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...
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Here's a method that requires no test equipment at all. It comes from G4DGU, who designed all the original muTek transverters and outboard preamps to have adjustable gain. This method uses the sharp threshold effect of FM detectors at low S/N ratios, and it allows you to optimize the preamp/transverter gain for your local band noise conditions.
1. Turn the transverter/preamp gain well up.
2. Find a very weak but steady unmodulated carrier (off-air, not from a signal generator or a local birdie). Rotate the antenna until you can just detect the signal in FM mode.
3. Reduce the preamp/transverter gain until you hear the noise increase. The FM threshold is sensitive to a small fraction of a dB in S/N.
4. Increase the gain just a little,to the point where you can't hear the quieting improve much.
5. Switch back to a real DX mode.
Remember that every dB of unnecessary preamp/transverter gain will probably subtract almost 1dB from your system intermod intercept!
The penalty of adjusting the gain correctly is that you're living just above the "knee" where S/N will begin to deteriorate rapidly if something changes. It's worthwhile to repeat this test every few months - especially just before a contest.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book' 'In Practice' columnist for RadCom (RSGB)
On 7/28/2020 9:28 AM, Leffke, Zachary via AMSAT-BB wrote:
An EXCELLENT topic of conversation, and one I keep circling back to over and over and over.......
-Zach, KJ4QLP
On July 28, 2020 7:45:50 AM MDT, Mike Seguin via AMSAT-BB [email protected] 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
participants (2)
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Mike Seguin -
Zach Metzinger