Duplexers are an important part of building a workable single band repeater. They are a set of filters in line that will let you have two transceivers on two different frequencies on the same antenna without overloading the other frontend. In order that the transmitter signal does not enter the receiver and overload the input and desensitise it, the signal from the repeater passes into a duplexer.
Since we will be running on amateur 440mhz fequencies, we can get one of the cheap celwave mobile duplexers from Ebay. I found one that came pre-tuned for any frequency that I wanted, so I set it up in the portable repeater range of 70cm.
I bought one, and it came in pretty quick. It had N Connectors for its LOW, ANT, and HIGH ports. Getting short pigtails to fit with the radios was pretty easy. I took it to a local radio club’s repeater specialist, and he checked the tuning for me. He even put it in an oven to heat it up then checked the tuning to make sure of the quality of everything. It passed with flying colors.
So now, we just have to plug the receiver into the LOW port, the transmitter into the HIGH port, and the antenna into the ANT port. We can power everything on and test it with a low power HT. When you key up the receiver frequency on your handheld radio, you shoud see the transmitter radio key up and transmit.
We tried to use this setup in California for Field Day, however, the receiver radio ended up dying on the trip out there. It will be rebuilt.