I've put together a bit of info on scanning for California Highway Patrol, for those that are interested. Programming for CHP is easy, they use single frequency analog in the 42 mhz range (as opposed to trunking systems which use banks of frequencies and talkgroups). To get the most out of mobile scanning CHP, you need a 49 inch whip, like this one. CHP splits channels into pairs, Mobile and Base (dispatch). Mobile units always transmit on the M channel, and receive on the B channel, and of course dispatch is the flip side of that, receiving M frequency and transmitting B. A series of localized repeaters are used throughout the system, the more difficult the terrain, the more densely packed the repeaters. Mobile units transmit around 100 watts. CHP almost always (99% of the time) simulcasts the mobile radio traffic over the Base channel, so when you are scanning, you need only program the B channel and hear both sides of the conversation. The simulcast of mobile radio traffic is a safety feature in case dispatch goes away, that all mobile units can hear one another, even if they are in different areas. (I heard this happen twice now, where dispatch was MIA for the better part of an hour, with some sort of equipment problem, but mobile units could hear one another over the simulcast). The other thing that occurs, is that the mobile simulcast is re-transmitted on all local repeaters. In flat terrain like the central valley, Los Angeles, SF Bay Area counties, you typically hear crisp reception for both mobile and base. In other areas where the dispatch zone covers mountainous areas, like the coastal hills, Sierra Nevada, you typically hear all mobile units clearly, but dispatch may be scratchy or not heard at all for all conversations. What happens is that the radio system chooses the closest repeater to the sender, based on signal strength received from the mobile unit. So when the dispatch is talking to a unit, it is only transmitting on the single repeater that is closest to that unit. If you are on the other side of the mountain, or too far away from that repeater, you will not hear, or hear scratchy dispatch calls - but only when dispatch is communicating with a distant unit. Because the MOBILE traffic is simulcast on all repeaters, you will hear the distant mobile unit as if he were next to you. If a mobile unit is within a few miles of you, or reasonably close, then when dispatch is communicating with that unit, you'll be getting clearer reception since the repeater chosen will be close to you. It's an odd system for sure, because the dispatcher might be talking to a lot of units, and you'll hear her/him clearly one second, then scratchy or not at all the next, due to this ping-pong of repeaters being active and not - again only for the base channels. Because CHP also use CTCSS tone frequencies, you can dial squelch to zero, as the (assuming you have a radio that can program the tone) radio will auto-squelch when the transmission ends. For car-to-car, the mobile units transmit low power (5 watts) on the BASE channel. Since dispatch is listening only on the M channel, they will not hear any low power base traffic (there are special cases to this, but mostly). In order to hear car-to-car, you need to be physically near them, within 1 or 2 miles, since none of these transmissions will be broadcast on the repeaters. I've not covered digital extenders, since they are not much use for mobile scanning as the base channel always simulcasts anyway, its a wasted effort IMHO to try and snag the close digital extender transmissions. Digital extenders may or may not be in use on a traffic stop. http://www.wb6nvh.com/CHP2001.htm From radioreference for CHP, Monterey Dispatch Center. Note the 'color' names given by CHP, which are paired mobile-base frequencies. These same pairs exist in other areas in the state, but spread far enough away to avoid interference. Monterey itself contains 2 pairs for 2 geographically different areas, but are served from the same physical dispatch center. In this example, the Black "channel" covers an area contained within the Monterey County, while the Green channel covers Santa Cruz County, and the southern half of Santa Clara County and northern San Benito County.