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Capacitance of a cylinder coil (and coil model)
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From: Antonio C. M. de Queiroz [SMTP:acmq-at-compuland-dot-com.br]
Sent: Thursday, April 09, 1998 1:37 AM
To: Tesla List
Subject: Re: Capacitance of a cylinder coil (and coil model)
Jim Lux wrote:
> Is that for a closewound or a spacewound coil? I assume it is some sort of
> empirical curve fit to the tabular data, much as the Wheeler equations are
> an empirical fit to inductance data.
Apparently, the formula is intended to work for any reasonable turn spacing.
Obviously it must fail for a single turn...
I don't have the original paper (see my other post) to see how was this
formula derived.
A large (actually, any) coil behaves as a transmission line, not as a simple
inductor with a parasitic capacitance, what is only a (good) approximation.
But the model is not a simple TEM mode transmission line. I have checked this
making measurements of the resonance frequencies (applying a grounded signal
generator with a resistor in series to the lower end of a vertical coil and
searching for the voltage minima at the base of the coil). I found that the
resonances are all significantly shifted to low frequency, not following the
relation f, 3f, 5f, 7f, ..., where f is the frequency of the first resonance,
predicted by the model for a TEM line with open end (no top load).
The experiment is simple, and it is easy to observe the standing wave pattern
in the coil by just sliding a finger along it. With the finger over the voltage
nodes the voltage at the base returns to (close to) the minimum value.
Could someone try to make this measurement? I would be interested in see what
are the resonances found and if the voltage nodes are at the expected positions
(in my tests they are).
Antonio Carlos M. de Queiroz
http://www.coe.ufrj.br/~acmq