Topband: 160 Vertical
Thu, 17 Oct 2002 21:54:34 -0500
> Bill, K4XS wrote:
> "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.
> My estimate of the resonant frequency of the structure is that it is
> below 900 kHz (electrically longer than 1/2 wave on 160m).
> 73, de Earl, K6SE
My estimate is similar to Earl's...I hope resonance is even lower, because:
If your tower is close to 1/2-wave, you may have trouble obtaining a
shunt-feed match without extremely high voltages, a big impedance
transformation, and sensitivity to small changes in the environment
(been there, done that, shunt feeding a 300-foot FM tower as an AM
radiator high in the band).
Although it may not work with your low Yagis, another alternative for
feeding a tall tower is to make a ground plane by attaching 4 to 8 sloping
radials at 60 or 70 feet and shunt-feeding the tower above that point.
Modeling suggests that this arrangement would be an excellent performer.
Suspending a 1/4-wave wire vertical away from the tower should also
work well, since a 1/2 wave tower is anti-resonant (nearly 'invisible').