Any idea how to slove intermod problems at repeater location, very close (
150 meters) to VHF repeater antenna is located FM-TV tower with 7 strong FM
channels, nominal power is from 5-10KW + antenna system gain. Bad thing is
that combination of 104.7 MHz + 145.7 MHz (repeater tx) - 105.3 MHz = 145.1
MHz ( same frequency for repeater input ) !
There is 3 +3 cavity duplexer pass/reject type and professional type of
vertical dipole with 0 dB, I`m sure if notch cavity can change this problem
or installing high quality FM Trap filter to existing setup.
The intermod could be arising in any number of locations. Some are
probably filterable, others probably aren't.
You could be getting intermod from:
- Mixing of the 2-meter TX with commercial FM, in your 2-meter TX
finals (FM "coming back down the cable"). The best solution to
this would probably be a ferrite circulator/isolator, located
between your TX output and the duplexer DX input... this will
divert any RF coming back down through the cable and duplexer into
a dummy load before it reaches your finals and mixes in.
- Overload of your receiver by incoming commercial FM - you might see
intermod or desense from this. You could try a notch filter tuned
to the commercial FM signal, or a high-pass filter to attenuate the
whole range below 144 MHz.
- Intermod created in the antenna system itself, due to some form of
nonlinearity (corroded connections or joints, presence of dissimilar-
metal junctions where plugs and jacks interconnect, etc.). In some
cases you may be able to get rid of this by cleaning up the
antenna... e.g. eliminate nickel-placed N/UHF connectors (use only
high-grade silverplated connectors), make sure that all cable
connections are tight and weatherproofed, check all bonding
connections to make sure that they are solid and not corroded, etc.
You can't get rid of this latter sort of intermod with a filter, as
it's being created outside of your electronics and the resulting
intermod signal is right on the frequency you want to receive.
Screening the antenna from the commercial tower, or placing your
antenna on that same tower but directly above or below the FM
antennas, may gain you enough reduction in commercial-FM signal
strength to bring the intermod to within tolerable proportions.
--
Dave Platt AE6EO
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