[amsat-bb] Re: Mirage Preamp Setting

Mike Seguin n1jez@verizon.net
Mon Aug 21 03:33:49 PDT 2006

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.

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
> Richard Lawn, Dean
> College of Performing Arts
> University of the Arts
> 215-717-6125

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