Bret, you might have your PA going in to oscillation creating the spurs due to a highly reactive duplexer.
We had a similar problem here many years ago and fixed it with a simple tuner on the TX similar ot the GE Z matcher . The one that we used was "Home Brew". When the tuner was adjusted for minimum VSWR, the spurs went away. 73 John VE3AMZ ----- Original Message ----- From: "brett" <brett_daw...@yahoo.com> To: <Repeater-Builder@yahoogroups.com> Sent: Saturday, September 04, 2010 7:26 AM Subject: [Repeater-Builder] Interference on VHF repeater > Hi all, > > I have come across an interesting problem which you may be able to shed some > light on. I have an intermod issue where my TX > sometimes opens up my RX. I have the distinctive hollow pipe sound. Both TX > and RX have the same CTCSS tone. The intermod > product is however not always present, and after looking at the RX output > from the duplexer with a SA I see a comb of products > that move slowly in time. When one of the products in the comb falls within > the RX bandwidth the RX opens, until it moves on. > > This is not a busy site, and I have been able to power down everything on > site except my repeater. Problem remains unchanged. > > I have also disconnected feeders from all other RF equipment on site - still > no change. > > The fact that the IM product frequency changes with time (drift rate is > roughly a few kHz's an hour) makes me think that there is > either another unknown source of RF on site which has poor freq stability > (pretty unlikley), or somehow my TX freq is involved in > producing this freq. > > I have inserted a 6dB pad in the antenna port of the duplexer and found that > the IM products drop 12dB, and also curiously, the > frequency of the products change. Removing the pad reverses this effect. I > have repeated this many times and the result was > always the same. It appears that the frequency of the IM product is > dependent on the strength of the radiated field from my > antenna. > > This is my question: I have read that it is possible for a strong EM field > to excite metal (eg tower member) such that > re-radiation will occur at a frequency which is different from that which > excited it. Can anyone confirm they have seen this, or > can anyone point me to a reference that talks about this? > > I should also mention there are multiple solar panels and associated > regulators on site. The regulators have been discounted as > possible sources, but the panels (given they may have bypass/blocking diodes) > may be a mixing location, however the source of the > drifting tone is still unclear. > > Thanks, > > Brett VK2CBD. > > > > > ------------------------------------ > > > > Yahoo! Groups Links > > >