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
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!