[Lowfer] Vertical antennas and top loading

[JD]

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