About the CHP system, what did they say?

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RedPenguin

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I was looking at this page:

http://www.radioreference.com/apps/db/?aid=995

And saw this:

"Note:

This may answer why you cannot hear the mobile units (officers), but you can hear the base (dispatchers).

The common configuration for CHP Base & Mobile operation is duplex. What this means is the radio site does not act as a repeater; the radio site has a transmitter on the Base frequency and a separate receiver on the Mobile which are not linked locally. The audio from the Mobile receiver is sent over the DGS microwave network to an audio switch. The Base transmitter receives its audio from this same switch also. This is an advanced setup where the audio switch can be controlled to repeat the Mobile audio back to the Base transmitter by splitting the audio path off. Dispatchers have this ability to enable splitting the Mobile audio path and sending it back to the Base radio at the site. In these situations because of the round-trip time it takes for the audio to be switched there can be delays causing rough audio. Often this is why a normally Mobile repeated Base will cease retransmitting Mobile audio. Also, CHP Dispatchers have the ability to see which site the unit was received from. They can choose to just transmit at that site (e.g., what's common for the Blue) or through the whole network.

The audio switch not only has access to CHP but other agencies also. Since you're dealing with audio paths they can be combined and split in various manners."

I re-read it like 3 times, but it just sounds confusing. I mean I thought I knew my radio scanner stuff but this is odd. I think I understand it for the most part, that it's not a normal repeater system, and it uses microwaves, but what's the point of doing all this in the first place and not just using a normal repeater?
 

45-70-405

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Microwave is normal...

It is very normal in public safety systems to use microwave. The CHP uses a voting system and multiple transmit sites. Voting is when there are many receive sites, linked by leased lines, RF links, or microwave back to a comparator. The comparator listens to all the receive sites and chooses the one with the best signal. It can swap sites many times during a transmission. If the system is adjusted properly, the swaps will not be noticed by the listener. The audio from the comparator is routed to the dispatcher or back to a transmit site (or sites in a simulcast system) to be heard or rebroadcast. The CHP does not use portables on low band, but on system that do, it takes a lot more receiving sites to hear a 5 watt portable than 100 watt base stations to transmit to the portable.

With a simulcast system, multiple transmitters on the same frequency all key at the same time with the same audio. Today, GPS frequency standards are used for transmit frequency control and CTCSS generation, while T1 multiplexers with time delay are used for audio timing. These multiplexers work over digital microwave or leased lines (if you don't care about reliability).

Also, many stand alone repeaters are controlled wireline controlled by the dispatcher. This method of control, along what is mentioned above, allows the dispatcher to listen and transmit at the same time. Many times, this wireline control is carried over microwave circuits.

CHP can selectivly enable repeater function using their duplex bases. They are also working more linking to local angencies via microwave(patch the sheriff;s tac channel to a CHP channel) Hopefully they will use E/M signalling and not VOX to do this. I have not got my equipment from the State yet for my agency, so I don't know how it is intended to work.

JH
 

ScannerDude244

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Also if their isn't microwave at transmit site they use 70mhz links mountain top to mountain top. PS you can listen to them if you have one the newer scanners.
 

emt_531

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Why would a new scanner be capable of listening to them. I can only pick them up in about a mile wide radius and I have a Pro96...
 

rockettradio

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Keep in mind the real basics of the CHP low band system; below 54MHz, it's a real pain to have a repeater Tx and Rx at the same site, due to freq spacing, you get desensitivity issues. Hence the microwave/70MHz links in the first place.
 

trooperdude

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rockettradio said:
Keep in mind the real basics of the CHP low band system; below 54MHz, it's a real pain to have a repeater Tx and Rx at the same site, due to freq spacing, you get desensitivity issues. Hence the microwave/70MHz links in the first place.
Duplexers and circulators for low band are about 6 FT high and take up about 2 racks worth of space.

Low band repeaters are NOT fun.

:D
 

emt_531

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So how come I can only hear them really close....I am specifically trying to listen to the CHP Fresno, Hanford, Coalinga, traffic.....
 

