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RE: Receiver impedance mismatch

> 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

Try also placing a 70cm notch filter at 2m transmitter.

-Jari OH3UW 
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