If the interference is the result of a third order IMD product,
each 3 dB of rejection will yield a 9 dB improvement in the
third order intercept point.

A modest filter might yield surprising results, it isn't a linear
relationship.

Pete k1zjh

"Surprising results" in a lab maybe. Still far short of real. An
attenuator would tell you how much you really need. I used to use an HT
with a dummy load instead of an antenna from Mt. Wilson to be able to talk
into a box on Santiago Pk. Otherwise the HT couldn't even hear 500 WERP
on-channel from the tower I could see with my own eyes. Be aware that you
might only have 30 to 60 db of bolt on attenuation before case or cable
leakage takes over.

I used an Alinco 2m HT with a two section helical resonator outboard (most
portable solution). There was 3 db of insertion loss and 20 db of rejection
outside of a 3 Mhz window. This was a packet radio and resulted in a 10db
improvement in performance on-channel, but this was a home station on a 6db
stick 20 ft in the air. Would have been far short on a mountain though. I
aslo used that combination for T-Hunting in addition to a fixed 60db pad and
a switched 20/20/10db pad with double shielded coax and 4 el. Quad. My best
solutions was to find places to listen from that were shielded from the
major mountain tops.

In article ,
Tio Pedro wrote:

If the interference is the result of a third order IMD product,
each 3 dB of rejection will yield a 9 dB improvement in the
third order intercept point.

A modest filter might yield surprising results, it isn't a linear
relationship.

For what it's worth: I used to have terrible pager-intermod problems
with my Yaesu VX-5, when used with any reasonably-efficent antenna
(e.g. J-pole)... pager-transmitter intermod drove it wild. This seems
to be a common problem with most current-generation HTs, with their
wide-open "DC to daylight" front ends whose high sensitivity (for use
with lossy rubber-duck antennas) leaves them prone to being badly
blasted by strong signals.

The solution I settled upon was the PAR Electronics VHFTN152-158, a
notch filter specifically tuned to eliminate the VHF paging band,
while passing other signals. PAR claims a notch depth of 50 dB
(typical) at pager frequencies, with low loss at 2M and 440
frequencies. From the look of the filter, I believe it's probably a
set of three helical resonators shunted across the line.

Problem solved - the VX-5 suffers no pager intermod at all that I can
hear.

The same filter did *not* help, though, in curing a desense problem
with our repeater's remote-link receiver, which was being blasted by a
newly-installed paging system located in the same building. The pager
was operating up in the mid-160MHz range, outside of the PAR filter's
notch. We installed a DCI cavity-bandpass filter and the problem went
away.

In re the OP's problem - I wonder whether it might be possible to
home-brew a moderate-Q helical filter to serve as a notch? The old
ARRL VHF handbook has some diagrams of this sort of thing. As Tio
points out, one might not need all that deep a notch to result in an
acceptable reduction in intermodulation and desense.

--
Dave Platt AE6EO
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