Topband: Tophat? Does it have to be at the top?

[Jon Zaimes AA1K]

I haven't modeled, but in two actual cases -- one with 70 foot of Rohn 25, the other with 100 feet -- I had  a 204BA on the mast a few  inches  above the tower. On the 70-footer I added a Wilson 4-el 15m yagi (17-ft boom) about 5 or 6 feet above the 204BA. On the 100-footer I added the same beam 12 feet above the 204BA. 

In neither case was there any noticable shift in resonant frequency of the tower when I added the upper beam.

Both towers were shunt fed, with extensive on-the-ground radial fields.

Perhaps adding a larger beam above a smaller one would make a difference.

73/Jon AA1K

.At 04:46 PM 6/8/04, Earl W Cunningham wrote:
>Bill, K4XS wrote:
>
>"I have several 200-foot towers with lots of HF stacks.  I asked that
>question about a year ago when I was considering using them as verticals.
> Almost unanimously, the replies were that it would not be an effective
>way to go.  The general consensus was that the first big antenna the
>tower saw, "would be the end of the line" as far as loading went."
>==========
>I also once thought that the bottom beam of a stack was the only one that
>was effective in top loading a shunt-fed tower.  I changed my opinion
>after modeling numerous shunt-fed towers and found that all beams in the
>stack make a significant difference in the amount of top loading effect,
>not just the lowest one.
>
>73, de Earl, K6SE
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