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Re: Calibrating E-field probes by simulation?

Original poster: Terry Fritz <teslalist@xxxxxxxxxxxxxxxxxxxxxxx>

Hi Steve,

I have not really tried this but I "think" it would work.

Set the coil and antenna up so they "look right". Don't let the antenna get hit by a streamer :o))

Hook a signal generator up to the base of the secondary and turn it to max known voltage.

With the scope, fiddle with the frequency to find the Q. Q = fo / (f2-f1). f2 and f1 are the -3dB frequencies.

The top voltage is Vo = Q x signal input. Note that at fo, the signal generator may be loaded down so the voltage at fo needs to be checked again there.

So if you put 10V into the base of the coil with a Q of say 150, the top voltage is 1500V. Then you can just up the range on the scope 100X or as needed.

Might need a frequency counter to get good frequency numbers but many plain meters do that now days.

Don't even think about the divider down the middle of the coil ;-))) That is a nightmare!!

There are all kinds of ways (I use HV DC power supplies as I think does Dr. Resonance). But the above is fast and simple with equipment you probably have. If the above works, I will do that too from now on. You can also use secondary base current to find the top voltage (jwFoL). I think you need to adjust the inductance factor though per FANTC.

I have done it with direct scope probes too but that only works with big coils and very high-Z little probes that don't affect things much (TEK P5100).

If you know the coil well, your can just trust what the models say ;-))) But that is easier once you "know" the voltage is true...

Cheers,

        Terry



At 07:46 AM 4/4/2005, you wrote: 
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
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