Hi Rick -
Measuring speaker voltage is an easy idea that could be refined by changing
the sig gen output by known amounts to get a "calibration" curve or, more
conveniently, put sig gen directly into the rx, at greatly reduced level, to
give the same audio output level, to get "calibration" curve.
Gene / W2LU
----- Original Message -----
From: "Richard (Rick) Karlquist" <richard@karlquist.com>
To: "Jim Thomson" <Jim.thom@telus.net>
Cc: "Gene Fuller" <w2lu@rochester.rr.com>; "Jim Walker"
<jim@walkersdomain.com>; <towertalk@contesting.com>
Sent: Wednesday, December 30, 2009 10:03 AM
Subject: Re: [TowerTalk] Motor Driven vac used in 40M DE?
>
>
> 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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