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Re: Mode J Recieve Desense Problems

>> Trouble is that I can't remember whether you want a quarter-wave or a half-
>> wave ... and whether its open-circuit or closed. But I'm sure someone else
>> on the -bb will know this and post.
>If the open end is, in fact *OPEN*, then use a quarter-wave.  If it's
>shorted, then use a half-wave.  You can "snip" the quarter-wave stub 
>a little at a time to find the "sweet spot", or insert a pin through 
>a half-wave stub to do likewise.  Either stub will represent a very low
>Z at the opposite end, and attenuate the undesired signal as a result.

Unfortunately, a stub is a "comb" filter not a bandstop filter. A quarter
wavelength stub at 145 MHz will not only attenuate the undesired signal,
it also pretty effective at attenuating the 435 MHz (desired) signal.

A good cheap filter is a 2m/70cm duplexor. Typically 50-60 dB out 
of band attenuation, not much insertion loss and not very expensive 
either. Many different models are available from HRO, AES etc.



Tony AA2TX@amsat.org
North Andover, MA

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