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San Francisco Bay Area Discussion Local area specific discussion for the San Francisco Bay area including the North, East and South Bays.

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Old 12-04-2013, 12:02 AM
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Default CHP Golden Gate-SF Region

Hey guys-why was I getting mobiles and base equally strong on 42.440 together for the longest time, and now since a few weeks ago, I get strong base on 42.440, and now mobiles on 42.760 with horrible reception?
I know those two frequencies were always base/mobile, but it never went like that before. Did they shut down a repeater that made mobiles as strong as base? What did I miss?
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Old 12-04-2013, 2:32 PM
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The mobiles can optionally be "re-transmitted" on the base frequency by the flip of a switch (at the dispatcher's whim). I always monitor the mobile frequencies to make sure I get both sides of a conversation (at least when I am close enough to do so).
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Old 12-05-2013, 8:18 AM
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Now that I did not know. Why wouldn't they use the best mode of transmit all the time-are they on some kind of energy saving budget? Those mobiles are now quite a challenge for the old d130j here.
BTW the other surrounding agencies like Oakland, Redwood City, Marin, I get mobiles all the time still. Just not the ones right under my nose. Figures.
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Old 12-06-2013, 9:28 PM
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I scan CHP daily on my drives in 3 counties, Santa Cruz, Santa Clara and San Mateo. I've found that the mobile units do not always transmit on the mobile channel, but I also scan both M and B as well. The San Jose units rarely use the mobile channel for whatever reason. As soon as I get into the Hayward or San Mateo regions, they usually transmit on the mobile channels. Not sure why one region is different, but its been that way for a pretty long time.
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Old 12-07-2013, 2:31 AM
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Quote:
Originally Posted by tahoekid77 View Post
I scan CHP daily on my drives in 3 counties, Santa Cruz, Santa Clara and San Mateo. I've found that the mobile units do not always transmit on the mobile channel, but I also scan both M and B as well. The San Jose units rarely use the mobile channel for whatever reason. As soon as I get into the Hayward or San Mateo regions, they usually transmit on the mobile channels. Not sure why one region is different, but its been that way for a pretty long time.
Not sure I follow you. The mobile units need to transmit on the mobile channel in order to activate the repeater or to be heard by dispatch. If they talk on the base channel, it's for car to car purposes.

Is it possible you just aren't close enough to hear them? They are probably only pushing 40 watts on the input or car to car channel.
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Old 12-07-2013, 10:42 AM
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So does anyone have the answer to my original question?
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Old 12-07-2013, 11:39 AM
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So does anyone have the answer to my original question?
Sorry ridge, the mobile channel is just the input to the repeater. Unless you are close you won't hear them that well.

The base channel Sometimes repeats the mobiles but it's selectable. So the link may just be off at the moment.
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Old 12-07-2013, 12:56 PM
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Sorry ridge, the mobile channel is just the input to the repeater. Unless you are close you won't hear them that well.

The base channel Sometimes repeats the mobiles but it's selectable. So the link may just be off at the moment.
Thanks for the reply. That must be it because I'm still hearing base to mobile comms on other base frequencies here in the bay area. I guess their repeater is down.
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Old 12-07-2013, 3:55 PM
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Or the repeater mode at the dispatchers console is turned off would be my guess. I know here in the Valley I can get both sides of Stockton and Tracy units 99% of the time. The Amador and Calaveras units are usually one side or the other. I would assume dispatcher preference or the type of incident(s) going on at the time.
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Old 12-07-2013, 7:54 PM
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I scan both M and B and I receive both in most regions, but not all. In the San Jose Golden Gate area I rarely or never hear the M channel, even if I am directly behind a trooper who is transmitting. I can only presume that not all regions are repeater driven. This is not the case in most areas, however.

I also get the car-to-car chatter when in close proximity to a unit, and they are transmitting low watt on the B channel.

I have noticed however, that in the last week that has changed somewhat, and they are using both channels. So to the OP, something HAS changed in the last week. (I've been scanning the south bay and peninsula CHP on a regular basis for about 10 years).

Last edited by tahoekid77; 12-07-2013 at 7:56 PM..
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Old 12-07-2013, 8:00 PM
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I believe in most cases all M channels are simulcast to the B channel over a repeater for safety reasons. I have heard in some rare cases the dispatch go down completely, but the mobile units could still talk to one another.
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Old 12-07-2013, 11:19 PM
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Thanks tahoekid-glad I'm not the only observer of this.
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Old 12-08-2013, 8:24 PM
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Quote:
Originally Posted by ridgescan View Post
Thanks tahoekid-glad I'm not the only observer of this.
I also heard San Mateo CHP dispatch and Monterey (Green) dispatch talking to mobile units about some problem with repeaters in the last 2 days or so.

