Hi John, I thought we may have a dubious TX issue - either spurs or as you say something unstable or oscillating.
I replaced the TX. It had no impact on the problem. The issue appears to be external to the TX. Also I have terminated the duplexer into a 50ohm load and looked at the RX port of the duplexer and found it to be clear of anything. Regards, Brett --- In [email protected], "John J. Riddell" <ve3...@...> wrote: > > 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...@...> > To: <[email protected]> > 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 > > > > > > >
