At 3/6/2008 09:23, you wrote: >Let me start by saying that I do this kind of thing all day long for a >living. I have seen it occasionally on some commercial systems with the >same antennas. I have all TX/RX 6 cavity duplexers, and the repeaters >are Motorola (CDR or R1225 type) with circulators and line matchers if >needed. I spent a few hours at the problem site the other night and no >matter how I was trouble shooting it, it pointed to the antenna system >(beyond my polyphaser lightning protection at the ground window/entry). >This is definitely an external mix (IM) that includes my transmit >signal. The mix is strong enough that I can see it on my service >monitor inside the concrete building. No problems at all with a dummy >load, and cutting my power output to exciter only reduces but does not >eliminate the problem.
I had this problem for a long time on my 440 system. There were 2 other transmitters involved, both on VHF HB & spaced 5 MHz apart. Even running split antennas with 60 ft. of vertical separation wasn't enough to lose it. The mix was in the tower itself, & could be temporarily reduced by painting over the tower joints with conductive paint. Eventually one of the TXs involved in the mix (a low power paging transmitter) went away, & I've never had any problems since. Bob NO6B
