> We have a 222 MHz repeater comprised of a converted Micor
> mobile, Telewave TPRD-2254 BpBr duplexer, AM-6155 PA modified
> for class C operation at 250W, and a DB-264JJ antenna at 80
> ft. fed by 1/2' Heliax on a commercial FM broadcast tower .
> With the duplexer terminated into a load, we have about 1 dB
> degradation in sensitivity when transmitting. However, with
> the antenna connected to the duplexer, we experience in
> excess 15 dB of desensitization. We have eliminated other
> narrowband transmitters and analog TV transmitters as
> contributing factors. We are left with a channel 12 digital
> TV transmitter at an adjacent site as a key contributor to
> the problem. Our hypothesis is that we have broadband IMD
> products from the mix of our transmitter and the DTV
> transmitter that are appearing in and near our receiver
> passband. Is it a "rusty bolt" problem or is there some other
> non-linear component somewhere on the site or in our system
> that is the mixing point - we don't know.
If you suspect IMD between the channel 12 DTV transmitter and your Tx
carrier, work on attenuating the DTV signal. Using cavity notch filters to
reject the entire 6 MHz of DTV isn't too practical, so instead, try adding
pass cavities on your Tx before the duplexer input. That will help
determine if the IM is originating in your PA. An isolator *may* help, but
with channel 12 being the better part of 20 MHz away (about 10%), it may not
afford full protection -- isolators don't have infinite bandwidth.
Likewise, finding a 250 watt 220 MHz isolator may not be easy. I think I
have some 220 isolators that came off a combiner (Sinclair), but doubt
they're good for 250 watts judging by their size.
If you suspect a "rusty bolt" mix, use an alternate antenna for testing. If
nothing else, try a quarter-wave whip (suitable for operation at your 250
watts TPO), even if it's just temporarily mounted on the tower (be sure it's
at a sufficient height to prevent desense due to close proximity to the
repeater itself).
Another good possibility is IM in your receiver front end (or preamp, if
you're using one). Again, pass cavities are your friend here. Attenuate
the channel 12 signal as much as possible and see if it makes a difference.
Have you looked at what sigs are reaching your receiver input on a spectrum
analyzer? With 15 dB of desense, you should be able to see the culprit(s);
it's not like they're going to be buried in the noise if it's causing 15 dB
of desense.
Even that 1 dB of desense would give me some agita. I'd verify that the
duplexers are properly tuned and the transmitter is clean before even
starting down any other paths related to the channel 12 issue. IIRC, the
Telewave cavities have adjustable coupling. If necessary, sacrifice a
little extra loss for additional rejection if necessary.
I also assume you're using all known-good interconnect cables (no foil+braid
or other cables not suitable for duplex operation).
Are you using a Polyphaser or other type of surge arrestor? If so, try
bypassing it. I've seen gas discharge tube type surge arrestors become
noisemakers after absorbing a strike.
Has the VSWR changed at all on your antenna? If so, it could indicate water
in a connector or the harness which will cause all kinds of grief, including
wideband noise.
Finally, does the desense change appreciably if you vary transmitter power
output (it probably will). Do you any have desense when running on just
exciter power?
--- Jeff WN3A
