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Re: [TCML] [Secondary wire length) question

Greg Westfall wrote:

> I found on one site that the length of wire on the secondary,
> that has always been 1/4 wavelength, is no longer valid,
> that it has been proved wrong.

It was never valid, only a myth, so there is no disproof to cite,
Just measure any Tesla coil to see that it doesn't work.

Further to Matt Siri's answer, here's a graph showing how the
velocity factor (with respect to the wire length) of the 1/4 wave
resonance varies with the ratio height/diameter of the winding.


Mutual inductive and capacitive coupling from one part of the
coil to another have significant effect on the propagation of
signals from one end to another.  The resonant frequency of the
wound wire is quite a lot higher than that of the straight wire,
at least for normal TC h/d ratios.

The velocity factor is roughly ln(h/d) * 0.39 + 1.19
where ln(h/d) is the natural log of the height/diameter ratio.

This formula can be turned around to give an expression for
resonant frequency given the h/d ratio and wire length,

  Fres = (0.39 * ln(h/d) + 1.19) * 75e6/wire_length

Similar curves exist for the overtone resonances, eg


A related myth is that you should add toploading to the coil to
pull its resonant frequency down to the frequency that its wire
would have if straightened out.   Nothing special happens if you
do that, although the resulting TC ends up with a large topload
which is desirable anyway for other reasons.
Paul Nicholson
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