Hi Mike, thanks for your post - I'll think about this a bit more. There is no isolator on the TX - will add one (and BP filter) when I next go to site this week.
The radio is synthesised. Regards, Brett --- In [email protected], "wb8vlc" <mas...@...> wrote: > > "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 [email protected], "brett" <brett_dawson@> 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. > > >
