Knowing what is on the nearby antenna is VERY important and is most likely involved in the problem!
I used to work at Sinclair as a systems engineer and dealt with solving IM problems and sources on a daily basis for customers' solutions so I know what I am asking WILL help in resolving the original problem of interference. Putting a pass can between the duplexer and receiver with a non optimized cable will help in describing the characteristic of the interference if the interference changes in RF level and then if it cures it then one can optimize the cable length. Critical length cable MEANS (to an experienced filter person) that the total response characteristics are optimized for a given response. That could be for VSWR shape or pass frequency response or even whether the notches add to more rejection than the individual notch responses or not. Also the effect of cavity interaction wrt tuning occurs with non optimized cables. Yes, I know they ran the frequencies - I can read. One cannot just look at the receive and transmit frequencies alone. If the combination isn't obvious with the differences in frequencies showing hits, you have to look at synthesizer frequencies, image frequencies, and other oscillator frequencies for the cause of the IM or spurious mixing. That is why I mentioned the 12.8MHz oscillator spur on the Mastr 3. Any non-linear junction in the presence of multiple RF signals will generate intermodulation. Even ferromagnetic materials like nickel will generate intermodulation. Characterizing the interference level changes with the application of a pass cans or attenuators can help in determining whether the source is internal to the receiver or external (cables, filters, antenna and beyond) Harold, VA3HF --- In [email protected], Ron Wright <[EMAIL PROTECTED]> wrote: > > I believe the inital posting stated no freq combination/mixing could be found. For the most part this would illimate an intermod problem. However, since there are so many txs in a typical city part of the problem could be from a distant unknown tx. > > 73, ron, n9ee/r > > > > >From: Harold Farrenkopf <[EMAIL PROTECTED]> > >Date: 2007/07/26 Thu PM 01:50:26 CDT > >To: [email protected] > >Subject: [Repeater-Builder] Re: Coax length between added cavity and duplexer > > > > >What are the frequencies of your repeater and the frequencies on the > >nearby antenna???? > > > >What are the radios used on the nearby antenna? > > > >Important information in determining the problem - I had one source of > >noise caused by a Mastr III that had a spur of considerable strength > >at 12.8MHz down from the transmitter carrier - synthesizer oscillator! > >It was received with the correct transmitter modulation as the carrier > >frequency and a pass on it or a notch at -12.8 MHz solved our problem. > > The level of the emission was within spec of the Mastr 3 but was > >still too strong for our antenna at 500' away. > > > >What is the configuration of their filters? > > > >4' horizontal separation is not much at all so filters should be > >designed to all the frequencies used to work together. > > > >Then, it only takes one transmitter to cause noise in an antenna if > >the antenna is "noisy". > > > >What does the interference sound like? > > > >Harold, VA3HF > > > >--- In [email protected], Ken Arck <ah6le@> wrote: > >> > >> At 11:06 AM 7/26/2007, you wrote: > >> > >> > >> > >> Intermod means there is mixing in YOUR PA with some outside RF energy. > >> > >> <----I should probably clarify that "intermod" is an often misused > >> term that has become a generic terms used to describe just about any > >> type of received interference. Ya know, kinda like Klenex has become > >> the name for any tissue. > >> > >> Intermod is a very specific type of phenomenon and for accuracy, > >> should only be used to describe intermodulation issues and not mixes, > >> adjacent channel interference, etc. > >> > >> (more of my 2 cents) > >> > >> Ken > >> > > > > > > > Ron Wright, N9EE > 727-376-6575 > MICRO COMPUTER CONCEPTS > Owner 146.64 repeater Tampa Bay, FL > No tone, all are welcome. >
