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

 To: towertalk@contesting.com [TowerTalk] Reverse-fed groundplane tower on 80m--sanity check K0GW k0gw@arrl.net Tue, 04 Nov 2008 12:41:18 -0600 mailto:towertalk@contesting.com>
 ```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 _______________________________________________ _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
 Current Thread [TowerTalk] Reverse-fed groundplane tower on 80m--sanity check, K0GW <= Re: [TowerTalk] Reverse-fed groundplane tower on 80m--sanity check, Pete Raymond, N4KW