Thanks to everyone for the replies so far. Please see comments embedded
below. I think that they address most of the comments, questions, and
recommendations posed by all to this point.
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.
[Stu] Agree, notches don't work well for that plus we don't have handy any
cavities that will tune there. Have used up to two BP cavities in the TX
path along with a 2-stage isolator. No difference in desense is observed.
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).
[Stu] We're presently on split antennas. One is at about 80 ft., the other
is at about 15 ft. This improves the desense on the order of 6 dB.
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.
[Stu] The desense is significant with or without a preamp. Worse with but I
can't find my notes to quote numbers. Used up to 2 BP cavities on RX with no
perceptable difference in desense. Have also installed a DCI 4-pole filter
on RX and TX with no effect. Have looked at the receiver input with a
spectrum analyzer. The most significant signal is the one FM broadcast
transmitter at the site. Running power down on it or turning it off has no
effect on the desense. Our TX signal at our RX input is consistent with our
measured duplexer isolation (about -88 dBc or -34 dBm). Within several
hundred kHz of the RX frequency there are no detectable narrowband signals.
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.
[Stu] I tuned the duplexer myself with a network analyzer and the
transmitter looks clean. I have coupling set where I get about 1 dB through
loss and the notches are at about 88 dB on TX and about 90 dB on RX.
I also assume you're using all known-good interconnect cables (no foil+braid
or other cables not suitable for duplex operation).
[Stu] All cables are either Heliax or double braided.
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.
[Stu] Yes but there is no difference in desense when it is removed.
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.
[Stu] The problem has existed since the repeater was installed. It exists
whether we duplex on a DB264 at 80 feet or a G7-220 at 15 feet, both fed
with Heliax
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?
[Stu] The desense is roughly proportional to transmit power. Barely
perceptable at 20W (exciter only) with split antennas. A little worse at 20W
(exciter only) using one antenna. Have tried both tube-type and solid state
amplifiers at various power levels.
--- Jeff WN3A
