Jim Brown wrote: > I have found that the main problem in using a talkie > as the receiver for tuning the notch in a duplexer is > the possible leakage of RF between the signal source > directly into the talkie. Most talkies are not > shielded at all, and any leakage will cause you to > tune a combination of the signal through the duplexer > and the direct signal leaking into the talkie.
Yes, I had that problem as well. When I built my last 2 meter repeater (a semi-homebrew using custom re-packaged GE modules, mostly) I crystalled the receiver for both the repeater receive and transmit frequencies with a service switch to select the second frequency. I also built a simple op amp DC amplifier sampling second limiter voltage at a metering point in the receiver and use it to drive a signal strength meter on the repeater front panel. Of course my custom chassis is very well shielded. The receiver is peaked for the repeater input frequency, so it isn't as sensitive on the other frequency; but it is good enough to see the depth of my duplexer notches. I use an old (cheap!) Boonton 202E generator as a signal source, and put 3 dB pads on the duplexer ports (and a 50 ohm termination on whatever port is not used at any specific point in the tune up process). The Boonton is surprisingly stable once it has warmed up for a couple of hours. I've had good luck with this simple setup. But adding the signal strength meter to many commercial repeaters (or worse yet mobiles converted to repeater) could be more of a challenge. Paul N1BUG
