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GMRS repeater help

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NathanPerr

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Apr 26, 2020
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Hello
I am trying to set up a GMRS repeater and do not own a duplexer. I was wondering what the spacing I would need between my tx and rx antennas? If anyone could help that would be great.
Thanks
 

RadioGuy7268

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Long answer short (and in reality, it will depend on many factors such as power, antenna type, and receiver selectivity) you need at least 15 feet between the antennas stacked vertically - and more separation is usually better. It's also better if you can place a bandpass cavity on the receiver. If you can measure any loss in receive sensitivity when the transmitter is keyed - you don't have enough separation.

When you calculate the costs of good antennas and quality coaxial cable - running 2 antennas is rarely the cheaper option. Duplexers are popular because they work in the vast majority of situations.
 

prcguy

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Some radios like an actual high end repeater can work with 15ft vertical separation between antennas. Other radios cannot and may never work full duplex with the antennas within a few blocks of each other. It would help to know what kind of repeater or radios the OP is going to use.

Long answer short (and in reality, it will depend on many factors such as power, antenna type, and receiver selectivity) you need at least 15 feet between the antennas stacked vertically - and more separation is usually better. It's also better if you can place a bandpass cavity on the receiver. If you can measure any loss in receive sensitivity when the transmitter is keyed - you don't have enough separation.

When you calculate the costs of good antennas and quality coaxial cable - running 2 antennas is rarely the cheaper option. Duplexers are popular because they work in the vast majority of situations.
 

n5ims

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Joined
Jul 25, 2004
Messages
3,937
And stay away from those cheap mobile duplexers that are available many places, especially ebay. While you might get lucky and find one that actually works OK, most won't work much better than the 15' vertical separation you've been warned about. Those mobile ones are designed for very low power transmitters and very good quality receivers. For a real repeater, you need a real duplexer and a quality repeater that's designed for repeater use. Just throwing a couple of radios together with a cable or even a controller will be cheap and sound (and work) like some cheap junk that was thrown together to do what it wasn't designed for.
 

vagrant

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I purchased one of those UHF mobile duplexers on eBay and it works. I typically do not put more than 20 watts into it, but I have tested with 50 watts and it worked fine. I tested 20 and 50 from the same location and relatively same time as I can remotely control the power output of the repeater. It can work fine for mobile use or in an environment that is not inundated with RF, as they are just notch filters.

As to LMR-400 vs 1/2 inch Heliax, there is indeed a noticeable difference. I experienced it when going from one to the other. Additionally, I saw an improvement when swapping out a dual band antenna to a single band UHF antenna I cut and tuned.
 
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