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Re: Have Distortion Problems-Can't Work FO-29



----- Original Message -----
From: "Greg D." <ko6th_greg@hotmail.com>
To: <domenico.i8cvs@tin.it>; <halpress@cinci.rr.com>; <amsat-bb@amsat.org>
Sent: Thursday, July 07, 2005 8:13 AM
Subject: Re: [amsat-bb] Have Distortion Problems-Can't Work FO-29

> Hi Domenico,
>
> You are, of course, correct that the 3rd harmonic is well outside the
> satellite downlink passband.  But never the less, I was fooled several
times
> while making the transition from the Easy Sat (RS-10) to FO-20.  At least,
> fooled by something.  The characteristic was a really bad audio, no
doppler,
> and (the little clue) reception after the satellite had set below the
> horizon.
>
> If it wasn't the 3rd harmonic, or something related to it, I can't think
of
> what else it could be.  My rig is a Yaesu FT-736R, and at the time I did
not
> have a preamp.  I think I did have a Mirage 2m power amp - the 2m uplink
> antenna wasn't very good, and I had 60' of RG-8 between the rig and
antenna.
>
> Maybe related, but I also had desense problems with AO-40 mode US,
whenever
> the satellite went behind some nearby trees.  Seemed like something from
the
> uplink was bouning back and messing with the receive side.  Maybe the
> problem is trees?
>
> Gerg  KO6TH

Hi Greg, KO6TH

I have not nearby trees but very often I noted similar desense with noise
and ghost signals while transmitting depending on the actual position and
orientation of the antenna related with other VHF/UHF antennas installed
on the roof.

When an antenna is aimed to the satellite but is looking to another close
antenna or any obstruction in general than the antenna impedance changes.

In general GaAsFET preamplifiers are only conditionally stable i.e. they
are stable when looking at 50 ohm but if the impedance of the antenna
connected to it changes with his orientation they fall in a unstable
condition ( K < 1) that probably generates the above mentioned problem.

Sufficient infact to move a little bit the antenna and the tendency to
unstability audible in the form of wrenching noise while transmitting
disappears.

73" de

i8CVS Domenico
----
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