Bottom line is always scan both B and M channels. You don't have a lot of control over anything else. (You can also scan extenders, for whatever that is worth).
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Old 12-08-2013, 8:26 PM
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The CHP system is not a "repeater" system like other such types. It was only a couple of years ago that the capability was added which enabled mobiles to be re-transmitted through the dispatcher's console over the base frequency. That function can be turned on and off at the whim of the dispatcher. You'll note that many (especially old timer) dispatchers still parrot mobiles, even with 'repeat' turned on.

Also, mobiles can talk car to car on the base frequency. This is a mode where the car radio's listen and talk frequencies are both set to the base frequency (normally the transmit is set to mobile frequency) and it transmits with less power. A lot of officers use this mode most, if not all, of the the time.

I suggest those that are truly interested read articles on this in the Wiki and/or listen exclusively to CHP (with proper antenna) for a day. You'll hear two mobiles talking to each other on the base frequency and then the dispatcher will transmit right on top of them like the dispatcher does not even hear them -- because the dispatcher mostly likely does not hear them (the dispatcher is typically listening to mobile frequencies).
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Old 12-09-2013, 12:57 AM
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When the dispatcher talks over two units having a conversation on the mobile channel, how does that go over?
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Old 12-09-2013, 5:22 AM
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When the dispatcher talks over two units having a conversation on the mobile channel, how does that go over?
They have to repeat what they said to the other unit. I've heard it happen. They know the drill and it's no big deal. The dispatcher has no way to to know if anyone is talking car-to-car at the time.

The CHP mobile radio microphones actually have two transmit switch positions. One labelled "C-S" and the other "C-C."

C-S stands for "Car-to-Station", which means the mobile radio is listening on the Base frequency and transmitting on the Mobile frequency (which is what the dispatcher hears).

C-C stands for "Car-to-Car", which means the mobile radio is both listening and transmitting on the Base frequency.

Quote:
Originally Posted by tahoekid77
In the San Jose Golden Gate area I rarely or never hear the M channel, even if I am directly behind a trooper who is transmitting. I can only presume that not all regions are repeater driven. This is not the case in most areas, however.
There is simply no way this is true, because that's not how the CHP radio infrastructure works. If the dispatcher hears the unit, they must be transmitting on the mobile frequency because that's what is monitored by the dispatcher. If you didn't hear them on the mobile, perhaps you weren't behind the right unit, or the mobile frequency was programmed incorrectly? Antenna also plays a huge role when monitoring CHP. If you don't have a low-band external scanner antenna on your vehicle (like the mag-mounts sold at radioshack for example), it's very easy to miss traffic on the mobile frequency even when the unit is very nearby. Stock rubber duck antennas are terrible for monitoring CHP, and barely receive the base frequency in many areas.

Last edited by inigo88; 12-09-2013 at 5:25 AM..
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Old 12-09-2013, 9:05 AM
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inigo88-to add your theory to my OP, how about taking a crack at MY original question with your expertise:
why was I hearing base and mobiles corresponding solely on the 42.440 frequency since like forever, then in the last couple weeks, it's base on 42.440, and now weak mobiles responding on 42.760?
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Old 12-09-2013, 12:00 PM
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Quote:
Originally Posted by ridgescan View Post
inigo88-to add your theory to my OP, how about taking a crack at MY original question with your expertise:
why was I hearing base and mobiles corresponding solely on the 42.440 frequency since like forever, then in the last couple weeks, it's base on 42.440, and now weak mobiles responding on 42.760?
It sounds to me like what you were hearing originally was the mobile re-transmit on the base channel, and you were within range of the transmitter site. The second instance the mobile re-transmit has been turned off, and you are only hearing the mobiles on their mobile frequency, and since (as explained above) they are lower power, you are not hearing them as well (or at all).

See my next post for a more detailed explanation of how the mobile re-transmit system works.
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Old 12-09-2013, 12:11 PM
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A bit of explanation on how the mobile re-transmit works:

The mobile repeat function is not routed through every transmitter site in each area. The mobile repeat in each area is routed through only select mountaintop transmitters to avoid simulcast problems.

For example: Here in my area (I-80 corridor Roseville to Truckee), CHP Green has eight transmitters on the Green channel scattered through El Dorado, Placer, and Sierra counties. The mobile audio is routed out through only one of these towers, Banner Mountain in Nevada City. This site wase chosen to give the widest coverage for the area without causing feedback problems common with simulcasting audio without synchronizing the patterns (I know I am not using the correct terminology, but the theory is sound). It is possible there is one more re-transmit site down in El Dorado County, but I'm not certain of it.

For the Truckee channel (Gray), they use two mobile audio re-transmit sites: Howell Hill in Colfax for the Dutch Flat units west of Donner Summit, and another site (unk which) in the Truckee area for those units east of the summit. Due to terrain, synchronization is not an issue.

This information came from one of our state radio techs, who maintains both our system and the CHP system.
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Old 12-09-2013, 2:39 PM
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Duster, thank you for your post.

For those who are seriously interested in CHP, I strongly suggest you read this.
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