequencies monitored by scanner listeners. Public safety activity occurs in two bands, 806 MHz to 824 MHz and 851 MHz to 869 MHz. According to FCC rules, current production scanners must block the cellular telephone frequencies that reside between these two bands.
Public safety and private systems are mixed in this band. Radio propagation for 800 MHz is good for in-building coverage, but requires more tower sites than VHF or UHF systems and thus is more expensive for an agency to install and maintain.
You can think of the 800 MHz as divided into two sections. The block between 806 MHz and 851 MHz are where mobiles transmit, and between 851 MHz and 896 MHz are where the corresponding base station transmits. Mobiles transmit exactly 45 MHz higher than base stations, but most scanner listeners are not close enough to the mobiles to capture the signal.
Spacing is either 12.5 kHz or 25 kHz with a bandwidth of 25 kHz.
Here are the general allocations for trunked systems:
There are a few other areas of the spectrum where you may come across trunked radio systems. These are less popular than the bands already mentioned, but are worth checking in your local area. If you hear trunked activity in these bands, please send me an e-mail and let me know what you find!
220 to 222 MHz
This band is shared with automatic vehicle location (AVL) telemetry data.
Base units transmit between 220.0025 MHz and 220.9975 MHz in 5 kHz steps. Mobile units transmit back exactly 1 MHz higher.
935 to 941 MHz
A number of private trunked systems operate between 935 MHz and 941 MHz, although it is not uncommon for public safety agencies to make use of these private systems.
Frequencies are assigned in 12.5 kHz steps.
As a popular scanner manufacturer states, "the future is wireless." With the explosion of wireless services comes the need for addition radio spectrum. To ease overcrowding and make some money for the U.S. Treasury, Congress has mandated that the FCC auction off additional bands that can be used for new services.
Broadcast television stations use up a lot of radio spectrum. A single UHF TV channel takes up 6 MHz, a block that could hold more than 200 voice radio channels. You may recall that the entire cellular telephone industry started in a portion of the radio spectrum left vacant when the FCC eliminated UHF TV channels 70 through 83 decades ago.
Under current FCC regulations, broadcasters on TV channels 60 through 69 must move to new frequencies more suitable for high-definition digital television by 2006. Out of these 10 channels, the FCC has set aside 24 MHz specifically for public safety. This represents the largest single allocation of spectrum for public safety ever made, and doubles current allocations.
Specifically, the plan allocates 764 MHz to 776 MHz (TV channels 63 and 64) for base-to-mobile transmissions and 794 MHz to 806 MHz (TV channels 68 and 69) for mobile-to-base. This should make it easier for equipment manufacturers to build 700 MHz-capable mobile radios, since the existing 800 MHz mobile frequencies begin at 806 MHz.
The FCC established two basic types of channels, a narrowband channel of 6.25 kHz and a wideband channel of 50 kHz. The FCC is betting technology will improve to the point where voice radio equipment can make use of channels only 6.25 kHz wide. The FCC calls this "spectrum efficiency," and it means more users will be packed into smaller channels without interfering with each other. Until technology catches up, users are expected to aggregate four of these slivers into larger 25 kHz channels that current generation equipment requires.
Any 700 MHz system that uses six or more narrowband channels must be trunked, although the FCC doesn't specify a standard. In addition, all radios must use digital modulation, and the FCC recommends the Project 25 Phase I voice standard (sometimes called APCO 25 -- see the June, 2000 Tracking the Trunks column for more information) be used. Mobile transmitters may have analog capability, but they will either be low power (2 watts or less) or be a secondary mode to the primary digital mode.
Radios that use the wideband channels will be required to move data very rapidly, up to 384 kilobits per second (kbps) in 150 kHz. The FCC anticipates public safety users will be transmitting video images and data across these frequencies, providing a real-time "on-scene" information to dispatch centers and other mobile users.
It will take several years for the 700 MHz band to start carrying public safety traffic, so there's no hurry to find a scanner to cover 700 MHz. The biggest hurdle for public safety agencies is to get the current users of the spectrum (what the FCC calls "incumbents") to leave before 2006. There are nearly 100 broadcast TV stations and more than 1,300 low power and translator TV stations licensed to operate on channels 60 to 69, and they're not in any hurry to leave.
That's all for this month. More information is available on my website at http://www.signalharbor.com, and I am reachable via electronic mail at firstname.lastname@example.org. Until next month, happy monitoring!
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