QST March 2004 page 34, upper left hand corner mentions that at 100 watts
the voltage at the base of an 8 foot mobile whip could exceed 1000 volts and
arc over a CB-ball. Can anyone plase show me the math?
tnx
Hank WD5JFR

"Henry Kolesnik" wrote in message
gy.com...
QST March 2004 page 34, upper left hand corner mentions that at 100 watts
the voltage at the base of an 8 foot mobile whip could exceed 1000 volts
and
arc over a CB-ball. Can anyone plase show me the math?
tnx
Hank WD5JFR

=========================
At what frequency ?

At a frequency of 10 KHz and 100 watts, an 8-foot vertical will have a
voltage across the feedpoint of many thousands of volts.

Calculation is easy. All you need to know is feedpoint reactance and
radiation resistance at the operating frequency. From then on its all Ohms
Law. Calculate the current which will generate 100 watts in the radiation
resistance. Multiply the current by the input reactance to give you the
volts.

The lower the frequency, the smaller the radiation resistance, the higher
the reactance and the higher the volts.
---
Reg, G4FGQ

Henry Kolesnik wrote:
QST March 2004 page 34, upper left hand corner mentions that at 100 watts
the voltage at the base of an 8 foot mobile whip could exceed 1000 volts and
arc over a CB-ball. Can anyone plase show me the math?

Assuming that an 8 ft. whip plus autotuner may be 0.5% efficient on 75m,
1/2 watt is reaching the 0.5-j2000 ohm feedpoint impedance predicted by
EZNEC. To get 1/2 watt into the 0.5 ohms requires a voltage of 0.5 volts
across the 0.5 ohms. It follows that the voltage across the -j2000 ohms
will be about 4000 times the voltage across the 0.5 ohms. So 4000 times
0.5 equals 2000 volts. That's obviously a ballpark figure but should
"show you the math".

On a damp morning at a 75m mobile shootout on the beach in CA, my 11 ft.
whip plus SGC-230 autotuner actually did arc across the antenna mount.

Question is: Why would anyone in their right mind tolerate 2000 volts
while radiating a measly 1/2 watt from a 100 watt transmitter?

Incidentally, that 11 ft. whip plus SGC-230 autotuner was a pretty
good mobile antenna system for 20m-10m.
--
73, Cecil http://www.qsl.net/w5dxp



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