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R: R: How far should one separate a LNA and itsassociated downconverter, and Why?



Jon,

I was trying to say that the input network of the preamplifier has been
adjusted for the lovest noise figure using a noise head having an impedance
of 50+j0 ohm because hopefully the antenna have the same impedance.

In operation,if the impedance of the antenna connected to the amplifier is
different in real part (resistive) or in imaginary part (reactive),than the
noise figure of the preamplifier increases.

The imput impedance of the device itself ( S11 parameter ) for the lowest NF
is not 50+j0 ohm and infact the typical input return loss of a GaAsFET
preamplifier is around 5 to 6 dB only.

In any case,if  you like not deteriorate the preamplifier NF and
additionally keep it conditionally stable than the antenna impedance or
LNA output impedance connected to its input must be 50+j0 ohm,no
matter what happens in to the many impedance transformations occurring
inside the preamplifier input matching network.

73" de i8CVS Domenico

----- Original Message -----
From: Jon Ogden <na9d-2@speakeasy.net>
To: i8cvs <domenico.i8cvs@tin.it>; <ericrawson@rawsonoptics.com>; AMSAT-BB
<amsat-bb@AMSAT.Org>
Cc: <kn6kc@arrl.net>
Sent: Friday, March 28, 2003 6:28 PM
Subject: Re: R: [amsat-bb] How far should one separate a LNA and
itsassociated downconverter, and Why?


> on 3/28/03 9:38 AM, i8cvs at domenico.i8cvs@tin.it wrote:
>
> > The input matching network of your downconverter has been adjusted for
> > the minimum noise figure connecting to its imput a noise head having a
> > purely resistive impedance of 50+j0  ohm.
> >
> > If the input matching network of your downconverter in operation don't
> > see the same impedance than its noise figure increases and probably it
> > starts to be unstable because the most part of  GaAsFET amplifiers are
only
> > conditionally stable i.e. they are stable only when the input and
output
> > matching networks are connected to 50+j0 ohm impedance.
>
>
> Well, all this depends and it depends on a number of things.  First of
all,
> the input match of the downconverter may NOT be 50 Ohms.  I think this is
> what Domenico was trying to say.  When matching an amplifier for the
lowest
> noise figure, 50 Ohms is NOT the optimum noise point.  If your d/c's input
> is turned for lowest noise figure, it may not be 50 Ohms.
>
> The output of your LNA is likely 50 Ohms and wants to see a 50 Ohm load.
So
> adding a length of coax from the output of your LNA to the input of your
DC
> will help the d/c's mismatched input look better due to the loss in the
> cable.  So Domenico's concerns are correct.  I don't know if you need to
do
> some impedance transformation, but a little loss in the cable will help
> minimize the mismatch.
>
> If your d/c has a 50 Ohm input and the output of the LNA is 50 Ohms, then
> there should be no reason you can't connect them directly together.  I
think
> it could be that another reason people say it is that by the time you add
an
> LNA and a d/c at the feedpoint of an antenna, it may be quite a lot of
> weight or leverage hanging off the feed.  You absolutely want the LNA at
the
> feed and it's better to do that and separate the LNA and d/c by coax
rather
> than putting the LNA and d/c together after a run of coax from the feed.
>
> Make sense?
>
> 73,
>
> Jon
> NA9D
>
> 73,
>
> Jon
> NA9D
> -------------------------------------
> Jon Ogden
> NA9D (ex: KE9NA)
>
> Citizen of the People's Democratic Republik of Illinois
>
> Life Member: ARRL, NRA
> Member:  AMSAT, DXCC
>
> http://www.qsl.net/na9d   <- Updated on 1/22/03!!!
>
> "A life lived in fear is a life half lived."
>
>










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