Hi Rick,
Here's a simple method for determining the proper amount of gain for a preamp. It does talk about preamps with continuously adjustable gain, but you should at least be able to get in the ballpark with your Mirage..... I've actually gone inside of some ARR's I've used and added a T pad to set the gain... YMMV.
73, Mike N1JEZ AMSAT #29649 "A closed mouth gathers no feet"
==== 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)
----- Original Message ----- From: "Richard Lawn" rlawn@uarts.edu Subject: [amsat-bb] Mirage Preamp Setting
I'm about to mast mount a new Mirage 70cm preamp. It has a high/low setting. Any suggestions from the group as I'm sure many of you are using this preamp. Is the high setting too much gain? I got tired of forgetting I had the ARR preamp on an frying it by sending too much RF into it. 73 Rick W2JAZ
Richard Lawn, Dean College of Performing Arts University of the Arts 215-717-6125