[amsat-bb] Re: New Preamp

Gary Memory gmemory at tks-net.com
Tue Sep 18 11:26:03 PDT 2007


Greetings once again.  You sent me a note some time ago that helped to
amplify my current belief concerning SNR.

I am willing to bet two things.

First, it is difficult (not impossible, just difficult) to obtain a better
front end noise figure than what the original receiver manufacturer has
created.  Almost certainly the addition of an outboard amp will add more
noise.  And this is not always a bad thing.  A preamp can be a great
addition despite the addition of more noise.  But then you get into a curve
of cost vs true value of gain.  Is it worth it?  Maybe so.

Second, it is very unlikely that maximum signal strength will be at the same
point as maximum signal to noise ratio (SNR).  Again, maybe this is not a
bad thing in any one individual case.  Almost certainly the best SNR will be
at a point of gain somewhat less than maximum.

I work with receivers and preamps that cost dozens of thousands of dollars.
Cost and receive noise figure is almost a logarithmic curve.  And it seems
that only with the very best front ends are SNR and gain at the same point.

Once you get the best receiver, coax and antenna you can afford, that is all
you can do....which seems like a dumb and obvious thing to say.  Beyond
that, true magic is hard to come by.  And I am not knocking the preamp.
Again, I own and use them all the time.



-----Original Message-----
From: amsat-bb-bounces at amsat.org [mailto:amsat-bb-bounces at amsat.org] On
Behalf Of i8cvs
Sent: Tuesday, September 18, 2007 6:17 PM
To: Gary Memory; 'Jim Jerzycke'; 'Bob Stewart'; 'Amsat-BB'
Subject: [amsat-bb] Re: New Preamp

----- Original Message -----
From: "Gary Memory" <gmemory at tks-net.com>
To: "'Jim Jerzycke'" <kq6ea at pacbell.net>; "'Bob Stewart'" <bob at evoria.net>;
"'Amsat-BB'" <amsat-bb at amsat.org>
Sent: Tuesday, September 18, 2007 8:22 AM
Subject: [amsat-bb] Re: New Preamp
> I have a WACCOM cavity filter for both VHF and UHF that I'd like to use at
> the mast head, but they are too big to get up there (and they are not
> weather proof).  Oh, and they are only a couple dozen KHz wide, which
> tuning to a different frequency a bummer.
> Gary, N7BRJ/DA1BRJ


For my similar situation in the bay of Naples wich is full of RF pollution
I use the following K3PGP home brewed filter at the mast head in front to
the low noise 70 cm preamplifier and a 70 cm cavity filter in the shack in
front of my 70 cm downconverter or 70 cm receiver.

Please read the article.


Similarly for 2 meters I have scaled the dimentions of the above filter wich
is antenna mounted in front of the 2 meters preamplifier and a 2 meters
cavity filter of the type used for the FM repeaters is in the shack in front
of my 2 meters downconverter or receiver.

I remember receiving OSCAR-13 on 145.950 MHz I was able to locally
translate the satellite at 145.250 MHz FM transmitting with 2.5 watt using
another antenna to show locally the satellite performance without desensing
the bird.

This setup is obviously permanently tuned and devoted for receiving the
70 cm and 2 meters satellite bands and it works.

73" de

i8CVS Domenico

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