Wi State patrol...

[OpSec]

The repeaters rebroadcast the field units audio, and the tower site closest to the unit is used by the communications center...this is why you can usually hear all field traffic but not all dispatch traffic.

I often have the opposite problem; I hear the dispatch and not the mobile. But this clears up some questions I've been having. So this system is what is called a "polling" system then, correct?

It's called a voting system, and our particular system works like this:

Each tower has a base unit that transmits on the base freq (output) and receives the mobile radio freq (input). Each tower is also linked via microwave to the other towers in the region, the region HQ and in most cases to other other towers outside the region. In short, all the primary towers in the state are linked together via a full-time hot microwave path.

Within the region, when a trooper keys the mobile radio, usually more than one tower hears it. Each tower's signal is routed to a signal comparator (voter) to pick the best received signal. This signal is then sent to the repeater, which is rebroadcast for all cars to hear. Keep in mind this is a real-time 'fluid' process, so as the trooper is talking the voter is constantly picking the best signal from all towers that hear the transmission. You can hear the voting sometimes, as audio levels and signal quality changes as the guy talks.

As for the dispatch traffic, it is only transmitted on one tower at a time...the one closest to the trooper. Dispatch audio is not repeated. There is an option to simulcast from more than one tower at a time, but this is rarely used as it causes more interference than it's worth.

Also, because of the hot microwave link there is no delay for the transmit and receive audio.

I'm going to try to record some transmissions to show what I'm hearing. Tonight it's been weird. The unit transmissions (on 155.445) are picket fenced. (I think that's the right term.) But they are strong, like S5 or so. Now the base transmissions are weak, but not picket fenced. They're only about S2. What I don't understand is the wavering transmissions if what I am hearing is repeated. (And it's not just a wavering sound... the S-meter fluctuates as well.) I've never heard a repeated signal that was like this.

The band is open tonight, and there is tropo ducting going on. As the ducting changes, the signal strength changes as you've noticed. It's got nothing to due with the repeater 8)

 

[djeplett]

You can hear the voting sometimes, as audio levels and signal quality changes as the guy talks.

Hmmm. Do you suppose the system is sometimes rapidly changing from one tower to another while it's transmitting? It would have to be a rapid change though...

The more I think about it the more I'm wondering if it's some 60Hz harmonic being introduced somehow in the tower that I'm hearing. It never fluctuates slower or faster... it's always at the same rate.

The band is open tonight, and there is tropo ducting going on. As the ducting changes, the signal strength changes as you've noticed. It's got nothing to due with the repeater 8)

It's hard to explain what it's like unless you can hear and see the reaction on my S-meter. Let me work on finding a way to record it. You've got to see it.

I've been picking up tropo a lot in the last couple weeks. I routinely hear county dispatches that are several counties away. It's been forcing me to use a PL instead of carrier squelch sometimes because I want to follow the action near me. But it has helped me find a couple new frequencies that weren't listed here on RR. (They were duly reported, of course. :wink: )

 

[OpSec]

You can hear the voting sometimes, as audio levels and signal quality changes as the guy talks.

Hmmm. Do you suppose the system is sometimes rapidly changing from one tower to another while it's transmitting? It would have to be a rapid change though...

That's what happens....as the S/N ratio changes, the voter changes which which audio path it uses. As I said, it happens the entire time the signal is being received...if someone is being a chatty chicken and babbling for 20 or 30 seconds, it's usually pretty noticable (if you are used to hearing it).

The tower that is the repeater transmitter never changes...it's the receive audio path thru the voter that changes. The repeater site stays the same...it just repeats whatever audio the voter sends it - good or bad.