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Re: R: 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?




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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