WayneH

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emt_531 said:
So how come I can only hear them really close....I am specifically trying to listen to the CHP Fresno, Hanford, Coalinga, traffic.....
Because they aren't using a repeated setup. When you can hear mobiles on the base frequency it's because the dispatcher had decided to patch the audio through (just as the note says).

If you're talking about problems hearing the dispatcher you need a better antenna setup. Hearing lowband is not as simple as VHF-hi or UHF. Something as simple as electrical noise can attenuate your signal and make them difficult to receive. The right antenna makes a difference; its location too.
 
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trooperdude

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And in my area I have to deal with out of phase simulcast from two base transmitters.

Low Band is an art.
 

gmclam

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emt_531 said:
So how come I can only hear them really close....I am specifically trying to listen to the CHP Fresno, Hanford, Coalinga, traffic.....
Another time you will hear them is when they are talking "car to car". You will hear them both on the "base" channel, but even in that situation you'll need to be close.
 

jlanfn

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emt_531 said:
Why would a new scanner be capable of listening to them. I can only pick them up in about a mile wide radius and I have a Pro96...
I think he was referring to the assumption that only newer scanners have the ability to scan the 70 MHz frequency range. They don't necessarily have the ability to pick up the signals better.

I recall there was another thread discussing the 70 MHz links a while back.
 

emt_531

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So anyone have any reccomendations as to what Frequencies I need to have in to listen to the Fresno Area CHP traffic, Air included.
 

trooperdude

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emt_531 said:
So anyone have any reccomendations as to what Frequencies I need to have in to listen to the Fresno Area CHP traffic, Air included.
Check the Radio Reference California Highway Patrol page for Fresno ?
:D
 

emt_531

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Well I figured that was a good start, but I want all the Air Ops frequencies as well
 

SCPD

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I agree that the explanation you quoted is confusing. I've ridden in CHP cars, been inside their radio facilities, have a ham license, purchased my first scanner in 1969, and I have a hard time following it also.

If there are database administrators who are interested I would be willing to draft up another explanation and post it for comments.
 

gmclam

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Fresno CHP

emt_531 said:
So anyone have any reccomendations as to what Frequencies I need to have in to listen to the Fresno Area CHP traffic, Air included.
I have the whole state programmed, but this is what I listen to when in the Fresno area:
SILVER base 42.080 146.2
SILVER mobile 42.280 146.2
LIME11 base 45.020
LIME11 mobile 42.780
BLUE base 42.340
BLUE mobile 42.180
Extenders 154.905
Aircraft 122.875 AM
 

emt_531

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Thanks for those frequencies, if anyone has anything else to add to it, Id be willing to accept it too.
 

WayneH

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Exsmokey said:
If there are database administrators who are interested I would be willing to draft up another explanation and post it for comments.
Yes, please go ahead. It's originally mine and I wanted something that explained plus gave a few more details on the why but it appears to be slightly complicated for those less interested in communications infrastructure.
 

GrumpyGuard

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If you have both frequencies in your scanner and a tall enough antenna you will hear both sides of the conversation.
Fresno uses the following freq's
Silver Base 42.0800 Mobile 42.2800
Hanford
Pink Base 42.4400 Mobile 42.7600

The vehicles also use 154.905 mobile repeater.

I hear the mobile half all the time when they are patrolling Hyw. 168 in Clovis. If you have any more questions feel free to email me directly.
 

SCPD

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wayne_h said:
Yes, please go ahead. It's originally mine and I wanted something that explained plus gave a few more details on the why but it appears to be slightly complicated for those less interested in communications infrastructure.
OK, Wayne, try this on for size. I've written it so that the most often asked question is anwered in the first paragraph and if the reader is interested they can read more about it in subsequent paragraphs. I've tried to keep this as basic as I can, as it is usually the least experienced scanner hobbyists that ask this question.

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 it the selection among multiple sites is the "vote" of 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.
I would have written this sooner, but have been using all my free time to program a new PSR-600 mobile for a trip I'm taking in another week.
 
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