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Re: Another interesting pre-amp story



Hi Joe, K0VTY

For GaAsFET protection it is a good practice to leave the device 
power supplyed and polarized while transmitting.

In general a GaAsFET preamplifier is not unconditionally stable and
the device can autooscillate if the input and output are not 50 ohm 
terminated.

For the above reason in TX conditions the input  and output of a 
conditionally stable GaAsFET preamplifier should be switched in 
to 50 ohm  terminations that in general are already  included by 
the manufacturer in to some type of switching coax relays.

If the input and output of a power supplyed GaAsFET preamplifier
are leaved open or are simply ground connected by relays instead to 
be 50 ohm terminated than the device can autooscillate producing 
effect similar to that described by KB7ADL

73" de

i8CVS Domenico
       
----- Original Message ----- 
From: <k0vty@juno.com>
To: <wa6fwf@amsat.org>; <amsat-bb@amsat.org>
Sent: Saturday, March 25, 2006 4:49 AM
Subject: Re: [amsat-bb] Another interesting pre-amp story


> Hi 
> 
> On the Landwehr VOX switching
> 
> I have been using the Landwehr preamps on 2 and 70 for over 16 years
> The text on the VOX operations:
> In transmitting mode a new designed RF-VOX becomes active . The
> switch-over 
> time is only determined by the magnetic energy , stored in the coils of
> the relays
> and not through a big capacitor. Thus there will be not clicking of
> relays 
> in the SSB mode.   The pre-amplifier will not be damaged during
> transmission even 
> if the supply voltage is connected.    Vox operation is permitted up to
> 150 watts
> Higher output power is handled with PTT
> 
> Note the RF-VOX with magnetic energy storage in the relay coils
> 
> I have run the preamps on pairs of KLM 22c and 40 cx's for many years.
> 16 foot fiber cross boom and factory power dividers..
> Currently I run only one each of those antennas.
> 
> Joe K0VTY
> Amsat # 860
> ===========================
> 
> 
> 
> On Fri, 24 Mar 2006 18:24:44 -0800 "wa6fwf@amsat.org"
> <kevinschuchmann@sbcglobal.net> writes:
> > Hi Vince,
> >    I don't have a Landwehr but I would bet it is RF switch capable, 
> > so they are tapping
> > off some of the RF, feeding it to a diode to get a signal to drive 
> > the automatic switching.
> >    Sending RF to a diode can create hash all over the place if it is 
> > not filtered well,you may
> > remember some people using a oscillator and a diode to make 2.4ghz 
> > signal sources!
> >   Just a thought of what the problem might be...
> > 
> > 73
> > Kevin WA6FWF
> > Amsat-UK #6505
> > 
> > ----- Original Message ----- 
> > From: "Vince Fiscus, KB7ADL" <vlfiscus@mcn.net>
> > To: <amsat-bb@amsat.org>
> > Sent: Friday, March 24, 2006 1:44 PM
> > Subject: [amsat-bb] Another interesting pre-amp story
> > 
> > 
> > >I used to have terrible mode J desense.  One day I decided to see 
> > if I could solve the problem.   I had the problem whether the 
> > >70cm pre-amp was in-line or removed.  I then removed the 2 meter 
> > pre-amp and the problem went away.  My pre-amps are Landwehrs. 
> > >Funny, it was the 2 meter one that was leaking the RF and noise.  I 
> > don't use a pre-amp on two meters now, but if we get a Mode B 
> > >HEO, I may need you figure out what to do.  Maybe a different two 
> > meter pre-amp will solve the problem.  I was surprised the two 
> > >meter was what was causing the problems.
> > >
> > > KB7ADL
> > > ----
> > > Sent via amsat-bb@amsat.org. Opinions expressed are those of the 
> > author.
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> > program!
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> > 
> > ----
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> > author.
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> ----
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