Repeater Antenna Placement HELP!!!

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WQOC472

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Hello all, I am putting up a 50 Watt GMRS repeater. I was given some antenna and coax gear.

I have two seperate antennas i was given, Here is the link to them:

Tram/Browning BR-6155 [Tram/Browning BR-6155] - $79.95 : The Antenna Farm :: , Your Two Way Radio Source! antennas

I was also given 50ft of Andrew LDF-450A 1/2" Heilax, and 65 ft of LMR-400

How should i place these antennas? At the repeater location there are two seperate towers i can place my antennas on.

The first mast is 50ft tall. The second is 35ft tall they are placed about 25ft apart.
 

zz0468

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Vertical separation is much more effective than horizontal separation, so try to get them on the same tower, with one directly above the other.
 

kb2vxa

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Not much. Picture the radiation patterns as two donuts placed horizontally one above the other. As long as they don't converge is all that matters. One minor issue, the one mounted atop the tower will have an omnidirectional pattern while the one side mounted below it will have a cardioid pattern with a shadow toward the opposite side of the tower. For that reason I suggest the side mounted one be the receive antenna to avoid confusion to the users.
 
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WQOC472

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if i mount the TX antenna at 50ft and the RX at 40ft would that work? Which antenna should i use the LDF-450A on the TX or RX?
 

mmckenna

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I think you will find you need more separation than you'll be able to get with a 50' tower. If you don't supply enough vertical separation, you will experience desensitizing of your receiver which will affect the ability for your repeater to hear.

Isolation beteween the repeater receiver and RF sources

Using the above site you would need to look at how much separation you'd need to keep your receiver happy.

The other issue is that LMR-400 has been reported to have some issues when used in a repeater environment. Might work fine for RX use, and I've never experienced the issue myself. It would be worth some research.

What I would suggest doing is what a lot of repeaters do, use your single good antenna and single run of good 1/2 heliax and get a good duplexer. That would allow you to run one good antenna as high up on the tower as you can. You can find UHF duplexers on e-Bay all the time. You will just need to get it tuned for your repeater pair. You need the right gear to set them up, and unless you have that equipment and knowledge to set it up right, you'd be best served by letting a tech set it up for you.
 

zz0468

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The actual separation needed depends on several things:

Transmitter power output
Transmitter noise output
Transmitter feedline loss
Receiver sensitivity
Receiver IF bandwidth
Receiver feedline loss


We have none of that information to provide a number of how much isolation you're going to need. Mmkenna linked to a chart that shows an approximation of the isolation you can get. And I emphatically agree with his comments against LMR400.

In your case, take as much separation as you can get, or better yet, install one antenna at the top, and use a duplexer.
 
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zz0468

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I think you will find you need more separation than you'll be able to get with a 50' tower. If you don't supply enough vertical separation, you will experience desensitizing of your receiver which will affect the ability for your repeater to hear.
I agree, but there is always the option of reducing transmitter power output until desense becomes acceptable. Not the best way, but lacking test equipment and the requisite RF skills to fix a desense problem, it's a viable option.
 

kayn1n32008

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+1 for using a duplexer and the 1/2" haedline. Way easier, and less issues with desense. +1 for staying away from LMR400, it is ok if the cable does not move but you will have noise issues if it is used on a tower.
 

popnokick

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Duplexer is key. eBay has lots of 'Em. Get it professionally tuned, though. My repeater uses mobile Duplexer and 30 ft of LMR400. Been operating for 3 years w/o trubs.
 
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