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[TowerTalk] finding resonant frequency of a shunt-fed tower for 160

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Subject: [TowerTalk] finding resonant frequency of a shunt-fed tower for 160
From: (Tom Rauch)
Date: Mon, 30 Jul 2001 14:13:05 -0400

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 

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 

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