I've recently become aware of Tom Russel (N4KG)'s elevated ground-plane
antenna article [QST, June 1994, pp 45-46] (courtesy of W0BV). I had
been toying with shunt feeding my (windmill) tower, but the reverse-fed
GP method seems preferable. In working through the calculations,
though, I've come up with something that seems a little
counter-intuitive, so I thought this forum might help me make sense of
what I've done so far.
I have a 40' windmill tower, which supports a 10' mast. At the very top
of the mast is a CushCraft ATB-34 (tribander; like a W4S, just older).
At the bottom of the mast is a 3-element 17m homebrew yagi. Three feet
above that (in between the tribander and 17m beam) is a 5-element
CushCraft 6m beam.
I calculate the areas of the big beams as approximately 508 and 338 sq
ft, top and bottom. I figure I can neglect the 6m beam since it's so
small by comparison. So now if I calculate sqrt (2 * area) following
the formula in N4KG's article, I get 32 and 26 sq ft respectively for
top and bottom beams. So, an 80 m vertical, at 65' for quarter-wave
would only need to be 65 - 32 - 26 = 7 ft high! Not only does this seem
ridiculously small, but mechanically it would put the attachment point
for the elevated radials in the middle of the mast section, which is
clearly impractical.
Now, this computation implicitly assumes that I can add the height
equivalent of the sq ft contributions of each of the beams. Looking at
this as a parallel combination of two capacitances seems to be sensible,
and of course in that case they would be additive. But the remaining
requirement of 7 feet is just too small.
So, considering the problem as I've laid it out, can anyone suggest
where I'm making my error (if I am), or suggest how to approach the
physical construction of such a beast (if my calculations are alright)?
I appreciate you considering this problem.
--
73,
Greg, KØGW
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