[Lowfer] Vertical antennas and top loading

[Bill Ashlock]

JD,

It always bothered me that most formulas found in antenna books for top hat capacity indicated it was a linear function of top hat diameter while engineering text books indicated the capacity between two closely-spaced conductive planes is a function of the area (or the square of the diameter). The discrepancy of course is the "closely spaced" condition that swamps out the fringe capacity effect. When I was experimenting with the placement of a counterpoise under the top hat of my Lowfer vertical as a way of preventing the antenna current from entering the highly lossy ground up here in the mountains, I found the counterpoise had to be at least twice the diameter of the top hat in order to capture 90% of the antenna current. This result got me to search back to one of my Fields and Waves text books from many years ago (and this was my poorest subject... complete with a professor that none of us could understand :)  Guess I'll have to revisit this whole topic some day as that vertical with is very low Rac beat the pants off my companion loop for radiated signal at 1w.

Bill

> From: listread at lwca.org
> To: lowfer at mailman.qth.net
> Date: Wed, 13 Jul 2011 23:12:45 -0500
> Subject: Re: [Lowfer] Vertical antennas and top loading
> 
> >>> It's the fringe field off the edge of the top hat, that is not small, 
> >>> and causes the complexity of the analysis.
> 
> Complex enough to put one's sanity in doubt for willingly working with such 
> things.  Hence the origin of the term "lunatic fringe," no doubt. :)
> 
> 
> 
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