Re: 8-9 RFI noise thoughts. 8-25 and still going!

Hi Alwyn,

>At 11:00 PM 08/24/1999 +0100, you wrote: 
>Hi Terry,
>Your reply is fascinating.  I will respond to some of  the points you have 
>You wrote:
>>Perhaps the high frequency signals, that the ferrites are there to stop, do
>>not have that much current behind them (although I bet they do!).
>>Apparently, the ferrites are doing something good but it is hard to say
>>what.  Perhaps they add just a little inductance or loss, which is all that
>>may be needed.  The models do suggest that even a small series inductance
>>can have a dramatic effect on the level of the high frequency signals.  But
>>that also assumes the effects are electronic in nature.
>Did the model you used for the firrite rings include the saturation, loss 
>and propagation effects. How did you model the SG closure.  This is very 
>important to any conclusion you draw from the results. does your software 
>have models of lines.

No, no, and no.  The gap is basically just a swich that has a series RLC
across it.  When the switch closes the LC rings.  MicroSim does do
transmission lines but the speed is much slower.

>You wrote:
>There is no doubt that standing waves can be set up in Tesla coils at a
>>number of harmonics.  What I would dispute is that there are significant
>>propagation and phase shift effects along the length of the coil.  If the
>>length of wire were a factor, and the current had to travel the length of
>>wire in the secondary, then the current would be delayed 90 degrees along
>>the coil's length.  However, it is easy to make Tesla coils with wire
>>lengths other than what the 1/4 wave propagation / wire length theory would
>>suggest.  In fact, a given coil can operate over a very wide frequency
>>range with ease given different top loads.  I suggest that the current at
>>the base of the coil and the current at the top of the coil are
>>magnetically linked.  This linkage simply overwhelms the effects of pure
>>wire length propagation.  If the wire were unwound and in a long straight
>>line, then it would act as a simple 1/4 wave antenna.  However, by winding
>>it up in a close wound coil, the currents in the wire are locked together
>>magnetically.  In a fairly similar fashion, the self-capacitance of the
>>coil is also locked.  Thus, the secondary system acts much more like a
>>simple lumped LC network rather than a 1/4 wave transmission line
>I suspect the key to our different views is in the first two sentences.  I 
>believe you get standing waves when two waves interfere. In this case the 
>forward wave and the reflected wave from the open circuit or C loaded end. 
>If standing waves can be set up along the coil how does a lumped L and C do 
>How can a standing wave be set up if there is no delay along the coil and 
>reflection at the end. 
>How can  a simple LC network resonate at several frequencies.
>I am not suggesting that any formula are wrong or in accurate.
>I am not suggesting that the resonance frequency can be calculated from the 
>length of wire and the propagation velocity of that OPEN wire with or with 
>out a top load. So that  comment appears to be irrelevant.  In the example I 
>gave if I remembered the frequency correctly the wire length was about 25% 
>of the 1/4 wave OPEN wire length so I agree the wire is not a 1/4 wave 
>length long.

Se my other results of testing I did tonight.  I have not had time to think
all this out yet but I will...  The voltage graphs do suggest that the
"lumped" model can support a chain of LC parrallel circuits that I "think"
is the solution...

>You wrote:
>There are computer programs now that can predict a coil's resonant
>>frequency with top load based on physical dimensions.  They do not depend
>>on wire length at all.  The programs calculate the self-capacitance of the
>>coil and the capacitance of the top load as a physical structure in space
>>(with a ground plane).  This capacitance is then combined with the measured
>>inductance to arrive at a resonant frequency.  The programs can get within
>>5%.  These programs are based on the voltage distribution along the coil's
>>length as an in-phase sine wave.  The current is a cosine wave that is
>>delayed in amplitude but not in actual phase (I need to find a better way
>>to explain that...).  Basically the current is maximum at the base of the
>>coil and is some lesser value (like 40%) at the top of the coil but still
>>in phase.
>It may be true that the formula does not contain a value defined as the 
>length of the wire that may just be how the formula is written.   For 
>example for  pitch, diameter and coil length read wire length. I assume you 
>don't believe that the inductance is not dependent on the wire length or 
>that if you half  the number of turns by halving the pitch the resonance 
>frequency will not change.

Do you mean capacitance??  I think a coil of a given height and diameter
will have a self capacitance and that is independant of the number of turns
or the pitch.  The inductance is obviously very dependant on the number of

