Topband: 160 Vertical

K9AY K9AY" <
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').

73, Gary