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[TowerTalk] measuring beam forward gain

To: <>
Subject: [TowerTalk] measuring beam forward gain
From: "Al Williams" <>
Date: Wed, 30 Dec 2009 09:05:59 -0800
List-post: <">>
What errors may be caused by:

The signal generator being too close (or too far away) --how far away is ok?
The signal generator being too low in height?

Although antenna modeling programs will display a 3d gain pattern, it seems 
difficult to me to quantify the gain.  Thus one uses an azimuth gain pattern 
for specific elevation angles. Can the rotational gain difference change for 
different elevation angles?


> My MonstIR experience is only a guide, since "YMMV" as they say.
> However, I found that I lost 2 dB of forward gain by being off by 100
> kHz on the parasitic elements.  It was quite a bit worse at 200 kHz off.
> There was a difference between being too high in frequency and being
> too low in frequency by the same amount, but I don't remember which
> degraded faster.  In any event, you need to test your own beam to see
> what it does.
> To do these tests, set up a sig gen as far away as you can from the
> beam, and listen to it with your receiver's AGC off and RF gain adjusted
> appropriately.  Tune in a nice CW tone, and connect an AC voltmeter
> to the speaker terminals.  Now you can measure AF voltage as the
> antenna rotates.  Since you are operating the receiver linearly,
> the AF voltage will track the RF voltage.  dB is 20 LOG V1/V2 as
> we all know, etc.  Some voltmeters can read directly in dB,
> such as the better HP models.  There is a computer program (somewhere in
> cyberspace :-) that uses your sound card as a voltmeter and gives
> a nice plot if your rotator speed is uniform.
> Rick N6RK


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