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Re: Receiver impedance mismatch



Thanks to all who have responded to my question. The basis for the 
query is a desense problem at the N6IJ club site where the FT847 70 
cm RX is overloaded by the 2 meter transmission. We have no preamp at 
the antenna and the problem is evident only when the RX preamp is on. 
Hopefully this is temporary as we have yet to raise the antennas to 
their final height but in the meanwhile it was fun to try different 
solutions.

Our first approach was to investigate the performance of a quarter 
wave 2 meter stub. This reduced the 2 meter level by 25 dB relative 
to 70 cm but it also dropped the 70 cm signal by 10 dB so this was 
not acceptable.

The next approach was to place a 5 element Chebyshev HPF right on the 
N connector of  the RX. This filter had a measured 50 dB loss at 145 
MHz and did reduce the desense but not entirely- possibly the problem 
is aggravated by common mode currents or a third harmonic component 
of 2 meters  This filter also had a pass-band loss of 2 dB so it was 
not really good enough.

We then tried a 70 cm cavity filter which completely eliminated the 
desense problem but the passband is so narrow that it can not cover 
the 70 cm range we require. No practical way of broadening the 
passband has come to light.

We next constructed a 7 element HPF and it tuned for minimum 
pass-band loss of less than 0.5 dB with loss at 2 meters of 60 dB. It 
was while tuning these filters that the question of impedance 
mismatch surfaced. What if filter performance in situ is nothing like 
what we measure on the bench?

A very cool TX/ RX diplexer design by OK1DNZ appeared in the Mar/Apr 
2002 issue of QEX  which is the next approach we will likely take.

-- 
73
Ron
W6WO
----
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