Greg, This is a great antenna and I used one for about 8 years. My
situation was a bit different, I had 105' AB105 tower and had two large
monobanders above it. I dropped a wire from the top of the tower to the
bottom with a two turn inductor at the bottom attached to the bottom of the
tower. I then dipped the tower and found it to resonate in the broadcast
band. I determined that I needed to go to the 45 foot mark of my tower and
attach 2in pvc tubes about 6 inches long to each tower leg.
at the other end I ran a solid copper wire #6 through the pvc tubes and
attached four 134' radials to this copper wire ring. I then attached my
coax center conductor to the ring and braid to the tower. I matched it with
two 1/4 wave length of coax in parallel and a piece of coax with 100 #73
beads to the feedpoint. It gave a wonderful match and worked very very
well. The reason I took it down was that I downsized and sold the tower. I
was told by a number of guys that antenna cannot work, well I am here to
tell you mine worked great. Good luck. The above is just a brief
description of what I did. If you have any questions drop me a note.
73 Pete N4KW
----- Original Message -----
From: "K0GW" <k0gw@arrl.net>
To: <towertalk@contesting.com>
Sent: Tuesday, November 04, 2008 1:41 PM
Subject: [TowerTalk] Reverse-fed groundplane tower on 80m--sanity check
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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