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Re: Another interesting preamp story



Hi Jim, WA4IVM

As you suggested the use of a Duplexer as a filter helps to suppress the third
harmonic of VHF getting into the passband of the UHF preamplifier but Wayne
says that sometime he hear a burst of noise that only occurs while
transmitting when the antennas point in certain direction.

It happens the same sometime to me transmitting in 2 meters and receiving in
70 cm when my antennas are looking at a very well know azimut with 0
elevation where I have many other nearby TV antennas installed over another
roof.

In the above antenna direction the noise level sometime changes while speaking
depending for example on the weather conditions with no problems if it's
raining.

My 2 meters TX has a homemade low pass filter in the output designed by W.
Cook AF9Y showing -20 dB attenuation for the second harmonic at 292 MHz and
-45 dB attenuation for the third harmonic at 438 MHz so that the level of my
harmonics are not too strong to overload or to compress the gain of my GaAsFET
preamplifier.

In addition my 70 cm homemade GaAsFET cavity preamplifier has in front of it a
one pole passband filter designed by John K3PGP as described in
http://www.k3pgp.org/432filter.htm

If  I change the GaAsFET preamplifier with another one using a bipolar
transistor in Norton circuit than the problem
desappears completely.

The GaAsFET preamplifier has a lower NF and show a greater gain with a lower
compression point but more important it is only stable if the impedance of the
antenna is the same 50+j0 ohm impedance to wich it has been calibrated for the
minimum NF and to the best stability.

The bipolar transistor in Norton circuit has a greater NF but has a greater 1
dB compression point and more important  it is unconditionally stable and so
it is less sensitive to the impedance changes of the antenna that come up in
different pointing conditions.

The above is my experience with this type of problem depending on GaAsFET
preamplifiers wich are difficult to get unconditionally stable.

If the impedance of the antenna changes or becames reactive for the above
mentioned reason than a GaAsFET preamplifiers connected to it becames unstable
and if overloaded by a strong nearby signal it tend to oscillate generating a
wide band white noise.


Best 73" de

i8CVS Domenico

  ----- Original Message -----
  From: Jim Wright
  To: i8cvs
  Cc: Wayne Estes ; AMSAT-BB
  Sent: Wednesday, March 29, 2006 6:52 PM
  Subject: Re: [amsat-bb] Another interesting preamp story


  You did not say which preamp is getting interferred with.  Don't ignore the
third harmonic of VHF getting into the passband of the UHF preamp.  A simple
DIPLEXER can help solve this problem.  Many manufacturers call them DUPLEXERS,
but they can be had for about $50.  I have one in front of my SSB SP7000 and
it solved my problem.  I feed the power over an old piece of coax to the
SO-239 connector on the preamp, so it is shielded and grounded, but had severe
desense until the DIPLEXER was added.
  73,
  Jim

  i8cvs wrote:

----- Original Message -----
From: "Wayne Estes" <w9ae@charter.net>
To: <amsat-bb@amsat.org>
Sent: Wednesday, March 29, 2006 8:22 AM
Subject: Re: [amsat-bb] Another interesting preamp story


Joe K0VTY wrote:

...Separate ground shielded coax for the DC to each unit...

Wayne W9AE replies:

Has anybody found that this is necessary for SSB Electronic preamps?  I
power my 2m and 70cm SSB preamps with (unshielded) rotor cable
conductors.  I never get desense on linear transponders, but sometimes I
hear a burst of noise on the downlink when I transmit at high power in
FM mode.  The problem goes away when I reduce the uplink power.  It only
occurs when the antennas point in certain directions.

Wayne Estes W9AE
Oakland, Oregon, USA, CN83ik


Hi Joe, K0VTY

The problem goes on because the impedance of your antenna changes
pointing it in different directions.
In certain antenna directions the input of your GaAsFET preamplifier
is looking at impedances that are slightly different from 50 ohm resistive
and the GaAsFET preamplifier becames unstable producing the above
effect no matter if the cables of rotator or preamplifier power supply
are shielded or not.

73" de

i8CVS Domenico
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