Conventional repeater systems

[vs1988]

My town PD uses a conventional repeaterized radio system. I understand the basics of repeater systems, but I have a few questions.

What are voters and reciever sites? Are these up on towers around the town?

I've heard radio tests in which the dispatcher switched to different sites. Is this done on the computer or by flipping a switch in the dispatch center?

Are the dispatch consoles connected to the repeater via a land "line" or does the base transmit to the tower which repeats the transmission?

During a thunderstorm a few months back the radio system was having some problems. The mobile units could not hear each other so dispatch had to relay whatever one unit was trying to say to another. Why/how would this happen on a repeaterized system?

Thanks a lot.

Vince

 

[biglaz]

Phew...bunch of questions I'm no expert but let's see here...

There are no "land lines" from the towers to the comm center; there would be a "link" frequency between each tower and the comm center. Each tower would have it's own link frequency. Each tower receives the same frequencies from the mobile units, so the tower close to a moble unit would receive a good signal but the tower 40mi away would have a much worse signal. If you mixed a good signal with a bad signal you wouldn't be able to understand much of anything. This is what a voter is for; only the "best" signal is sent to the console and also transmitted out the repeater.

So basically every tower will listen on the same frequency (the repeater input freq), but they all retransmit on a different link frequency back to the repeater site. A voter accepts the signals from all the link frequencies and sends out only the "best" signal to the repeater. Though there are often other steps between the voter and the repeater/transmitter such as equalizers, audio compressors, etc. And if the repeater wasn't at the transmitting tower, then there would be another link frequency to get it there.

There's no simple answer to the thunderstorm. Maybe a tower was struck. Maybe a dispatcher turned the repeater off by mistake

Edit: The links between towers can be wire links, but more often than not they're either UHF, 900MHz, or microwave signals.

 

[N4DES]

The information on pages 7 & 8 should answer a lot of your questions:

http://www.repeater-builder.com/ge/lbi-library/lbi-39149.pdf#search='voter%20%20radio%20%20system'

 

[nd5y]

It certainly IS possible to use land lines to link radio systems together. It is done all the time.

 

[SLWilson]

Satellite RX sites

What are voters and reciever sites? Are these up on towers around the town?

Are the dispatch consoles connected to the repeater via a land "line" or does the base transmit to the tower which repeats the transmission?

Thanks a lot.
Vince

Hi Vince, in our area, the sheriff uses a single transmit site for their repeater. They have FOUR receiver sites throughout the county. Yes, we use various "locations" to hang the antennas on. Not ALWAYS a tower, one is a rural water tank that is 90 feet tall. They use ma bell to connect the RX sites to the repeater. (yes, there IS a monthly recurring phone bill cost). The dispatcher doesn't have any way of "testing" any certain site. A radio tech has to do that at the repeater site....It works well with this setup. Steve/KB8FAR

 

[Al42]

Phew...bunch of questions I'm no expert but let's see here...

There are no "land lines" from the towers to the comm center

Land-line control is very common - both DC and tone. I'd guess that there are still some "ghost" control lines being used.

there would be a "link" frequency between each tower and the comm center.

Which could simply be (and very often is) the normal input frequency. IOW, the dispatch radio is the same as a mobile radio, but with a desk mic and an AC power supply.

Each tower receives the same frequencies from the mobile units, so the tower close to a moble unit would receive a good signal but the tower 40mi away would have a much worse signal. If you mixed a good signal with a bad signal you wouldn't be able to understand much of anything.

Since there's an effect in FM receivers known as "capture", you'd never know that there was a weak signal on the frequency - as long as one signal was more than the capture ratio above the other signal (in strength - and how much that is depends on the receiver), you'd only hear the stronger signal.

Which is one reason aircraft communication is, and will probably always be, AM, not FM. If a plane has an emergency and goes down, even if there was other traffic on the frequency that was stronger, the weaker signal was there, and audio filtering techniques can isolate that weak signal, so it may be possible to figure out what happened.

This is what a voter is for; only the "best" signal is sent to the console and also transmitted out the repeater.

Sent out the repeater yes, but the console position may have access to all the voting receiver outputs. If no signal is clean enough to be repeated the dispatcher may still be able to understand what's being said. The brain is pretty good at integrating a few bad signals and coming up with an intellgiible transmission.

So basically every tower will listen on the same frequency (the repeater input freq), but they all retransmit on a different link frequency back to the repeater site.

Or send the audio back to the dispatch point by wire line, from where it's sent to the repeater.

And if the repeater wasn't at the transmitting tower, then there would be another link frequency to get it there.

The "repeater" is a receiver (or more than one), a transmitter and control circuitry. The "repeater site" is usually considered the transmitter site. It's a semantic thing, but people don't usually refer to one of the receiver sites, or the dispatch point, as "the repeater site". So having the transmitter and "the repeater" at different locations is a bit like holding your left elbow in your left hand.

