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We made a plane wave antenna for high voltage peak potential
measurements by using a 30 x 30 inch piece of sheet copper. Near the
center portion we suspended another smaller 4 x 4 inch piece of sheet copper on
4 small ceramic standoff insulators. This center section fed the center of
a 50 ft long coax cable which the large copper piece fed the outer sheath.
This was all supported on a few corner insulating pieces of PE which held the
copper sheet 1/4 inch off the supporting wood 2 x 4 structure. We put it
on wheels for easy mobility around the lab.
Termination was into a precision 50 Ohm resistor. I
suppose one could also use a trim pot and trimmer capacitor if absolute accuracy
is required but since we are performing a calibration with a HV DC power supply
on each type of terminal we test (toroid, sphere, or oblate) we didn't feel this
extra work necessary.
We did the calibration using a 75 kV DC power supply and also
did a separate calibration with a 200 kV DC power supply with similar
results. We usually run a separate calibration for small coils with a 30
kV DC supply calibrated to NBS standards and use the 75 kV supply for the larger
coils and Van de Graaffs.
We operated this approx 12-25 feet away from larger coil and
6-8 feet away from smaller coils and obtained very consistent and reliable
results.
It measures the peak AC potential and the peak DC
potential. To keep the load small we also experimented with a small
electrostatic field mill voltmeter but due to the high impedance of modern
oscopes there was not a detectable difference.
We tested our "simple contraption" against one of Terry's
earlier plane wave antennas and there was no noticable difference. With a
small op amp inserted we were even able to measure charge on passing clouds on a
non-stormy day. During a storm the op amp was not necessary.
Dr. Resonance
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