Cecil Moore wrote:
Tom Donaly wrote:

What is the characteristic impedance of Tom's coil?


A few thousand ohms. Use equation 50 at:

http://www.ttr.com/TELSIKS2001-MASTER-1.pdf

What's your formula for the velocity factor of Tom's coil? Is it from
the same Tesla coil crackpot you quoted in previous posts?


Do you reject all IEEE white papers? The formula
is equation 32.


Ahem...I'm quite familiar with that paper from work with Tesla coils,
and I have had some conversations a few years ago with Jim Corum.

That's a conference paper, so I wouldn't vouch for it's extensive peer
review.

The Corum's analysis is an attempt to fit transmission line behavior to
what is essentially a lumped system (Tesla coils can be very well
modeled as lumped systems). While the model is certainly valid within
their stated limitations, the real question that arises is "why". A
useful model makes useful predictions, and simple lumped models make
adequate predictions of tesla coil performance.

However, their analysis might have value for higher frequencies, where
the coil is a bigger fraction of a freespace wavelength. A typical
tesla coil runs at a few hundred kHz (lambda= 10-20 km), and a
positively huge one might have a secondary perhaps 2-3 meters long (i.e.
the coil is 1/10,000th wavelength long.
Furthermore, people HAVE made current measurements at the top and bottom
of a large tesla coil and found very small phase differences, indicating
that there is little or no deviation from a lumped model. One might
want to look at
http://www.abelian.demon.co.uk/tssp/

Compare this to a loading coil that is 30 cm long on an antenna for
40m: 1:120th wavelength.

Jim Lux wrote:
While the model is certainly valid within
their stated limitations, the real question that arises is "why".

Because some people are claiming a 3 nS delay through
a 75m mobile loading coil. Corum's VF estimate says
it is more like 40 degrees rather than 4.5 degrees.

Furthermore, people HAVE made current measurements at the top and bottom
of a large tesla coil and found very small phase differences, indicating
that there is little or no deviation from a lumped model.

There is virtually no phase difference in standing-wave
current which is what was being measured. Standing-wave
current cannot be used to measure the delay through a
loading coil.

If the loading coil is located in a traveling-wave
environment, the delay through the coil is obvious
by the phase shift through the coil.
--
73, Cecil http://www.w5dxp.com