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Re: [TowerTalk] Motor Driven vac used in 40M DE?

To: Jim Thomson <>
Subject: Re: [TowerTalk] Motor Driven vac used in 40M DE?
From: "Richard (Rick) Karlquist" <>
Date: Wed, 30 Dec 2009 07:03:59 -0800
List-post: <">>

Jim Thomson wrote:
> ###  OK.   The consensus is the  F/B  will  degrade  when trying to cover a 
> major portion of the band.
>   But what about the forward gain ??     Does it degrade greatly... or just 
> a little bit ?????
> IOW..  I can live with  greatly degraded FB...  but if the  forward  gain 
> falls off over the cliff,  that's  unacceptable.
> Does the F/S  degrade as well ???     F/S  at this location is MUCH more 
> important than  F/B

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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