Department of the Highway Patrol (CHP) (CA)
From The RadioReference Wiki
A Beginner's Guide to the CHP Radio System:
The CHP employs two frequencies for communications between units in the field and their dispatch centers. One frequency is used by base stations, located at electronic sites, most of which are placed on mountain peaks or other high positions. Cars, or mobile units, use a second frequency to talk with these base stations. On the list below the base frequency is shown as the "frequency" and the mobile frequency is shown as "input." It is important to realize that base stations transmit from higher elevations and at much greater power than mobile units do. For this reason the base frequency can be heard over long distances, while the lower powered ground level mobile units can only be received if they are close by. The dispatch centers use one or more electronic sites to communicate with mobile units. Remember that high locations are ideal for radio reception as they can "see" more territory than ground level sites. This is why the dispatcher can receive most of the mobile traffic, while the scanner listener cannot. Electronic sites are linked via microwave or 72 MHz frequencies and the dispatcher's console is controlled by a type of computer that chooses the best signal to send to the dispatcher if more than one site is receiving a mobile unit's signal at the same time. This computer is called a voter, as the selection among multiple sites is "voted" for the best signal. When transmitting to a mobile unit the dispatcher can choose the electronic site they want to use, and normally choose the same site the voter chose when replying to a mobile unit. This is why you may hear the base frequency come in well at times when a nearby site is being used and not so well when another, more distant site is being used. When units want to communicate directly with each other, commonly referred to as "direct" or "car to car", they both switch their radios to another channel where the radio is both receiving and transmitting on the base frequency. One disadvantage to this is the cars can have the same experience that scanner listeners have, which is that they cannot hear other mobile units that are located some distance away.
In some locations within the state, mostly in urban areas, and mostly in southern California, the mobile frequency is patched to the base frequency on the most often used electronic site within a dispatch area. Cars are now able to hear each other from longer distances that they could before. This works the best in dispatch areas that are small and mostly urban as in these areas most mobile units can receive the same electronic site. In rural areas the dispatch area can be quite large with as many as 6-12 sites and mobile units may only be in range of one of those sites. Since the advantage of having the other mobile units hear each other is lost, the mobile traffic is not transmitted over the base frequency in rural areas.
Mobile Extender Handheld Radios
When a CHP officer intends to get out of the patrol vehicle or gets out of the patrol vehicle, the officer activates the mobile extender unit to hear radio traffic and transmit radio traffic over the car radio (higher power) via the officer's handheld portable radio.
The mobile extender unit operates on 154.905 MHz, and normally listens for transmissions on that frequency from a nearby handheld portable which is then re-transmitted through the car radio mobile out on the car-to-station channel (42 MHz). When the car radio is receiving traffic (on 42 MHz), that traffic is re-broadcast through the extender unit (if it is enabled) on 154.905 MHz.
The handheld radios are programmed with other frequencies that do not make use of the mobile extender radio, allowing the CHP officer to talk to other CHP handheld portable radios and other non-CHP radio systems. Channels 1 through 6 are listed; however, the handheld portable radios also have channels 7-16, which are normally programmed locally to communicate with other public safety systems on the same VHF band within the same geographical area as the radio being used.
Hint: if you are monitoring 154.905 MHz on a CA highway and you hear traffic (CHP), there is a good chance you are within 2 to 3 miles of that CHP unit. This is the same technology used in Uniden's Beartracker radios.