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[TowerTalk] Reverse-fed groundplane tower on 80m--sanity check

Subject: [TowerTalk] Reverse-fed groundplane tower on 80m--sanity check
From: K0GW <>
Date: Tue, 04 Nov 2008 12:41:18 -0600
List-post: <">>
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.

        Greg, KØGW


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