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Re: Bang the rocks together harder lumpophiles

Hi Bob,

At 10:43 PM 04/17/2000 -0400, you wrote:
>Hi Terry,
>
>When you say the phase is consistent along the modelled coax do you mean
>there was no phase shift?

Once the standing wave is set up,  It's magnitude overwhelms the driving
signal and it acts like "Malcolm's ruler" in that the coax's signal phase
is basically zero along the length of the coax.  It took a real RF guy like
yourself to bring that situation to light for me!  However, that phase is
90 degrees out from the generator.  I am not sure the exact point of the
phase shift but I imagine it is close to the base termination of low
impedance.  It is, in fact, needed for power transfer to take place.  I ran
through that model fairly quickly so perhaps I misinterpreted something?

>
>Yes you can mount the coil vertically preferable say one diameter above the
>ground plain.  But your making it difficult to get a short ground connection
>for termination's and probes at the top end but I think it will be OK up to
>a perhaps a meg. Good practice is to  have the probe tip on the signal point
>and take the 0v connection vie a lead to the common ground point.

The probe "tip" has a ground reference back to the scope that is grounded
to the common point.  The TEK probe wire is 2 meters long so perhaps that
was affecting something??  I want to keep the secondary configuration
comparable to a typical Tesla coil or all this looses the "golden" meaning.

>
>The isolator is good practice.

That's a darn neat thing!!

>
>You suggest you should drive the coil from a very low impedance because the
>Q is high why? what are you trying to measure?  NOT PROPAGATION DELAY I HOPE

As the voltage builds on the low loss coil, the currents reflected back to
the generator can trigger it's over current function.  The isolator stops
this while also providing a low Z termination at the base of the coil.

>
>How you drive the coil depends on what you are trying to measure.
>
>If your measuring  propagation time,  the impedance looking into the
>isolator from the coil must match the coil characteristic impedance and
>output of the coil must be terminated in the to coil characteristic
>impedance, as I have already said.

The characteristic "equivalent" impedance of my coil is 31,000 ohms.  I
don't not think adding a 31K source and load impedance would be realistic
for a "real" Tesla coil.

>
>If you are trying to make the coil resonate it should be terminated in an
>impedance that is low with respect to its characteristic impedance which may
>hundreds of ohms so 4.7 is fine but it depends on what you are going to
>measure.

My 10 ohm resistor provides reliable signals strength above the noise and
is 1/3100th the characteristic Z so I should be fine.  For simply finding
the resonance, this resistance seems to be very insensitive.

>
>If you your performing a propagation experiment the probe input C (X10),
>even at 1mHz is 15kohm so it want effect the termination significantly
>provided your characteristic impedance is less than 1500ohm.
>
>If your doing resonance experiments think of it as a 10pf  top load.

The probe impedance makes a "BIG" difference.  I try to use a 2.5pF 10Meg
TEKP6009 probe I borrow from work.  I really need to get a TEK P5100 for
myself since I can use it often.  Simply changing probes changes the
measured signals drastically but somewhat predictably.

>
>Yes you can drive the coil with an impulse or noise why? what are you trying
>to measure?  but you will need a spectrum analyser or transfer function
>analyser to work out whats going.

If I can get "any" model to reproduce what I see.  I will be able to do
wonderful things.  Neither the lumped or typical T-line model works well
for this subtle stuff.  The equivalent simple T-line model does not exist.
However, a multi element lumped model should be able to reproduce the
situation simply by virtue of the computer being able to crunch through the
math...

>
>I suggest you stick to the propagation test with continuous sinewaves (at
>least initially) at the resonant frequency of you coil assuming  you are
>trying to measure the delay at the working frequency of a coil.

But the square waves give a great deal more information!  The added fun of
the step response excites and amplifies the differences in the coil's
characteristics.  This provides a much more strict standard that or
"little" models have to meet.  The goal is to come up with a model that
provides the right answers all the time.  Rest assured that the "right
answer" looks very much like a transmission line, but it will take a
complex lumped model to use.  Sort of beautiful isn't it ;-))

>
>As can be seen by reading the large number of posts there's a lot coilers
>doing all kinds of things.  Few appear to be well set up propagation
>measurements. A critical point to the interpretation of many of the
>experiments is their not using continuous sine waves and with various input
>and output termination's this coupled with the high dispersive proprieties
>of the coil makes a very complicated response and very difficult to
>interpret.  I suggest they start with a standard propagation test (correctly
>terminated input and output) and when they can show the delay or 90deg phase
>shift at the resonate frequency their ready to move on.

Most anyone can do these tests.  However, the low impedance probe seems to
be a rather critical factor and they are not cheap!  It is really important
to reduce seeing the "probe" and increase seeing the "coil" in these tests.
 With the coil having only 9.3pF to start with, the probe has to be very
special.  Dropping a 16.6pF probe across the coil really messes things up.
Perhaps you know if a simple very high Z probe adapter.  I think we can
increase the coil voltage to get a meaningful signal to a much higher Z
probe...

>
>But I am totally perplexed as to why with so many examples of standing waves
>with no phase shift that its difficult for so many to accept its true for a
>Tesla coil and presumable most on the list are engineers or at least the
>ones with scopes and things. You must have done those experiments at school
>with the lycopodium powder and tubes what about the string expeiments with
>bits of paper. Can somebody explain it.  What's special about a Tesla  coil.
>I am starting to feel its a religious believe and I am a heretic. Just do
>the propagation measurement  with a matched  terminated coil as I have
>described.

I would submit that Tesla coils deal in the world of fast transients.
Standing waves are an over simplification of a system that defies a simple
continuous time model.  A coil that is driving from a sine generator is not
the same as a coil driven by a spark gap.  That nasty transient response is
a very key factor!  A simple RLCG T-line does not explain anything.  Us
lumpophiles have neat computer toys that have drilled the practical things
to a fine science using just an inductor and two capacitors.  However, even
"we" know that it can't be that simple and we are surprised "that" lumped
model works so well.   Thus is the reason we are turning our vast resources
to the "T-line side" (the dark side ;-)) to help you poor guys out in
explaining the finest details.  You know your are right (and so do we), but
you don't have the tools to explain it.  Rest assured, "we" have the tools
and we will drill this thing down even to the transmission line theory in
search of the "simple" truth.  Richard Hull thought this quest would drive
us nuts.  But he underestimated the lengths we would go to get there ;-)) 

BTW - The C component of a secondary's T-line model appears to be
'negative' for the first 1/3 of the coil's length.  Ain't that sweet!! ;-))

Cheers,

	Terry


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