[amsat-bb] Re: 70cm preamp

Gary Memory gmemory at compuserve.com
Wed Sep 27 18:20:13 PDT 2006

I'm sure there are other and perhaps better ways, but in my work (similar
but not FO29) I use a single cavity filter.  I had WAACOMM build one for me
to my spec, but I note they are now out of business.  The can (for 2 meters)
is about 8 inches in diameter and about 2.5 feet tall.  N connector in and a
N connector out.  I can move my simplex frequency around about 20 - 30KHz
before I need to retune the filter.

Yes, this is a brute force and expensive method, but by golly, it sure works
well for me.  This filter creates a nice, low noise-floor with just about
any on-air transmitter in my shack.  I loved it so much; I bought another
(smaller) filter for 440 as well.

There are other filters out there as well...but the method I've chosen works
best by far for me.

Finally, I've found that for any one given situation, an attenuator is as
useful as a preamp.  It all depends on your total noise figure and link
budget.  Also, I feel antenna gain is "more better" than a pre or power amp.

In the end, it sounds like that unless you can add gain or a brute force
filter, reduced power in your nearby TX is possibly the only other method.
But then I'm sure others on this list will tell me I'm full of crap.

Good luck es 73!!

Gary Memory, N7BRJ

-----Original Message-----
From: amsat-bb-bounces at amsat.org [mailto:amsat-bb-bounces at amsat.org] On
Behalf Of Bill Bruno
Sent: Wednesday, September 27, 2006 7:04 PM
To: Amsat BB
Subject: [amsat-bb] 70cm preamp

On my V/U setup I have a SP7000 70cm preamp. Working FO29(JAS2) my 2 meter 
signal overloads the preamp, my signal splattering like an adjacent strong 
station. With preamp off the problem is not there. In testing I found the 
overload happens in any combination of 70cm 2m frequency pairs. I can 
eliminate the problem by lowering the tx power, but by doing so I have a 
hard time copying my own signal. How can I eliminate the 2m sig overloading 
the preamp while still maintaining  adequate power?
Antennas m2 436cp30,  Gulf Alpha 2m5el, both right hand polarization.
I could use some suggestions.
Thanks Bill 

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