>You wrote:
>I have probed along coils at resonance driven by a signal generator.  One
>>has to be sure to use a properly terminated antenna or the capacitance and
>>coax loading will mess up the phase of the measurement.  I use a short 50
>>ohm antenna (cell phone or scanner type), a length of coax, and a 50ohm
>>terminating resistor at the scope end.  Although the amplitude of the
>>signal along the secondary definitely rises and falls along the length, the
>>phase of that signal stays in phase.
>This had me worried at first but thinking about this a little more.  Its a 
>standing wave.  It does not move it only varies in amplitude because its two 
>superimposed waves that are travelling in opposite directions.  If you want 
>to see the delay remove the reflection by correctly terminating both ends.
>I suggest you double check your phase measurements. I predict that at 
>resonance the current at the top leads the current at the bottom.  Note: 
>This can only be checked when the coil is driven at its lower end not 
>inductively coupled. My previous comment on this subject was rubbish so I 
>have learned something. 
>Oh just one more point that I can not verify at the moment. You can 
>calculate the first resonant frequency of a open circuit line by using its L 
>and C with a fixed factor.  I suspect that's all the Medhurst calculation 
>does, ie for a close wound coil the C to ground can be calculated from the 
>size add the measured L and a fiddle factor (may be unity) and you have the 
>first resonant frequency.  
>I assume I am not the only one who thinks the analogy is an open organ pipe. 
>The guy who elaborated on my analogy of the double pendulum may be able to 
>add the primary coupling to this. 
>If your LED experiment did not convince you its a 1/4 wave effect we will  
>have to agree to differ. I rest my case on that subject.
>.Regards Alwyn (FL)

If you have not seen it yet, you may want to see my paper on top and bottom
phase at:
(The peakpeak server seems a little busy lately so try back if it does not

Also, here is a repost of something I spent a long time writing previously
that explains my veiws on all this well.  Of course, my new tests prove
that there is "still more to be discovered" as I stated at the end.  All
the past theories of Tesla coil behavior seem to have one thing in common,
they don't last long.  I try not to forget that... ;-)




Hi Jim,

	It is very true that using the three lumped parameters (Lsec., Cself, and
Ctop) will predict Fo, voltages, and currents well.  However, the secondary
system is not just three simple components.

	First I'll mention the problems with the old 1/4 wave and wire length
theory.  It was assumed that for electricity to travel the length of the
secondary, that the electrons would travel through the whole length of the
wire.  Thus, the electrons would flow from the base, to the terminal, and
back.  This is very similar to a 1/4 wave antenna.  The resonant frequency
is simply determined by the time it takes for light to travel the length of
the wire.  This seemed like a reasonable way to determine Fo and explain
why a bare coil resonates at such a low frequency.

	However, there were problems.  Medhurst found that an inductor's Fo
frequency was related to the inductance and dimensions of the coil instead
of the wire length.  Given a coil of a fixed height and diameter, it could
be shown that a single capacitance could be used to determine Fo along with
the coils inductance regardless of the length of the wire.  The wire
lengths did not have any correlation and, indeed, it was simple to wind
coils that violated a simple wire length model.

	The next problem was the extreme sensitivity of Fo to external influences.
 If just the wire length were controlling Fo, then simply placing ones hand
near the coil should not alter the frequency to the extreme degree it does.
 It was obvious that the external fields around the coil were having a very
profound influence on the resonating system.  Medhurst put these two
together into the Cself of the coil.  The coil was now an inductor that had
a "self capacitance".  If you know the inductance and the self capacitance,
you know Fo.  Medhurst did his excellent work and found an empirical
equation to find Cself based only on coil dimensions.  It is well known
that this equation (the Medhurst equation) can predict the coils Fo
frequency with remarkable accuracy.

	However, there was still a belief that the 1/4 wave model was valid.  No
one really had any reason not to think that the secondary still acted like
a 1/4 wave antenna instead of a simple inductor in parallel with a
capacitor.  Indeed, the distributed capacitance along the coil fit an
antenna model better than a lumped model.  Problem was, no one could find
the parameters that would govern this system.  The Corums changed that.
Their "Slow Wave Helical Resonator Theory" adapted transmission line theory
to the Tesla coil.  They used standard transmission line parameters and
equations to form a model that had distributed components of inductance,
capacitance, and losses.  This model also allowed for 1/4 wave phenomena to
still exist.  However, there was a problem.  No one could ever find the
parameters needed to use the equations.  Every coil seemed to have it's own
parameters.  The Corums often sighted the formulas by Kandoian and Sichak
for finding the transmission line parameters but, in practice, those
parameters never worked well.  A small error would give vastly different
results.  It could also be argued that the use of this empirical formula
was not appropriate.  However, this theory gave a whole new set of
equations and even introduced the smith chart to Tesla coiling.  Sounded
good, had all the "stuff", based on true electrical engineering theory...
Just didn't work.  They wrote their work up and it was widely distributed.
Few people really understood both Tesla coils and transmission line theory
well so the problems were not easy to discover.  Most people just read the
papers, looked at the equations, fearlessly tried to calculate something
with them, and concluded that it looks great and must be true despite the
fact that they personally did not have the skills to use the darn thing.

	So, you assumed that the Corum's model was correct but used Medhurst to
find Fo because all that Smith chart stuff was impossible to understand.
However, many people realized that "something was wrong"...

