[amsat-bb] Re: fun with preamps

n1jez@burlingtontelecom.net n1jez at burlingtontelecom.net
Tue Mar 16 03:28:12 PDT 2010

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)

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

More information about the AMSAT-BB mailing list