RadioBanter - View Single Post

June 17th 06, 07:26 AM posted to rec.radio.amateur.antenna

On 16 Jun 2006 19:18:44 -0700, wrote:

Thanks Richard and Tom D.

I was worried for a while mythology and the magic dust (not Cheech and
Chong style, but Texas style) would overshadow what really goes on.

Richard Clark wrote:
"few persons realize that when a person stands
in an open field on a clear day, his head has a
potential approximately 300V more positive than his feet
and .... The gradient averages about 180 V/m over land
in the summertime."

We have to be careful with that!

Actually the impedance of the field is very high. When a person stands
in an open field he actually perturbs the electric field very close to
him because his body resistance is very very low compared to the
impedance of the electric field. His feet are not really 300V more
negative than his head unless you would replace everything below his
eyebrows and above his ankles with a very good insulator.

Hi Tom,

Quite true. Its as if a needle has penetrated the equipotential
shells over that 300V span. Streamlines would probably reveal a dead
short to what are in the distance 300V/90aA = 3,333,333 GOhm resistive
paths.

The author's intent was to design a plane's auto leveling (in flight)
mechanism by sensing the voltage differential while banking. He
accomplished this by placing sensors at each wing tip turning the span
into a bridge circuit. He wasn't sensing voltage, but instead using
current. The plane would be already shorting out the potential, so he
had to turn this lemon into lemonade.

The problem was that the sensors had to grab hold of the current
before the massive short hogged it. The solution was to use mildly
radioactive isotopes to "make good electrical contact with samples of
air at two different points in the vertical potential gradient...."
Those radioactive sources were 500 µCurie Polonium Alpha emitters -
the kind I used to remove static from records back in the early 70s
(your solution to taking care of paper static, Tom D.).

73's
Richard Clark, KB7QHC