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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There is a current null at the end of the antenna, so the voltage is highest
there.
With 1/4 wave there is a current max at the base, voltage null.


"Bill Turner" wrote in message
...
On Wed, 10 Mar 2004 19:33:35 GMT, "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?

__________________________________________________ _______

That voltage is present at the END of the whip, not at the base.

--
Bill, W6WRT
QSLs via LoTW

Bill Turner wrote:
That voltage is present at the END of the whip, not at the base.

Bill, the voltage at the END of the whip and at the BASE of the whip
is not all that different for 8 foot whips used on 75m. That's why
the SGC-230 doesn't have a coax connector on its output and instead
has a porcelain insulator capable of withstanding thousands of volts.
--
73, Cecil http://www.qsl.net/w5dxp



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John Smith wrote:
There is a current null at the end of the antenna, so the voltage is highest
there. With 1/4 wave there is a current max at the base, voltage null.

One thing the poster forgot to mention is that the article is about
an 8 foot whip used on 75m (or other low frequency) a far cry from
"1/4 wave".
--
73, Cecil http://www.qsl.net/w5dxp



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Bill Turner wrote:
On Wed, 10 Mar 2004 14:37:30 -0600, Cecil Moore
wrote:
Bill, the voltage at the END of the whip and at the BASE of the whip
is not all that different for 8 foot whips used on 75m. That's why
the SGC-230 doesn't have a coax connector on its output and instead
has a porcelain insulator capable of withstanding thousands of volts.

That's true if you use a tuner like the SGC, which is separate from the
whip itself. I'm speaking of the classic whip with loading at either
the base or center, and fed at the bottom of the whip, BELOW the loading
coil. On those, the voltage at the base is quite low, relatively.

True, but irrelevant to the "March 2004 page 34 QST article" which is
the original subject.
--
73, Cecil http://www.qsl.net/w5dxp



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