Re: Calibrating E-field probes by simulation?

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Original poster: "Dr. Resonance" <resonance@xxxxxxxxxx>



This works fine if you fire it in a single pulse mode, ie, diode string and
a string of hi-meg resistors so it fires once every 5-10 sec.

Dr. Resonance
>
> 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
> FANTC.
>
> 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
> instance:
>
> 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
> etc.
>
> 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
>
>
>