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




----- Original Message -----
From: Eric Rawson <ericrawson@rawsonoptics.com>
To: <amsat-bb@AMSAT.Org>
Cc: <kn6kc@arrl.net>
Sent: Friday, March 28, 2003 8:32 AM
Subject: [amsat-bb] How far should one separate a LNA and its associated
downconverter, and Why?


> I own a 2.4 GHz LNA (DEM 13ULNA) mounted at the focus of a 2' x 3' BBQ
dish.
> The dish is modified for circular polarization (aluminum screen covering,
> 5-turn helical feed).  The LNA connects via a short run (~2') of coax to
my
> downconverter (DEM 2400-144).  When I bought these last year, I was told
> that the coax run between the LNA and the downconverter should be "more
than
> a few feet".  When I asked why, I was given a mumble about
> "interaction"...it was rather vague.
>
> Hence, my question.  Is it really necessary to separate an LNA from its
> associated downconverter with some length of coax? If so, why?  What,
> exactly, might happen if you, for example, mounted them within inches of
> each other?  And how far apart must they be separated (i.e., how many feet
> of coax) to avoid this problem, whatever it is?
>
> Eric Rawson, KN6KC
>

Eric,

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.

If the output impedance of your LNA is not 50+j0 ohm at 2400 MHz than
the coax cable connecting the LNA output to your downconverter input
operates infinite impedance transformations every 1/2 electrical wavelenghts
of coax cable as you can see rotating by 360 degrees  in to a  Smith Chart.

For this reason,every lenght of coax cable connecting your LNA to your
downconverter creates a different impedance transformation depending on
the mismatch existing (VSWR) between LNA output and  downconverter
input.

If the mismatch is very small or non existing than you are free to directly
connect the LNA output to your downconverter input because they will
work in a conditionally stable condition.

If the LNA or downconverter starts to be unstable than you must cut and try
to chouse a coax lenght or longher or shorter such that the impedance
transformation is still tolerated by the only conditionally stable
amplifiers characteristics.

In general,if the coax cable is long than its loss increases and it is like
to connect a 50 ohm attenuator between the LNA and downconverter and
it helps to better see an impedance of 50 ohm but at the expences of
gain from the LNA preamplifier.

For the above reason how far apart the LNA and downconverter must be
connected  is very difficult to say.

73" de i8CVS Domenico


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