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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