Tom,
Yes, you are right. I think my feedpoint resistance scenario is
oversimplified. But we ought to bring it back to an answer for Scott. I
have a feeling he is itching to get something on the air. So let me state
that where the hamstick radial idea might make a kind of sense on a well
elevated vertical, it would be mostly useless on a ground mounted vertical.
This is because radials mostly serve a different purpose in the two
different cases.
So, let me suggest that in the case of a ground mounted full size 1/4 wave
vertical, as you add ground radials, the feedpoint resistance will drop as I
said and it will be a good indicator of increasing efficiency. You will
reach a point where further gains per radial are small by that measure.
This is a reasonable point for most of us to stop adding radials. If you
bring in Tom's cases, my proposal starts to break down. For example, field
strength readings are the only way to measure improvements in a heavily
loaded short mobile antenna.
Tom, if you feel that even in my full size grounded vertical case the
feedpoint readings don't tell the story, I am inclined to believe you. But
then what is a practical way for us weekend hackers to measure field
strength in a way that translates to real performance improvements globally?
Are the near field readings near the ground sufficient to indicate useful
improvements in efficiency, or do we need far field measurements. For
example, can I put an FS meter on a tree stump on the other end of my 4 acre
property and go by that? What would you recommend?
Dudley - WA1X
-----Original Message-----
From: Tom Rauch [mailto:w8ji@contesting.com]
Sent: Friday, October 22, 2004 3:34 PM
To: Dudley Chapman; towertalk@contesting.com
Subject: Re: [TowerTalk] RE: TowerTalk Digest, Vol 22, Issue 80
> One caveat, though. The scenario above, of bringing 60
ohms down to 32
> ohms applies to a vertical element that is physically
close to 1/4 wave. In
> that case, the approx. 32 radiation resistance of the
vertical element
> radiates half the power and the rest goes into heating up
the 30+ ohms of
> ground resistance.
We have to be very careful assuming distributed losses are
directly conveyed to the feedpoint and that they are always
conveyed without modification by standing waves in the
ground system! There are a lot of things going on in that
"soup" we call ground.
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|