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Re: mode J filter



On Tuesday 04 October 2005 19:34, you wrote:
> I was surprised to see the 2M port "left unterminated". I'd expect
> terminating it to 50ohm would provide, on average, the *best* possible
> isolation. (The power gets absorbed-away, not left in hi-Z state.)
> For the same reason, users with small antenna separation (< 1M?) may also
> notice a difference on the 2M signal (azimuthal pattern effect towards 70cm
> antenna)), between terminated vs unterminated.
>
> So I suggest that when testing, try both terminated and unterminated to see
> which is better.
>
> Dave M.
> VK5DSM.
>

 Hi Dave, 
   its my understanding that a diplexer  has two sections of low and high pass 
filters, in this example you are only using one half of the filter, the 70cm 
side, so you have a high pass section that is allowing 70cm signals through 
and you have a filter or filters in the 70 cm path  that is attenuating the 
2m signal.  
   The 2m port that you are worrying about doesn't even come into use in this 
example,  so it being terminated or not doesn't matter.

   You might want to check out QEX March/April 2002 if you can get it,  it has 
a diplexer built by OK1DNZ and explains the theory and tuning.

73
Kevin WA6FWF
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