Hi Bob,
Here's a method I've used for setting preamp gain. Got it from GM3SEK.
===== 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) ====
73, Mike, N1JEZ AMSAT 29649 "A closed mouth gathers no feet"
Hello
Just installed a AG-25 (Icom preamp) on my 2M cp yagi. No preamp noise
Smeter=0, preamp on Smeter=7. Signals "appear" better after backing down the volume with the preamp. I have 8-10 feet of LMR400uf from antenna to preamp, 50 feet of LMR400 to shack. I suppose I was expecting more "dramatic" results, I realize the AG-25 isn't the greatest and best, but it works well with the 910. Are these real world results for what I have. S7 noise level seems a bit high.
as always-thanks
73 Bob W7LRD