On July 28, 2020 7:45:50 AM MDT, Mike Seguin via AMSAT-BB amsat-bb@amsat.org wrote:
And here's a short text from Ian, GM3SEK about adjusting preamp gain. No test equipment needed. Too much gain can be a bad thing...
I completely agree. Too much gain can crush the front end mixer of your receiver.
One also has to make sure that out of band QRM isn't the cause. Case in point: we live about 1 mile from a water tower which has an impressive array of cellular antennas on its perimeter. I've measured approximately -50 dBm, preamp bypassed and at the end of the coax in the shack, at 700-800 MHz as received by the 70cm passive Lindenblad. Obviously, some low-loss filtering before the LNA was in order, and I'm even contemplating a SAW filter on the output to really clean it up. I've got plenty of spare gain.
Also, don't forget the ATT button on your radio! If you click on the attenuator and the noise floor drops significantly but the signal drops down much less, you're overloading the front end of your rig.
--- Zach N0ZGO