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.



Tnx for any help !

Dear "Lex--Lutor" (no call sign):

1. In North America, the frequency spacing between FM broadcast
transmitters in the same area is usually 800 kHz. 105.3 MHz - 104.7 MHz =
600 kHz. Are you sure about the two FM broadcast frequencies?

2. IM is produced from a nonlinear response in something. The odds are
strong that the 145.1 MHz signal is being produced in one or more of the
three transmitters - most likely one or both of the FM broadcast
transmitters. Adding filtering in your system will have no effect if the IM
is produced outside.

3. It is a good idea to perform a classical IM analysis (up to at least
seventh order) that involves all seven of the broadcast frequencies and to
use the analysis to select a repeater pair that does not involve an IM
frequency.

4. Theoretically, you could place a filter or isolator in the broadcast
transmission lines.

73, Mac N8TT

cross posting has been changed
--
J. Mc Laughlin; Michigan U.S.A.
Home:
"Lex-Lutor" wrote in message
...
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.



Tnx for any help !

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
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

1. In North America, the frequency spacing between FM broadcast
transmitters in the same area is usually 800 kHz. 105.3 MHz - 104.7 MHz =
600 kHz. Are you sure about the two FM broadcast frequencies?

Hy,

Yes I`m sure, in EU we have RX feq - 600 KHz for TX on 2m Ham band, for
PMR band it is RX + 4.5 MHz for TX ..


2. IM is produced from a nonlinear response in something. The odds are
strong that the 145.1 MHz signal is being produced in one or more of the
three transmitters - most likely one or both of the FM broadcast
transmitters. Adding filtering in your system will have no effect if the
IM
is produced outside.

I made a spectrum check at RX side after duplexer, there is lot of junk but
145.1 is clear, than I put repater in TX mode with spectrum connected at
duplexer RX port and you can see a modulated carrier - 95 to -90 dbm. In
demodulation mode (Wide) you can hear mixed program of this two
radiostations. I repat this test few times on two different spectrum
analyzers with and WO external attenuators in input line with same results.

I`m sure where is source of IM, repater antenna, coax line, or duplexer?
Maybe all 3-parts togother since duplexer is not Hi.quality, it is made from
aluminium cavity and Cu plunger, no sliver plating, RX and TX side are
separated so there is no good DC connection !


3. It is a good idea to perform a classical IM analysis (up to at least
seventh order) that involves all seven of the broadcast frequencies and to
use the analysis to select a repeater pair that does not involve an IM
frequency.

:-(( very hard, only if we change SHIFT



4. Theoretically, you could place a filter or isolator in the broadcast
transmission lines.

:-))

TNX Mac for all.....