There's no simple answer to the thunderstorm. Maybe a tower was struck. Maybe a dispatcher turned the repeater off by mistake

Maybe the signal was so noisy (FM receivers can be affected by static - just not as badly as AM receivers) that, even though the dispatcher could understand most transmissions, people in the field couldn't understand most of them.

Edit: The links between towers can be wire links, but more often than not they're either UHF, 900MHz, or microwave signals.

You can't say that without doing a pretty extensive survey. In my experience, most links are wire, but I'm not expanding that to say most links are wired, just that most commercial links I've had anything to do with have been by wire, both dispatch to transmitter and voting receiver to voting controller. (Ham radio uses RF links much more than wire links, but we usually speak about PS systems here.)

 

[biglaz]

Good to know, thanks. I wasn't trying to say there could not be wire links, though I guess it looked that way. The few I have seen, however, were not wire links.

 

[kb2vxa]

Hi VS and all,

I see some confusion so here's hoping I can untangle it with some clear, concice answeres.

"What are voters and reciever sites? Are these up on towers around the town?"

In a small area such as a city or town the remote receiver "satellites" are usually connected to the voter at the repeater site via class A telephone lines or the Internet. That means they run direct for signalling purposes rather than going through the central switch (exchange) while multiplexed digital audio and control can be accomplished via the Internet. For a small town it's telco lines, Internet linking is cost prohibitive. The voter selects the receiver with the strongest signal and feeds it into the controller. The sats aren't necissarily on towers, they can be anywhere and usually can't be so identified.

FIY, radio links are used to connect widely spaced repeaters in a network since Class As are impractical. An alternate method is Internet linking for the same reason, only a local full time dedicated telephone connection is required.

"I've heard radio tests in which the dispatcher switched to different sites. Is this done on the computer or by flipping a switch in the dispatch center?"

That depends on how the dispacher communicates with the repeater, land line or radio. If by land line it goes into the voter at each site just like a sat receiver and site selectable on the console. If by radio it's by switching PL tones at the console. Usually a computer is the heart of the system.

"Are the dispatch consoles connected to the repeater via a land "line" or does the base transmit to the tower which repeats the transmission?"

It can go either way but if the tower is located at the dispatch point it's most assuredly by local building wiring, land line. This is a combined base/repeater of the sort I used to operate, the repeater can be shut off to kill idle chatter the dispatcher may find annoying or distracting.

"During a thunderstorm a few months back the radio system was having some problems. The mobile units could not hear each other so dispatch had to relay whatever one unit was trying to say to another. Why/how would this happen on a repeaterized system?"

The repeater function was either disabled by lightning or the dispatcher's accidentally pressing the wrong button. Lightning could have easily taken out the controller leaving the rest of the system intact.

For what it's worth, the correct term is simply "repeater", not "repeaterized".

 

[pro961]

repeater

Vince,
What town are you talking about and what service. Police, Fire?

I run a dispatch center in CT.
If it's my town I could give you specific info.

 

[garys]

In large services conventional radio systems are very complex. The service I'm familiar with uses mostly leased phone lines, but they all go through Central Offices. There are both two wire and four wire phone lines, but the phone company is trying to phase out four wire lines. Even "land line" phones go over microwave links in many cases.

Our dispatch consoles do not go to receiver sites, they go directly to the transmitters. It would make no sense to run transmit audio to a receiver site. We run 10 or so receivers for each channel. We don't operate repeaters in the ham radio sense. Even the reciever located at the transmitter site routes it's signal back to the voter/comparator.

Since the reciever sites are listening for mobiles on the input frequency and the mobiles are scattered all over the city, the receiver with the best signal is the one that is repeated. Interestingly, that is not always the site closest to the mobile. For purposes of this discussion, mobile includes portable radios.

Selecting bases can be done by hard wire switch, but again on big enough systems, the console is computer controlled, so the effect is the same.

If you read all of the messages on this thread, you'll see that there are a number of ways to repeat a signal. Which one is used depends on the size and complexity of the system, where it's located, what resources are available, and so on.

Gary

 

[vs1988]

Hey all, thanks for the responses. I went to the old Fordyce site (cache on google) and found two 900 mhz freqs. listed as microwave links. Do you think this is for the reciever sites or the MDT network?

Vince,

I run a dispatch center in CT.
If it's my town I could give you specific info.

Hey pro961,

what dispatch center do you work for?

Vince

 

[kb2vxa]

Hi again,

"The service I'm familiar with uses mostly leased phone lines, but they all go through Central Offices."

Let's get our terms straight here. Class A lines are connected directly at "frame" bypassing the "CO switch" being they're unswitched permanent connections point A to point B. Sure, everything goes to the central office, that's where frame is but let's not confuse people letting them think the connections are handled like ordinary phone calls. FYI for the non phone phreaks, the "central office" more often than not is unmanned, hardly an office in the usual sense of the word.

