Just make sure to pull the unused loop out of the cavity.
Steve / K6SCA
Gary Schafer <[EMAIL PROTECTED]> wrote:
If you have an extra band pass cavity then you also have a notch
cavity. Just connect a T to one port of the band pass cavity and ignore the
other port on the cavity. This will work as a notch cavity for your testing.
73
Gary K4FMX
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From: Repeater-Builder@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of
Stu Benner
Sent: Friday, January 25, 2008 4:54 PM
To: Repeater-Builder@yahoogroups.com
Subject: RE: [Repeater-Builder] Desense Problem on 222 MHz Repeater
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
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