[Lowfer] Vertical antennas and top loading

[Bill Ashlock]

"The biggest logical flaw is the apparent assumption of uniform capacitance 
per linear foot of conductor, no matter where it is located in relation to 
the ground!"

I buy that 100%, JD. I have read numerous Lowfer antenna analyses that make this mistake. The nature of the electric field from the upper section must be determined in order to predict the capacity to ground and this is not simple. Even the capacity a simple horizontal plane to the ground plane involves some complicated math. If the spacing between these two planes is a small fraction of the diameter of the top hat a simple formula can be used.... but that's never the case for real antennas. It's the fringe field off the edge of the top hat, that is not small, and causes the complexity of the analysis. 

Bill

> From: listread at lwca.org
> To: lowfer at mailman.qth.net
> Date: Wed, 13 Jul 2011 18:21:36 -0500
> Subject: Re: [Lowfer] Vertical antennas and top loading
> 
> I was a bit taken aback by the imprecision of the very first sentence in the 
> white paper (a rather less definite meaning of the word "defined" than one 
> might expect from an engineer), but continued to read with interest anyway. 
> Electromagnetics is admittedly not my strongest subject, but in the end, 
> while there is still reason to believe the Nautel approach is preferable to 
> the Southern Avionics design, I'd have to fault the author's logic in 
> support thereof.
> 
> The biggest logical flaw is the apparent assumption of uniform capacitance 
> per linear foot of conductor, no matter where it is located in relation to 
> the ground!
> 
> In the SAC diamond configuration, the top half of each conductor is at least 
> partially shielded from interaction with earth by its lower half, so unlike 
> a single vertical conductor, the whole thing could be expected to exhibit an 
> effective height of LESS than half the physical length.  It's not a very 
> good arrangement.
> 
> On the other hand, the Nautel configuration is akin to accepted and 
> long-proven LowFER practice; but the calculation of effective height based 
> on those assumed Cm and Ch values is a poor one that does not take into 
> account the vector components of current flow in the individual elements 
> (that is, the "downward" flow in the conductive section of the guy wires 
> cancels part of the radiation from the "upward" flow in the upper part of 
> the mast).  In practice, the true effective height will be somewhat less 
> than that calculated by the author's method.
> 
> (Consider: if the proportion of top hat capacitance to vertical mast 
> capacitance were really the key, then taking it to the logical conclusion, 
> you would run the conductive part of the guy wires almost to ground and tie 
> them together there.  Maximum Ch/Cm ratio that way, but very little 
> radiation!  Empirically, Mike Mideke and others have found that a drooping 
> top hat which obscures more than the top half of the mast becomes 
> counterproductive.  Some question whether it should obscure much more than 
> the top third for maximum Rr.)
> 
> Although the Nautel chap's reasoning is flawed, it errs in the same 
> direction for both antennas being compared...so it is still true that one 
> performs better than the other, just not necessarily by the amounts claimed. 
> In other words, follow the general principle illustrated, but don't plug the 
> numbers into any further design computations and expect precise solutions.
> 
> John
> 
> 
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