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Calibrating E-field probes by simulation?

Original poster: "Steve Conner" <steve.conner@xxxxxxxxxxx>

Hi all,

I was just thinking about ways of measuring the output voltage of a coil.
E-field probes like Terry's planar antenna are a nice solution but the
problem is how to calibrate them. It struck me that if you were to use a
probe with an easily modelled geometry (a small toroid or sphere?) then the
calibration could be done using a finite element modelling program like

All you would need to do is have the program calculate the capacitance
between your coil's topload (plus some fraction of the secondary self-
capacitance I suppose?) and the probe. The division ratio of the probe is
approximately (capacitance between probe and coil)/(capacitance between
probe and ground including cable and scope input cap)

This may only have to be done one time to generate a calibration table. For

To get a 100000:1 ratio, place the probe
6 feet from a 12" toroid
9 feet from a 24" toroid
11 feet from a 36" toroid

Can anyone comment on the feasibility of this, and likely sources of error?
Streamer capacitance is an obvious one but I assume you could use a breakout
point to divert streamers as far from the probe as possible.

An interesting way of calibrating E-field probes by experiment might be to
drive the base of your secondary from a Marx generator. You could assume the
topload will "resonant charge" to twice the Marx output voltage.

Another possibility for voltage measurement might be to build a capacitive
divider using a long string of small capacitors down the inside of the
secondary itself, rather like the load capacitor Terry built for his
magnifier. Maybe 100x 100pF 10kV ceramics in series, with a 0.1uF to ground,
to give a 100k:1 ratio. The divider would be shielded against stray
capacitance errors this way since the voltage gradient down the middle of
the secondary should be almost the same as the gradient down the divider
(both in space and in time). IOW, you're using the secondary as a guard.

But I don't know what you would ground the bottom of the divider to. High
dv/dt on the toroid would induce high currents through the capacitor string
and maybe spike the ground to dangerous levels.

Anyone have any thoughts on this?

Steve Conner