GMRS Repeater Build

I’m new to this site and new to the FRS/GMRS world. I do have a lot of experience with radios from my time in the military and as a first responder, but only to a certain level.
I’ve received my GMRS license and I’m looking for information on how to build a repeater.
I have 2 CDM1250 radios that I plan to use for the repeater. I’ve scoured the interwebs looking for information on how to use them. I’ve come up empty so I thought I’d see if anyone has an idea as to how.
Any and all help is appreciated!!


There is a great site called repeater builder. The thing about repeaters is performance. Hams have had cross band UHF to VHF repeaters that are really simple for years. You pick two frequencies that are not harmonically related so interference is minimised connect the audio out to audio in, a little bit of switching, usually a line that goes high when one radio’s squelch opens that switches the other unit’s transmit on. If you (for maths example only) transmitted on 150MHz and received on 450MHz then the third harmonic of 150 would wreck it as the vhf radio would have enough output at 450 to make the receiver think a valid signal was coming in!

In band repeaters, on licenced frequencies means filters. Multi cavity devices that let through one frequency but block everything else, while at the same time allowing the other frequency to come in but blocking the one going out. These usually allow a single antenna but others require two antennas. They are needed because of a phenomena called desensitisation. If a receiver can hear somebody a fair way away on frequency 1, as soon as the transmitter on the repeater fires up it partially blocks that distant station, making the system deaf. The filters try to reduce this. The closer the receive and transmit are to each other, in frequency the better filters have to be. They need tuning and that is not simple unless you have an analyser with a tracking generator. If you have access to the kit, they’re not hard to tune. Some folk try it with less complex gear and it’s possible with skill and experience. I found an old repeater in my store I tuned years ago, and put the cavity on test. It’s performance was pretty poor. It was never very good on range, put down to location, but the problem was receive loss. The filtering was actually ok but the receive losses were quite bad.

That’s a quick trip to get you started.

Thanks for all the info!! It seems i may have bit off more than I can chew lol
I’m super new to radios as far as programming them and setting this build up. I’ll take a look at that website and hopefully it’ll be easier than I’m expecting.
Thanks again!!


Repeaters can be very simple - in general, the most important requirement is the frequency split. My repeater here in the UK runs 453.3375MHz transmit and 459.8375MHz receive. 6.5MHz split - which is manageable on the less efficient smaller filters on one antenna. It looks like 5MHz split for the GMRS repeaters via Google - which is tighter but with care can be done - although I’d probably have two antennas and two filters for this for maximum performance.

I’ll have to have hire someone to help me then. I have zero programming software and experience. I’m pretty sure our ham guys in the area can partake in my fun lol
The only items i have for a list of needed parts is a duplexer, 2 antennas, 2 filters, and that’s it, plus my 2 cdm1250’s thanks to you!
BTW, i’ve got a friend in northamptonshire. He works for Her Majesty’s Mail Service i think it’s called. I’m trying to plan a vacation/holiday to go visit hopefully soon.

Royal Mail. Check the duplexer. Two antenna duplexers are less common. Tuning duplexers or any filter is not an everyday ham thing. There are some YouTube videos and you can see what you need to do. Maybe you could borrow the equipment? The process is not actually difficult. Tools wise a couple of spanner’s/wrenches (US?) and a screwdriver, and an analyser with tracking generator.

Thanks for the additional!!
I’ll look at anything i can to learn.
Yes, wrench is correct and actually spanner is too. In the fire service, we use spanners/ spanner wrenches to tighten or loosen a hose connection from one hose to another or from hose to nozzle or from hose to pump etc…