I have a problem that most guys wish they were blessed with. I have a 200
foot tower which is pretty much in the clear and is guyed with Phillystran at
all points. It has a 5 element KLM 20 meter beam which of course has all
elements insulated from the boom. It also has a bunch of other big Yagis at
various heights from 27 to 165 feet on it.
I am aware that the 5 element beam on the top will further lengthen the
effective height from the 200 feet by (just estimating here) around 20-25
more feet. Will the other antennas which are mounted lower on the tower have
any loading effect? They are all element insulated from the boom and are 10
and 20 meter yagis. I realize any numbers are estimates due to the
complexity of the antennas mounted on the tower.
I would like to feed the tower as a vertical but am concerned about several
things.The effective height of the tower will be in excess of 1/4 wave, but I
believe even with the top loading probably is slightly less than 1/2
wavelength. I realize too, that generally the effective height of the tower
the more important it is to extend the radials beyond 1/4 wavelength. I am
limited to around 110 to 130 feet on 180 degrees of the antenna base and can
go somewhat farther on the other 180 degrees. Putting down 60 or more
radials would be no problem ( except for the work). I am aware of the need
to add capacitance and have a huge variable capacitor arond that looks like
it came from a California KW. My soil situation is kinda different. Near the
antenna it is a mix of sandy loam, and clay, probably average at best in
conductivity. However the water table of fresh water is never more than
three or four feet below the surface and I have cypress swamps as well as
drainage canals within 150 feet of the tower.
How effective would this antenna be compared to a regular 1/4 wave veritcial
with the same ground system? How do you think it would play compared to a
suspended wire slightly top loaded 1/4 wavelength long with an elevated
ground plane of eight wire at a base height of 50 or 60 feet? What do you
estimate the impedence of the shunt fed tower to be?
Thanks for any help.