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RE: mode J filter



At 03:40 PM 10/5/2005 +0930, MERRETT, David wrote:

>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)

Hi David,

The explanations offered by Kevin and Diane (thank you Kevin
and Diane) are the correct ones.

There is really nothing magic about a "duplexer" (diplexer,) it
is just a couple of filters. If the 2m port could affect the
70cm port, then it would not be a very good diplexer. In real-world
operation, you have no idea what impedance will be presented by
the other port so a good diplexer has to be designed to work under
those conditions including a high-Z or even a short-circuit on
the other port.

Just to put these miconceptions to rest, I tested the COMET CF-416C
mentioned in the write-up both with and without a 50 ohm termination
on the 2m port. The difference in 2m attenuation on the 70cm port
was less then 1 dB.

Note that even if the attenuation was an extra 10dB higher (say 70dB)
it would not matter because all you need to do is prevent the preamp
from overloading. Once you achieve this, additional attenuation does
not help unless you want to run 2Kw on your 2m uplink antenna.
fortunately, this would be unlikely on a satellite uplink (maybe on Field day!)

I have personally used the COMET unit in several installations and it
has always worked nicely so I can highly recommend it.

73,
Tony AA2TX
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