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[TowerTalk] Shunt Feed Capacitor for Tower

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Subject: [TowerTalk] Shunt Feed Capacitor for Tower
From: (Tom Rauch)
Date: Fri, 8 Feb 2002 19:51:15 -0500
> With only a 4 to 1 frequency ratio, I expect it may be a little hard
> to make even an 'ideal' choke that will look like a short at 1.8 MHz
> and an open at 7 MHz since X = 2 pi F L.  At 14 MHz, you have an 8 to
> 1 ratio to work with which is quite a bit better, as long as you be
> careful to keep the self resonant frequency of the choke above 28 MHz.

Keep in mind the impedances, and the problem gets easy to solve.

The center point of the elements has such a low impedance is 
makes virtually no difference at all if it is directly grounded. You 
can verify that in a model. 

That being the case, even a hundred ohms or so reactance at the 
elements lowest operating frequency has no effect at all on the 
yagi (with the exception of driven elements, which can be grounded 
effectively with an air-core choke balun of several hundred ohms 

On 160 meters, the current flowing into the element is low, so even 
a moderate reactance does a VERY effective job of keeping voltage 
nearly at zero volts. Several hundred ohms of reactance would take 
voltage to very low levels.

As an additional benefit, the element has a static drain and won't 
"charge up" and flash across the insulation.

The best solution electrically and mechanically is to use an RF 
choke. It's simple, reliable, automatic, and more rugged and has no 
ill effects on any band.
73, Tom W8JI 

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