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

[Peter Sears]

Back 15yrs ago when I first installed my 130' rotating tower I first put 
up a 4 element 20M beam at 60'. I  installed a shunt feed 2' away from 
the tower and the tap at 58'.  This tuned very easily with my 500pF 
variable air cap on 160M. I then installed an identical 20M 4 element 
beam at 120' and a 40-2CD 10' above the top 20M beam at 130'. The top 
two beams really messed up the match and made the tower look extremely 
long. I took down the shunt feed and erected my 4 square without regret.

Peter
W5PS

Jon Zaimes 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.
>
>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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>
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