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Re: Lumped vs. T-line - You be the judge...

Hi All,

	I reran my tests with a TEKP6009 probe that has a loading of 2.5pF/10Meg
ohm.  This probe has about 1/6 the loading of a regular scope probe.

A MicroSim model is here:
http://www.peakpeak.com/~terryf/tesla/misc/T-line01.gif

With the expected outputs here:
http://www.peakpeak.com/~terryf/tesla/misc/T-line02.gif

The two top models are for an ideal and lossy transmission line,
MicroSim's lossy line model is probably a little more accurate but the
outputs are almost identical in this case.  I have also allowed for the
limited bandwidth of the scope at very high gain to get a bit better
accuracy of the measurement system (it makes little difference).

The expected outputs are very different now with the much lower loading of
the probe.  The lumped case is a very nice sine function and the T-line
case is a fairly good square wave with some increasing reflections.  they
are now both very distinct

The actual measured output is at:
http://www.peakpeak.com/~terryf/tesla/misc/T-line03.gif

The cleaner Excel plot is at:
http://www.peakpeak.com/~terryf/tesla/misc/T-line04.gif

Looks like it is really trying to be the lumped case but there are
definitely some T-line qualities to it also.  Notice the ragged area of the
output in the first 800nS.  That must be the current tearing it's way up
the coil by induction hitting all the little resonances and such along the
way.  There is a definite initial spike and another about 600nS latter.
Those are very real and repeatable.

I am sure a very short inductor would looked lumped and a straight wire
would look like a T-line.  It looks like a Tesla coils is somewhere in
between with the lumped case being quite dominant.  Antonio and Malcolm
have suspected this and it certainly looks like they are right.

So it looks like all the equations and theory behind lumped modeling and
Tesla coil calculations is very solid.  However, those that like
transmission lines now have me convinced that there are a few little T-line
effects too.  I can't think of a situation were T-line modeling would have
any advantage but at least we know better what we are dealing with.  I
think Antonio has really summed the situation up as follows:

"The differences from the ideal transmission line are due to the fact 
that a coil is not a TEM transmission line (a line where the electric 
and magnetic fields are perpendicular to the direction of the line). 
The magnetic coupling along the coil, predicted in the lumped model, 
causes that initial rise. The transmission lines incorporated in 
simulators, and treated in usual books, are all TEM models. They model 
very well cables, but don't work very well for coils. Maybe a waveguide 
model can be a better approximation."

Cheers,

	Terry