Several months ago I put a six meter machine on the air in my area. It is a GE Master Pro tuned for 52.810 out and 51.110 in. One of the things still nagging me is some sort of desense or RF phase noise, let me explain. After tuning up the duplexers into a dummy load and running some tests I experienced no desense all the way down to about .15uV. I moved the dummy load to the end of the transmission line just to be sure and again the same results. When I put the antenna in line and run the same tests this is what occurs. When I key the transmitter and set the output of the signal generator from a starting point of say 100 uV I hear what sounds like phase noise or just plain static just slightly in the background. As I bring the signal generator output down the background noise gets louder but it never wipes out or overloads the receiver altogether as I can still hear the generator and the background noise and this is down to the same squelch threshold I get when on the dummy load. I am hesitant to call this desense as say when one of the duplexer cavities isn't tuned correctly. Then it is obvious because the transmitter totally wipes out the signal I am feeding it from the signal generator. I thought perhaps the preamp was the culprit so I took it out of line but sill experience the same issue. I am thinking that possibly the repeater output from the antenna is getting back into the repeater cabinet? I took a handheld scanner and set it on the same frequency as the receiver and connected directly to the rx port on the duplexer and can hear the noise there as well. I do hear a slight buzzing in the audio of the receiver almost like 60hz whenever I key the transmitter with the squelch wide open and no input signal present using the antenna. I don't hear it when using the dummy load though. I would like to think that the duplexer is tuned correctly or fairly close as there isn't any desense when terminated into a load. The last thing there is a single phase 7200 volt primary line servicing our neighborhood probably less than 100 feet from our house that I wonder is the culprit. I don't hear any arcing or power line noise with just the receiver squelch open but maybe when I transmit there is some mixing going on?
Thank You Tom Elmore KA1NVZ Anchorage, Alaska
