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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