Pete,
You can not *easily* find the resonant frequency of your grounded
tower. You could measure it by using making a clamp-on
transformer to fit around the tower, but the thing would need a large
core!
You might get away with connecting a short wire several feet above
ground through the analyzer or with a few turn link to a grid dip
meter, the inductance of that wire will move the reading slightly but
not far.
But who cares? The only problem would be if the tower was
electrically way too long..more than 3/8th wl or so.
> I have a C31XR at 72 ft - The elements are -not- grounded. Thus I assume
> much less (or none?) capacitance hat effect. I therefore assume I can
> still work out a shunt feed for -some- frequency(s), but they will likely
> be higher freq than if I had grounded elements and therefore more
> capacitance at the top?
Be careful of that! The voltage between the elements and the boom
can be extremely high. I modelled that for someone on the topband
reflector, and it could be in the kilovolt range.
Many people burn up insulators on antennas or choke baluns by
shunt feeding towers.
I'd be absolutely sure to ground all the elements through a small
choke to the boom.
You want the reactance to be less than a few thousand ohms on
160, and more than a few dozen ohms on the bands the yagi is
built for. You can get the formulas from the Handbook, I'd use at
least number 14 wire in a nice single layer. The exact values are
not critical...probably values as low as a couple dozen ohms (up to
many hundreds of ohms) reactance will work.
73, Tom W8JI
W8JI@contesting.com
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