adding a duplexer to the receive side of a repeater only, can it be done?

[WRQK570]

I am doing some configurations to my repeater, and i would like the tx to do a full 25, and i have 2 comet antennas, one is a 6 dbi (for tx) and one is a 11 db gain(for rx and is set lower and 35 feet away from the tx antenna) 

if i add a duplexer, on the receive side to so i will not have as i call it "drown out" will it work right??

thanks for the answers in advance!

[WRTC928]

17 minutes ago, WRQK570 said:

I am doing some configurations to my repeater, and i would like the tx to do a full 25, and i have 2 comet antennas, one is a 6 dbi (for tx) and one is a 11 db gain(for rx and is set lower and 35 feet away from the tx antenna) 

if i add a duplexer, on the receive side to so i will not have as i call it "drown out" will it work right??

thanks for the answers in advance!

If I understand correctly, you don't want to use a duplexer into a single antenna (the usual arrangement) because there will be some signal lost in the duplexer? Have you tested or done calculations to see how much you would lose? 

[OffRoaderX]

6 minutes ago, WRTC928 said:

Have you tested or done calculations to see how much you would lose? 

I am pretty sure that if he knew how to do that he wouldn't be asking this question.

[WRQK570]

i have, and i am loosing about 30% so 5.5 watts, and its not bad, but my swr is high, at a 2.3 usable, and till i get a new duplexer, and not a China cheap flat pack.

[WRQK570]

14 minutes ago, OffRoaderX said:

I am pretty sure that if he knew how to do that he wouldn't be asking this question.

"Some People" huh sir?

[gortex2]

As I normally tell people "It depends". The only advantage is it should notch out the TX frequency from the receiver, but will only do this if tuned correctly. Without the prpoper test equipment its only try it and see what happens. In real LMR we use receive multicouplers that do what you are trying to do. Most have either window filters to filter only a certain block of channels or notch to block TX only. Most are pass and only pass the stuff we want (465-470) mhz for examble and all other frequencies are essentially blocked. Alot of TX sites have similar on the TX side but not all. My SAR UHF stuff has 3 repeates on a multicoupler, with 2 sets of window filters. All TX goes to individual antenna's, not because I needed them but the combiner was a bit more than we could spend. I have the luxery at this site to have them on seperate antenna;s.

[WRTC928]

If it's just a matter of power, you'd probably have better success adding an amplifier to your repeater. It's not as simple as putting an amplifier in line between the repeater and the antenna, but apparently it can be done.

Videos here and here

[MSnow]

On 1/11/2026 at 4:02 PM, WRQK570 said:

I am doing some configurations to my repeater, and i would like the tx to do a full 25, and i have 2 comet antennas, one is a 6 dbi (for tx) and one is a 11 db gain(for rx and is set lower and 35 feet away from the tx antenna) 

if i add a duplexer, on the receive side to so i will not have as i call it "drown out" will it work right??

thanks for the answers in advance!

I don't think adding a duplexer is going to give you what your trying to do. I think I understand your logic, but even the open unused receive port on the duplexer would still receive the TX signal from the other antenna. you either have to get the antenna's farther apart, or you can run one antenna on the duplexer, and that for sure solves your problems. As for the loss through a duplexer you can get some that have lower loss, I run a Sinclair Q3220E which factory spec says around .8db loss you can tune it and get it a bit better I think I got mine down to .6. Going with a duplexer you will have to get a good SA or VNA to tune it, and you'll want to tune it on site. The Rigol dsa815-tg is a common one due to it being somewhat affordable. Still not going to get full 25 watts if that's your repeaters output power because your getting loss in all the cables, adapters, splices, and lightning arrestors as well. Best bet would be to start with more power so that after all your loss you end up with 25watts at the antenna. Toronto Surplus is a good place to pick up duplexers that are used for a decent price FWIW. 

[gortex2]

If your going to do as asked you would terminate the TX port on the duplexer with a dummy load.