[amsat-bb] Re: Filters and Preamps Question

i8cvs domenico.i8cvs at tin.it
Mon Oct 19 18:37:52 PDT 2009

----- Original Message -----
From: "Joe Fitzgerald" <jfitzgerald at alum.wpi.edu>
Cc: <amsat-bb at amsat.org>
Sent: Tuesday, October 20, 2009 1:33 AM
Subject: [amsat-bb] Re: Filters and Preamps Question
> >  what's the preferred
> > order?
> As ever "that depends".  In the specific case I was involved in at
> Worcester Polytech, we found that we could get the best trade off by
> putting the preamp at the antenna then knocking down the out of band stuff
> with the filter between the preamp and the receiver.   The losses in the
> filter hurt your system noise figure much more if you go ant -> filter ->
> preamp -> receiver
> If the out of band stuff is desensing your preamp, well, that is another
> story.  At +12 dBm for the ARR GaAs FET we were using there was no
> problem, even in an urban environment.
> -Joe KM1P

Hi Joe, KM1P

You are right and in fact:

It is a common EME procedure to put a passband cavity filter or an
interdigital passband filter at the output of the antenna mounted low
noise preamplifier because the insertion loss of it in dB decreases only
the gain of the preamplifier by the same amount in dB so that the
Noise Figure of the preamplifier is not degraded by the insertion loss
of the filter.

In this configuration we have  --> antenna --->a T/R coax relay---->
preamplifier ---->cavity filter all antenna mounted and a coax cable
connected to the output of the preamplifier devoted only to receive
and directly going down the shack ------>to the receiver.

In more details the antenna is connected to the common port of the
T/R coax relay while the preamplifier is connected to the normally
closed NC contact of the T/R relay

In order to transmit the TX power amplifier is connected directly to
another coax  cable dedicated only to transmit and it is connected at
the antenna via the normally open NO contact of the T/R relay.

The above configuration using a filter at the output of a preamplifier
is the best one because it do not degrade the preamplifier Noise Figure
but reduces the bandwidth providing a bonus in selectivity.

It is possible to improve the above configuration introducing between
the antenna and input of the preamplifier a filter plus a preamplifier
protection device designed by K3PGP showing a very low insertion
loss less than 0.1 dB at 432 MHz that without to deteriorate the
preamplifier Noise Figure provides in addition another bonus of
selectivity at the preamplifier input as you can realize reading the
following page:


>From 1977 I was a 432 MHz EME operator so that I actually use for
satellite the above discussed setup both for 2 meters and 70 cm
because I live in the center of the gulf on Naples with a terrific RF
pollution in every amateur and out of amateur band so that a filter
at the preamplifier input and output is also mandatory to reduce the
preamplifiers gain compression due of out of band strong nearby

It is evident that using a filter at the output of a preamplifier it becames
a complicated matter using a commercial preamplifier with two coax
relays into it and by using only one transmission line to receive and to
transmit because the preamplifier output filter must be a big one to
pass all the transmitter power and particularly it must be tuned in such
a way as not to introduce a high VSWR into the TX transmission line.

Best 73" de

i8CVS Domenico

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