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[CQ-Contest] Voltage Requirement for Variable Capacitor for

Subject: [CQ-Contest] Voltage Requirement for Variable Capacitor for
From: w8ji@contesting.com (Tom Rauch)
Date: Wed Jan 10 11:36:39 2001
Shunt   Feeding Tower
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 > Unfortunately You didn't explain if you are going to use a Gamma or an
 > Omega match, how tall is the tower and where you want to tap. In case of
 > gamma, the voltage of capacitor depends by various factors, mostly by
 > where the tap for shunt feeding is connected to the tower and also by the
 > height of tower and the amount of top loading. Roughly, as much the tap
 > point is raised up in the tower the voltage increases, as much as the tap
 > is lowered the voltage decreases but the required capacity increases. A
 > capacitor like those used for plate tuning in tube amplifiers could be
 > adequate, in terms of voltage isolation in most shunt feed applications.
 > 73,
 > Mauri I4JMY

Hi Mauri,

While everything you say is true in a general manner, we have to
be careful.

We must not confuse the across and through vectors!

The capacitor shunting the feedline can not possibly have more
voltage that seen across the feedline, because it is directly in
parallel (across) with the feedline.

The capacitor in series with the gamma wire see a voltage that is
reactance times the current because it is in series. Those are the
only things important.

Lower capacitance values in series with the gamma  will have more
voltage across them for a given current into the gamma no matter
where the tap point is.

I can have a tap point at 15 feet, and by using a cage of wires
reduce the capacitor voltage substantially with the same power!

73, Tom W8JI

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