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Old 05-17-2011, 3:03 AM
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Default Questions about Radio Systems

1. for a small city about 6 square miles? Would one repeater work for the whole city if it was TXing at 100W 300 feet up? With all flat land.
2. How does a conventional radio system work with many repeaters? Does the signal go to the closest repeater and then it's transmitted back to the main repeater then back to the other radios or?
3. How does a five watt radio reach the repeater? Does the repeater put out a signal that reaches out to the radio?
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Old 05-17-2011, 10:35 AM
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Location: NJ USA (Republic of NJ)
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You don't mention which band, vhf or uhf??

In general I will try and answer your questions below:

1. for a small city about 6 square miles? Would one repeater work for the whole city if it was TXing at 100W 300 feet up? With all flat land.

That would probably work too well. You could cover a city that small with flat terrain with an
good quality antenna much lower on a tower or tall building, probably at about the 100 to 150
foot level. It would also probably cover outside the city area as well but how far depends a lot
on the surrounding terrain and band(vhf/uhf) used along with many other factors.

2. How does a conventional radio system work with many repeaters? Does the signal go to the closest repeater and then it's transmitted back to the main repeater then back to the other radios or?

There are no smarts in a conventional repeater system unless you build it into the system
such as voting receivers. Hard to answer your question since you don't have any parameters
but it can get complicated but as it does so does the cost rise.

3. How does a five watt radio reach the repeater? Does the repeater put out a signal that reaches out to the radio?

The repeater site usually has a tower or tall building. On top or close to the top would be a good
quality repeater grade antenna. The antenna would be fed with a qood quality commercial grade
hard line with low loss and durable to time and weather considerations. The antenna at that
height would have a line of sight to the immediate city area and most portables and mobiles
rf signals would have no problem reaching the antenna at more than adaquate strength. The
further away they get the lower the signal strength into the repeater receiver. The receiver is
connected to the repeater transmitter via a control unit of some type and the audio is transmitted
at a much higher power and distributed back to the antenna. The transmit and receive happens
at the same time due to a duplexer allowing use of the receiver and transmitter at the same
time. The repeater duplexer keeps the transmitter from interfering with the receiver and makes
this possible. There are also lightning protection add ons built into the hard line system that
does its best to protect the repeater equipment when storms pass through.

Thats the simple scenario but it can get much more complicated the more bells & whistles you add into
your want list.

EDIT: Forgot to mention that the transmit and receive also happens on seperate frequencies.
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Old 05-18-2011, 12:36 AM
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Location: California
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Quote:
Originally Posted by sepura View Post
1. for a small city about 6 square miles? Would one repeater work for the whole city if it was TXing at 100W 300 feet up? With all flat land.
Probably, but there are a lot of variable you've left out. Frequency? Density of the buildings, expected grade of service, etc. RF propagation is statistical in nature, and coverage will be less than 100%. How much less, we don't enough to make anything more than a guess.

Quote:
Originally Posted by sepura View Post
2. How does a conventional radio system work with many repeaters? Does the signal go to the closest repeater and then it's transmitted back to the main repeater then back to the other radios or?
The best (and most expensive) solution is to simulcast the repeater transmitters, and vote the receivers. The voting comparator makes the selection on which receiver is used, based on signal to noise ratio.

A cheaper alternative to simulcasting the transmitters would be transmitter steering, which can be rather inelegant. Another viable method is to use a single high power repeater transmitter, and multiple receivers to provide portable talk-in coverage. Of all the methods, this probably gives you the most bang for the buck.

Quote:
Originally Posted by sepura View Post
3. How does a five watt radio reach the repeater?
That's a pretty vague question. I'm not really sure what it is you're asking.

Quote:
Originally Posted by sepura View Post
Does the repeater put out a signal that reaches out to the radio?
That's the general idea, yes.
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Old 05-22-2011, 8:56 AM
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Quote:
Originally Posted by sepura View Post
1. for a small city about 6 square miles? Would one repeater work for the whole city if it was TXing at 100W 300 feet up? With all flat land.
2. How does a conventional radio system work with many repeaters? Does the signal go to the closest repeater and then it's transmitted back to the main repeater then back to the other radios or?
3. How does a five watt radio reach the repeater? Does the repeater put out a signal that reaches out to the radio?
That's about the dimension of my home town in VA where I was a police/fire dispatcher and a volunteer firefighterback in the 70's-80's. The PD used a UHF system, with the repeater on top of a water tank in the middle of town. Dispatch from HQ was covered by runnung a tranceiver from the board and simply pointing a yagi to the repeater. (the boad had panels for PD1, PD2, FD, and the statewide lo band freq.) The UHF system worked great within town, even on the HT220 HTs.

The FD used a VHF system with the repeater at HQ and the antenna atop a 100' tower. The footprint of this system went well beyond the city, with mobiles able to hit the repeater as much as 20 miles or more out.

What was interesting was that on PD1 - the repeater freq for the police, HQ was like just another mobile unit....the repeater itself was across town. The FD repeater, on the other hand, was at HQ, so i was operating the through the repeater itself. Perhaps this was the optimal set up since we used that freq for the FD alert tones and paging, the tone board fed right into the controller. The downside is that I only heard what mobiles transmitted on FD1 - the repeater channel, because if they went to FD2 - simplex on the mobile RX freq, I could not hear them at HQ. (The undercover cops sometimes used FD2 as a tactical freq, as their conversations were not captured on tape at HQ, lol.)

My ham club's repeater system where I live now in NY is VHF (2m) with one transmitter - antenna on top of a 300'+ tower, and about 5 receive sites around the county. The repeater uses a voting system to determine what signal is selected. The county PD uses a similar system on VHF and there are known dead spots in the county, but this is way more than 6 miles square.

The problem with VHF - in the past and now, is problems with coordination and interference from band openings. Back in the day, my FD in VA often had Camden, NJ booming in. The PD of the county where I live now in NY has had a long running interference issue with a neighbor in NJ. I haven't experienced this on UHF.

If you are the guy in charge of designing a system for your town, you probably need to work with someone who can model your communications requirements (e.g., departments, talk groups, security/encryption, specs like citywide portable reliability, etc.) and topography against various system variables, like band, power, receivers, transmitting antenna height, etc., etc., etc.....not mention your budget!

Sounds like a fun project...enjoy!!
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