[Amps] Voltage at feed point

[Roger D Johnson]

Paul Christensen wrote:
>>Anyone know a formula for calculating the voltage at the feed point of a
>>vertical if you know the resistive and reactive values and the power?
>>Example:  160 meter inverted L, R = 80 ohms, X = +420j & P = 1500
>>watts.  73, Rob, NC0B.
> 
> 
> The resulting Z is 428-ohm.  At 1500 watts, I calculate 800 volts at the 
> base.
> 
> I first solved for Z by taking the overall square root of the R-square + 
> X-square value.  I then solved for E through P = E-square x Z.
> 
> Correct?
> 
> Paul, W9AC 
> 
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Hi Rob....

Paul's analysis is correct as far as it goes. However, I presume
you will be using some sort of matching system at the base of the
antenna. Let's assume you use a series capacitor of -j420 to cancel
out the inductive reactance of the antenna. This leaves you with a
resistive load of 80 Ohms and a current into the antenna of 4.33
Amps RMS. This current flowing through the -420 Ohms reactance of
the capacitor will produce a voltage drop of 1819 Volts RMS or 2572
volts peak. The capacitor and base insulator will have to be rated
to withstand this voltage.

73, Roger

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