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Shorted top turn
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From: Gary Johnson [SMTP:gjohnson-at-ksu.edu]
Sent: Thursday, February 19, 1998 1:37 PM
To: tesla-at-pupman-dot-com
Subject: Shorted top turn
I recently made some puzzling measurements, and am wondering if anyone else
has noticed this effect.
I am measuring the input impedance and the top voltage of a coil above a
ground plane. (Think of a vertical quarter-wave antenna above a ground
plane). The coil is 387 turns of 14 ga magnet wire on a polyethylene form.
The diameter is 15.5 inches and the length is 45.5 inches. The coil is
space wound. The measured inductance is 17.0 mH. I am driving the coil
with a function generator at about 30 volts. At resonance, current is in
phase with the voltage, so by simply measuring the voltage and current, one
can calculate the input impedance.
To measure the top voltage, I use a capacitor divider inside the coil. I
lay a conducting lattice over the top of the coil and connect it to the top
turn, to form the top plate. One or more toroids are placed on top the
lattice for top loading. The column of air in the coil is the dielectric of
the high voltage capacitor. The lower plate is a small sphere mounted on a
metal box centered in the bottom of the coil. The box contains the low end
capacitor of the divider, a buffer, 9 V batteries, and a fiber optic
transmitter to send the signal to a matching receiver in the instrument room.
I mounted the lattice in a loop (turn) of one inch copper tubing, which then
forms turn no. 388 when installed. The ends of this turn are about one
eighth inch apart, so the turn is easy to short by wedging in a piece of
copper. The turn can be thought of as a skinny toroid.
Anyhow, what I observed when I shorted the top turn was the following:
1. No change in resonant frequency.
2. Input impedance decreases about 10 percent. With a strong voltage
source, this means the current and power input increase about 10 percent.
3. The top voltage increases about 10 percent, maybe even 15 percent.
The last item was a surprise to me. It is standard wisdom that a shorted
turn at the bottom is bad. It would seem then that a standard toroid should
also be bad. I have read arguments that the magnetic fields and currents
are small enough at the top of the coil that it really doesn't make much
difference, so go ahead and use the solid toroid. This makes good sense, so
I was not expecting to see an improvement. My results imply that one could
place a shorted loop of quarter inch copper tubing right on the top of the
coil and see greater power input and longer sparks with no change in tuning.
Has anyone else seen this? If it is real, can anyone explain the phenomenon
to me?
Thanks,
Gary Johnson