CHP radios and scanners   (This seems to be the same type of system that ISP uses with 155.505)

What is a "CHiPs detector"?  What's the complete story on CHP radios?    from Chucko@charon.arc.nasa.gov (Chuck Fry):

That's right.  Many CHP cars are equipped with repeaters so that when the officers get out of their cars, their walkie-talkies need only reach the car    instead of the base station.  The CHiPs Detector (tm) takes advantage of the fact that the CHiPpies rarely turn off the repeater when they're IN the car.  Note that the CHP may change this frequency at any time, although they're Not likely to.

   The disadvantages are that you just know at least one CHiPpie is in the area, not how close, what they're doing, or whether they're after you; and    there's no signal transmitted from the repeater when the base station is silent.  So it's hardly foolproof.

   morris@grian.cps.altadena.ca.us (Mike Morris) posted on 12 oct 1991: 
   The following info was compiled from several sources, none of which have 1st-hand knowledge of the new CHP radios, but what I have been able to put together seems to agree.  So with that caveat, ...     The older Motorola Micor mobile radios had "mobile extenders" by GE.  These    extenders were 1/4 watt transmitting units that repeated the audio from the 42mhz CHP mobile radio to 154.905mhz.  The mobile extender time-sliced the channel to transmit for roughly 9/10 second and receive for 1/10 of a
second to see if the officer was replying.  Hence the "yakyakyak-chuff-yakyakyak-    chuff-yakyakyak-chuff-..." sound of the repeated traffic on the 154mhz channel.  There was a writeup of the single-channel time-slicing technique    in a ham radio magazine back in the early 70s, and the technique has taken    off tremendously.  It has mostly been used to allow single-channel radio-to-telephone interconnects called simplex autopatches (because they use one channel - a "simplex" channel.)

   Anyway the mobile extender technique works very well, and allows the officer to use a relatively low power high-band hand-held to communicate with the dispatcher via the > 100w low-band mobile radio in his/her patrol car with very little trouble.

   A low-band hand-held would have to use a 6' antenna to be resonant, or a "rubber duckie" over a foot long.  And the hand-held couldn't have enough RF power to reach the dispatch center in 99% of the state.  Hence a 150mhz handheld (where a 18" antenna is the norm, and a "rubber duck" is <9") and a mobile extender. 

   A bit of history:    The bid for mobile radios was won by Motorola around 10 years ago.   The mobile extenders were an afterthought, and that bid/contract was won by GE.   The user interface was a simple on/off switch, and the state radio shop
   people mounted it in the Motorola control head.  It was a toggle switch labeled with a Dymo tape "repeat enable/disable" (or "extender on/off" or "portable on/off").

   The average officer soon discovered that leaving the extender switch in the "on" position worked just fine.  They turned off the hand-held to shut off  the "repeater", not realizing that the mobile side of the extender was still on.  Probably 99% of the CHP officers left it on for the entire shift.  With the almost constant activity on the CHP dispatch channels, this 154.905mhz   vehicle transmitter behaved like a 1/4w beacon, providing between 1/4 to 1mile notification of the location of a patrol car.

   Now the spoiler: The CHP is replacing (has replaced here in my area) _all_ of their Motorola Micor/GE extender radio packages.  The new radios are all GE, with CHP-designed control groups.  (The state Red Cross got 90% of the radios for the 47.42 - 47.62 freqs.  A few of them went to other state agencies, like the Office of Emergency Services).  The 1991 Southern California edition of the "Police Call" frequency listing has a nice writeup on the CHP-designed control groups, as I remember.  They even got 90%
of it right.  

The new design forces the officers to disable the extender when they are in the patrol car.  Listening to 154.905 while mobile now just tells you where a CHP car is _stopped_, with the officer out of the car, as opposed to  before when it would tell you where a stopped or a moving one was... 

   Here is the frequency map of the CHP hand-helds as I have it.

   F1: 154.905  with the primary tone.  (NOTE 1)
   F2:  same         1st alternate tone
   F3:  same         2nd alternate tone
   F4: 154.920 (CLEMARS 1) - Base side of CLEMARS
   F5: 154.935 (CLEMARS 2) - Mobile/Portable CLEMARS
   F6: 156.075 (CALCORD) (NOTE 2)
   F7: 155.475 (CLEMARS 3 / NALEMARS) (NOTE 3)

   Abbreviations: CLEMARS: California Law Enforcement Mutual Radio System.
                  CALCORD: California Coordination - a statewide "on-scene" channel
                  NALEMARS: National Law Enforcement.... A federal version of CLEMARS.

Note 1: With the old hand-helds (2 freqs - 154.905 and 154.920) there was no way that two units from different areas (i.e. different dispatch frequencies) could have their extenders operational at an out-of-vehicle scene -- when an officer transmitted, both mobiles would be brought up. The remainder of Note 1 explanation is from the post of scotto@ipars.cts.com (Scott O'Connell) on 14 oct 1991:

The receiver of the extender has an attenuator making a low wattage HT usable for only a short distance (typically less than 50yds).  To make sure there is only one extender being used within close proximity each vehicle extender sends a short burst tone to see if others are active.  If it is within range of another active extender it doesn't turn on at all.   The HT is then using the other vehicle radio (the one that was already turned on).

Now for the PL explanation.  There are three channels on CHP HT's that relate directly to the extender.  Channel 1 (also called PP or Person to Person) does not transmit any tone nor does it decode.  It is meant for HT  to HT use.  Channel 2 has a subaudible tone on transmit allowing the officer to talk to dispatch. (ie, transmits on the input freq of the lowband radio)  Channel 3 has a different subaudible tone on transmit allowing the officer  to talk to other officers. (ie, transmits on the output of the lowband radio) All channels are carrier squelch on the receive so that PP can be heard regardless of other traffic.

I hope this clears up why there are three 154.905 channels on the HT's.

Note 2: 156.075 is also the Ship TX side of Marine channel 61 (paired with 160.675 Ship RX).  I understand some re-thinking of the use of this frequency is going on.  It seems to be pretty useless in coastal areas.

Note 3: 155.475 I have been told that this channel has multiple PL tones.  I have also been told that the CHP handheld is 10 freq - capable.  Maybe this channel has multiple appearances like F1-F2-F3.  More info is needed.

Another rehash of the low band channels is in the works since LA County Sheriffs is moving to 800 or 900 mhz.  The CHP has acquired all of the 39mhz LASO channels and is slowly moving to change all of the low band dispatch operation to full repeat.  My sources do not know if the mobiles will be   transmitting on 39mhz and listening on 42mhz or vice versa.  It does not make much difference to the GE mobiles since they cover the full 30-50 mhz  just fine (as opposed to the old Micors that covered 42mhz to 50mhz only.  Does anybody have any info?