	I am an electrical engineer and I do understand Tesla coils and
transmission lines.  So "no problem"!  I'll just click up MathCad, punch in
the Corum's equations, and I'll be set...  Two weeks later, nothing was
working.  The equations gave all kinds of results depending on all kinds of
things.  I started to listen to the "other guys".  Malcolm was the first
person I remember that seemed to have a problem with the 1/4 wave theories.
 It was the coherence theory of the Corum's.  Sounded good on paper, but
was never demonstrated and violated the conservation of energy.  Once I saw
that the Corums could be wrong(!), I started going back and checking
everything I could.  Basically, the 1/4 wave theories didn't work and the
lumped models did...

	The fiber optic probes were coming together about this time (they were
originally made to study this problem), so I hooked them up to the coil and
made a bunch of measurements.  They were easily able to resolve and measure
the phase shift from the top to the bottom of the secondary which would
once and for all determine how much phase shift was going on.  The results
were that there was practically no phase shift!  Exactly what the lumped
models predict!  I wrote up the "top and bottom" paper and started the
"thrash the 1/4 wave theory" thread on this list.  For me, the 1/4 wave
theory fell apart like a house of cards.  I was finally able to find the
"true" transmission line parameters for a coil and it looked like an
extremely short antenna "stub" with lots of L and C.  Resonant yes, 1/4
wave, not at all...  It was like using a 1/4 inch wire in place of a true
108 inch CB antenna.  It was simply far far shorter than needed to support
1/4 wave effects.  It was far closer to a lumped inductor and capacitor
than a 1/4 wave antenna.

	Soooooo...  Where are we now...

	I now believe that the secondary "can" be modeled as an antenna.  A very
very short one.  My coil has a wavelength of a few thousand feet and it
looks like an antenna 2.5 feet long.  It does not transmit well at all.
There is hardly any phase shift from top to bottom.  The reason the wire
length theory is not valid is because, unlike in a straight wire antenna,
the windings are all magnetically linked to each other.  Current at the
base of the coil does not need to travel a few thousand feet to reach the
top.  It only has to travel 2.5 feet and magnetically couple to the top.  A
far shorter path and I would submit the "coffin nail" for the 1/4 wave
theory.  The Tesla coil is not a straight wire in free space.  It is a coil
in free space.  The current does not travel trough the wire but is induced
in the coil's turns by magnetic induction.  Current in the bottom turn can
induce current in the top turn in a few nanoseconds.  The effects are, for
all practical purposes, lumped.

	The secondary is an inductor, It sits is space and therefor has
capacitance due to its physical dimensions as any conductor would.  Voltage
on the coil stores charge in this capacitance in the space surrounding the
secondary.  In can do this in a fraction of a nanosecond so the time delays
are not significant.  This capacitance is linked to the coil's turns all
along the coil's length.  This capacitance is distributed in that way, but
there is practically no time delay involved.  Medhurst's equation does
accurately predict this capacitance given the dimensions of the cylinder.
When you place a top terminal on a coil, it adds capacitance to the
structure thus lowering the resonant frequency.  The voltage along the
secondary is distributed as a sine function with the base being at zero
volts and the top being at full voltage.  The current is a "delayed" cosine
wave.  Full current at the base but only maybe 30% of that current is seen
at the top of the coil.  The current divides between the self capacitance
and the top capacitance.

	Wheeler's equations can tell you the inductance of the coil as can any LCR
meter.  There are no significant exceptions there.  Medhurst's equations
can tell you the self capacitance of the inductor also, without exception.
Fields programs (like my own E-Tesla3) can determine the resonant frequency
of a coil with a top terminal using the rules in the last paragraph along
with field theory and Gausses law.  It can also generate arrays that show
the voltage distribution around a given coil.  The accuracy is within 5%
despite the fact that I wrote it :-))  Programs like MicroSim can be used
to very accurately model Tesla coil systems using lumped parameters
although it does not show the inner workings of the secondary coil as we
have discussed.  

	So, I think we really do know a lot and we are on the right track.  Of
course, that's what Tesla, the Corums, and who knows who else thought...
The difference is, our ideas seem to work in every situation for everybody.

	I do not claim to have "invented" any of this.  I may have helped to
develop some of this or demonstrate something here or there, but many
people have contributed to all this, not just me.   Indeed, I suspect that
Medhurst could have told us all this long ago....

There is still more to be discovered.....


At 09:01 PM 5/16/99 -0700, you wrote:
>Terry, Malcolm et all,
>    At the risk of sounding ill informed, I'd like to play Devil's advocate 
>on the 1/4 wave theory. While there is no dispute that lumped parameters 
>"get the job done", perhaps some subtlety has
>been overlooked. Could the discrepancy between the wire length and actual 
>resonant frequency be a function of velocity? Coil geometry
>has a great deal to do with the final outcome for many reasons, not
>the least being distributed capacitance. It sounds reasonable that the
>more "media" the electrons have to slog through, the slower the velocity. 
>Different combinations of LCR will render the pure length
>parameter meaningless, yet it still could be a 1/4 wave phenomena.
>   Now that I've braced myself, go ahead and blast me!
>Jim McVey