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

[Earl W Cunningham]

Jon, AA1K wrote:

"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."
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I can't explain your findings, Jon.  Those that I modeled for with
stacked beams said the shunt feed parameters I gave them worked out
exactly correct.
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"Perhaps adding a larger beam above a smaller one would make a
difference."
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I've never modeled an "upside-down Christmas tree" like that, but I agree
that the difference will be even more pronounced in that case.

The models I've done have the smaller beams above the larger ones, some
with even a 2-meter Yagi on top (which also makes a significant
difference).  Keep in mind that the higher beams in a stack have more
effect on top loading than the lower ones (which also makes logical
sense) even if they are a bit smaller in size.

Even beams that have their parasitic elements insulated from the boom
(where only the boom and driven element provide any top loading) make a
significant top loading difference above a beam with all parasitic
elements grounded to the boom.

73, de Earl, K6SE