N4XD@aol.com wrote:
>
> Here we go again!! Who gives a RAT if the antenna was tested by Los Alamos??
> As I recall the original question was if the antenna would work. Can't we
> keep the comments limited to that??
Hi XD,
My irritation is not with Uni-hat, it is with McCoys review and his
claims about radiation resistance. Such rubbish causes people trying to
understand antennas to walk away with total misconceptions of how things
work.
There is no reason for these things to turn into "squabbles". Facts are
facts, and are available in dozens of engineering and Physics textbooks.
Of course the antenna will radiate, but it will radiate about the same
as any other 31 foot high antenna with a 15 foot diameter hat. Some
might be better, and some might be worse, but NONE will be as good as a
full size antenna over the same ground system....(And I'll bet money on
that.)
It's no secret that a large hat on top improves a vertical. The
improvement in radiation resistance is caused by the change in current
distribution in the radiating structure. Maximum radiation resistance
occurs when the current distribution is uniform and in-phase over the
entire area of the radiator.
Without a hat, the current distribuition would almost be triangular in
shape. With a lumped capacitance at the open end that is much larger
than the distributed capacitance of the radiator, most of the current
terminates in the end capacitance and the entire vertical radiator
carries uniform current and radiates equally throughout its length.
In a short antenna, radiation resistance differences between the
extremes given above are a factor of approximately four times. The loop
radiation resistance of a 31 ft vertical on 160 is about 1.3 ohms with
no top loading, and about 5 with top loading. Folding the wires like a
folded unipole causes no impovement in ground loss or loop radiation
resistance. As a matter of fact making the radiator THICKER actually
DECREASES radiation resistance and increases ground loss because it
increases current taper in the element!
Claims "increased radiation resistance" results from "folded dipole or
unipole effects", a thicker element, and/or a shunt coil at the bottom
are technically false.
Either that or Kraus, Jasik, Jordan, Balmain, Termin, and others right
back to Maxwell are all wet, and all the Physics books are wrong.
While I DO AGREE this antenna is very well constructed and a good basic
design, it is no better efficiency wise than any other hat loaded
vertical of the same dimensions could be.
In any case, if I had an antenna with a loop radiation resistance
between 1 and 5 ohms I would put a ***VERY*** good ground system.
Without a good ground, no matter how many zig-zags of wires run up and
down the radiator, you can bet efficiency will stink.
73 Tom
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