Of course telco is phasing out 4 wire lines, the second pair used for DC signalling isn't needed when the more modern DTMF control has been around for years. Frankly I'm surprised to learn the last wasn't pulled decades ago. In a pinch an engineer can manually control the site from any ordinary telephone by pressing the right keys. I know how to control the transmitter of a local radio station but I won't tell YOU. (;->)

"I went to the old Fordyce site (cache on google) and found two 900 mhz freqs. listed as microwave links. Do you think this is for the reciever sites or the MDT network?"

MDTs operate in the same frequency range as the standard voice comms usually, only they use a separate radio with a terminal node controller (TNC) and may even share a common antenna. They may be distinguished by the back and forth "braaaap braaaap" of packets being exchanged when communicating which is brief and infrequent. 900MHz is a garbage band shared by many services from radio station studio to transmitter links (STLs) to the rare Amateur Radio operator venturing into "CB Land".

BTW, MDTs always work flawlessly on COPS but seldom in the real world, or at least the local gendarmarie complains about them often enough. (;->)

 

[Al42]

Of course telco is phasing out 4 wire lines, the second pair used for DC signalling isn't needed when the more modern DTMF control has been around for years. Frankly I'm surprised to learn the last wasn't pulled decades ago. In a pinch an engineer can manually control the site from any ordinary telephone by pressing the right keys.

There are still systems using DC control, connected by 76 (or 96) PLs (private lines)

 

[garys]

Hi again,

Let's get our terms straight here. Class A lines are connected directly at "frame" bypassing the "CO switch" being they're unswitched permanent connections point A to point B. Sure, everything goes to the central office, that's where frame is but let's not confuse people letting them think the connections are handled like ordinary phone calls. FYI for the non phone phreaks, the "central office" more often than not is unmanned, hardly an office in the usual sense of the word.

Good points, all. There was a time when COs were manned, but those days are gone.

Of course telco is phasing out 4 wire lines, the second pair used for DC signalling isn't needed when the more modern DTMF control has been around for years. Frankly I'm surprised to learn the last wasn't pulled decades ago. In a pinch an engineer can manually control the site from any ordinary telephone by pressing the right keys. I know how to control the transmitter of a local radio station but I won't tell YOU. (;->)

LOL! Our 4 lines were installed in the mid 1970s, and I think were actually to allow for full duplex audio. DC control was phased out in this system before my time, which was 1978. During my initial training we had a lot of education on keying tones, channel select tones, and so on. Even though I was only going to be a dispatcher, they wanted me to know that stuff. I haven't been in that end of the business for years, so I'm not all that current on it. All of our consoles are whatever the current version of Centracom is, some have CRT displays, but most don't.

MDTs operate in the same frequency range as the standard voice comms usually, only they use a separate radio with a terminal node controller (TNC) and may even share a common antenna. They may be distinguished by the back and forth "braaaap braaaap" of packets being exchanged when communicating which is brief and infrequent. 900MHz is a garbage band shared by many services from radio station studio to transmitter links (STLs) to the rare Amateur Radio operator venturing into "CB Land".

I think that might be highly variable. We use UHF for our voice radios, but 800 for data and AVL (two separate radios). Well, supervisors and above have 800 trunk radios, but no AVL, but still operate almost exclusively on UHF. Motorola VRM 650s or 800s, for data, in case you're that interested.

BTW, MDTs always work flawlessly on COPS but seldom in the real world, or at least the local gendarmarie complains about them often enough. (;->)

In our case, the system works very well, it's individual computers that seem to have problems. The MW-520s are rock solid, but the Datalux units seem to have had issues. Too early to tell about the ML800s(?).

Gary

 

[tfr]

BTW, MDTs always work flawlessly on COPS but seldom in the real world, or at least the local gendarmarie complains about them often enough. (;->)

The MDTs here seem to be working just fine. These things are quite a marvel on their own:

But I suppose that's a topic for a thread of its own.

 

[biglaz]

So what happens when the airbag deploys?

 

[hcsd35-32]

So what happens when the airbag deploys?

Don't know about other departments, but when the department I work for installed MDT's, they also installed switches that can disable the air bag.

 

[kb2vxa]

Hi Gary and readers,

Methinks you're a bit confused. I wrote about 900MHz and you came back with 800MHz. (;->)

As for the MDTs, I was being sarcastic. Like Mae West, when they're good they're very good, when they're bad they're better. Yeah, more sarcasm based on what I hear on the scanner. Then there's this dispatcher that follows every 10 code with plain language, makes you wonder. It's either that this is one of the lowest crime areas in the country or something's in the water, tastes funny too. I'm having mental images of Barney searching his pockets for the bullet. (;->)

I better stop before I describe alternate uses for the air bags.

 

[tfr]

So what happens when the airbag deploys?

The passenger side air bags are disabled.

 

[wYtEbOyFoLyPh]

That is kind of funny; it's like a wannabe police car. I tell ya, I'd be embarrassed to drive that around. I'd be afraid someone'd see me.