Re: [amsat-bb] AO-51 Mode J Desense Solution

["hasan schiers" <schiers@xxxxxxxxxx>]

Deon,

(No...not if you put the duplexer as high pass filter ahead of the preamp)

Sure it could happen, IF there is no protection ahead of the gasfet. My
preamp has a diode in front of the gasfet to protect it, but that very same
diode is what gets caved in from high rf producing all the "hash", "noise"
etc as it is protecting the gasfet.

The solution is exactly what we have seen posted multiple times;

Put a 2/70cm duplexer in front of the gasfet input, using the 70cm port of
the duplexer as a high pass filter. This works perfectly and only adds 0.25
dB loss ahead of the preamp. This is inconsequential for satellite work, and
protects the input of the 70cm preamp. I have not had a problem with 70cm tx
getting into a 2m preamp rx, but always had problems with 2m tx getting into
70cm rx preamp or no preamp.

The duplexer as high pass filter is inexpensive, effective and readily
available. I don't think we would have seen so many posts about the topic,
were it not for one person saying the filter belongs "after" the
preamp...which is just nuts for the problem that was trying to be solved.
Suffice it to say, scores of people have implemented the solution as
described above, with total success. (and yes, it will protect the preamp
from being fried as well). I have tested this combination with 150w output
on 2m located only 6 inches from the 70cm feedpoint. There is no desense, or
any other rf interference on the 70cm preamp, even with all this 2m rf
located so close. The Comet duplexer as high pass filter does the job. Other
duplexers may do it as well, I can only speak for the one I actually used.

73,

....hasan, N0AN
----- Original Message ----- 
From: "Deon Erwin" <zs1zl@qsl.net>
To: "hasan schiers" <schiers@netins.net>
Sent: Monday, August 23, 2004 9:26 PM
Subject: Re: [amsat-bb] AO-51 Mode J Desense Solution


> Hi Hasan
>
> Talking about strong 2m and 70cm signals in close proximity ...
>
> Can the 2m or 70cm pre-amp be damaged during full-duplex operation, when
the
> antennas are mounted very close to each other and the 2m or 70cm
transmitter
> is running at full power?
>
> Thanks
> Deon ZS1ZL
>
> ----- Original Message -----
> From: "hasan schiers" <schiers@netins.net>
> To: "John Mock" <kd6pag@qsl.net>; <amsat-bb@AMSAT.Org>
> Sent: Monday, August 02, 2004 1:25 AM
> Subject: Re: [amsat-bb] AO-51 Mode J Desense Solution
----
Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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