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UHF Antenna Separation

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BlueDevil

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I am in the process of building a UHF Repeater. For this specific repeater I am not going to be using a Duplexer.

How much (or how little) horizontal separation can I get away with while limiting desense of the receiver? Obviously there are some variables to consider. Lets say the transmitter is running 30watts with a 3db gain omni directional antenna.
 

jonwienke

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Get a duplexer, even if you have to go with one of the cheap Chinese ones. If you separate the antennas enough to eliminate desense, the coax run will eat all the signal. You'd have to have the antennas several hundred feet apart to be effective.
 

kayn1n32008

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I am in the process of building a UHF Repeater. For this specific repeater I am not going to be using a Duplexer.

How much (or how little) horizontal separation can I get away with while limiting desense of the receiver? Obviously there are some variables to consider. Lets say the transmitter is running 30watts with a 3db gain omni directional antenna.

It is significantly easier to use vertical separation for in-band repeaters. Is there a particular reason that you do not want to use a duplexer?

As jonwienke says, it will take significant horizontal separation to get a 2 antenna set-up to work properly.
 

RFI-EMI-GUY

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As said before, horizontal yields little isolation, however vertical seperation impacts coverage.

Invest in a decent duplexer.



Sent from my SM-T350 using Tapatalk
 

prcguy

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At the minimum you probably need at least 60dB isolation and maybe more for reliable desense free operation. Its impossible to get that with antenna separation unless you can optimize height and patterns and find a freakish null while measuring antenna isolation with a spectrum analyzer. You would certainly have to supplement the antenna isolation with a transmit and receive band pass filter cavity.

A cheap mobile notch only duplexer will get you more than 60dB isolation and cost less than a pair of band pass cavities. Are there any other repeaters or UHF radios nearby? If so you would need band pass filtering or a Bp Br duplexer. If you are on the same tower as other repeaters you would also need an isolator.

Using a 3dB gain antenna on a repeater is a waste of a repeater unless you only need to cover a very small area. The antenna height along with the antenna gain is responsible for most of the repeater coverage.
prcguy
 

Project25_MASTR

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At the minimum you probably need at least 60dB isolation and maybe more for reliable desense free operation. Its impossible to get that with antenna separation unless you can optimize height and patterns and find a freakish null while measuring antenna isolation with a spectrum analyzer. You would certainly have to supplement the antenna isolation with a transmit and receive band pass filter cavity.

A cheap mobile notch only duplexer will get you more than 60dB isolation and cost less than a pair of band pass cavities. Are there any other repeaters or UHF radios nearby? If so you would need band pass filtering or a Bp Br duplexer. If you are on the same tower as other repeaters you would also need an isolator.

Using a 3dB gain antenna on a repeater is a waste of a repeater unless you only need to cover a very small area. The antenna height along with the antenna gain is responsible for most of the repeater coverage.
prcguy

I think I calculated around 64 dB of isolation would be needed (assuming the receiver has 90 dB of "adjacent" channel rejection 5 MHz away, -120 dBm receive sensitivity and 20W ERP). Still was looking at something like 84 meters of horizontal separation needed to make that up.
 

lmrtek

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The more the better but 30 feet of vertical would likely be enough.
.........
I was able to run a 75 watt repeater with -124db sensitivity with 50 feet between the station masters
.........
It would take 20 times as much horizontal separation to get the same results
 
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BlueDevil

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Brandon, Get a REAL duplexer and antenna. Don't fool around like last time.

This is kind of a unique system I am considering putting together. I am just trying to explore all the options before I dive into the project.

Thanks for everyone's input and help. I will try and find a way to make it work with a duplexer.
 
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