"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."
The above says that it's a 2nd order mix, F1+F2, F1-F2, 2F1 or 2F2. Since it looks to be a 2nd order product, proven by your 6 dB attenuator causing a 12 dB drop, whereas a 3rd order IM product would drop the product by 18 dB. Th 2nd order mix indicates that it could be combination of an internally generated signal from your equipment F1, Probably in the receiver input stage itself and an outside signal source F2 from an external transmitter, yours or another adjacent one. A pure 3rd order IM product is typically an indication of an internally generated source in the receiver input stages itself without any externally generated sources, but not always. Also do you have an isolator on the TX output along with a Low Pass filter after the isolator ? Is this a synthesized exciter or crystal controlled ? If you can identify all the signals present on Spectrum Analyzer then with the above 2nd order formulas you could probably zero down the 2 signals causing the problem. Mike --- In Repeater-Builder@yahoogroups.com, "brett" <brett_daw...@...> wrote: > > 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. >
