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RE: mode J filter



I understand how the 2-M section isnt really being "used".
I only suggested this because it could possibly make the difference between an
*extra*, say, 10 dB of isolation.(EG 60 to 70 dB)
The 70cm section probably works by presenting a Hi-Z to the 2M band.
If 2M  port is left "open circuit", then 2M signals at the common-port end
(after sneaking onto antenna) are also in a high impedance state. (more
voltage, less current).

Anoother way to look at it is that the 2M signals are going to get onto your
antenna whether you like it or not.
(They havent even 'seen' the diplexer yet)... what do they do then? They go
"Oh great!.. A nice 2M band pass filter!"... off they go down that, past the
70 cm Bandpass filter.
Suddenly they get to the unterminated end.. and go "Damn... well have to go
back!"... back they go...(they cant radiate out of an open plug)
NOW they end up back at the common port.. and have a choice... either back out
the antenna, or try to get into the 70cm pass-filter.
The problem is the BOTH these ports ( the antenna, and 70cm pass filter) look
~like an open circuit... so they are try to split both ways...hence the
overall 2M signal voltage level at the COMMON port is more likely to be higher
if its 2M port is unterminated.

Using this explanation, by not letting them travel back through the 2m filter
( IE Terminating 2M port) , they can never build up voltage at the common
port, hence the signal output at 70cm port is less.
In reality, things are more complex of course (EG phasing and coupling
effects), and I know Ive waffled on a bit, buts its my coffee break.

Dave M.
VK5DSM.


-----Original Message-----
From: owner-AMSAT-BB@amsat.org [mailto:owner-AMSAT-BB@amsat.org]On
Behalf Of kevin schuchmann
Sent: Wednesday, 5 October 2005 13:08
To: amsat-bb@amsat.org
Subject: Re: [amsat-bb] 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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