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Re: Bang the rocks together harder lumpophiles
Hi Terry,
When you say the phase is consistent along the modelled coax do you mean
there was no phase shift?
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 isolator is good practice.
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
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.
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.
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.
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.
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.
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.
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 think Terry has contributed significantly to this mass rejection of reason
(the duff phase stuff) combined with the failure of the standard equations
for resonance in a transmission line (there just not the right equations due
to the mutual coupling). I hope Terry will be equally vocal when the
propagation delay is finally measured and its 90deg.
I still don't think your banging the rocks together hard enough lumpophiles.
Regards Bob
-----Original Message-----
From: Tesla List <tesla-at-pupman-dot-com>
To: tesla-at-pupman-dot-com <tesla-at-pupman-dot-com>
Date: 17 April 2000 06:41
Subject: Re: Bang the rocks together harder lumpophiles
>Original Poster: Terry Fritz <twftesla-at-uswest-dot-net>
>
>Hi Bob,
>
> I have been looking over your test idea and had a few questions.
>
>I can set up the aluminum foil on a concrete floor easily enough but I
>assume I should slit it so as not to have the eddy current / shorted turn
>effects. I figure the ground point can be in the center. I want to keep
>the coil vertical to represent a real Tesla coil case.
>
>I have a <1 ohm output generator but it is 1 volt at 20mA so driving a coil
>directly over currents it. It is active output so there is no real output
>impedance as a normal generator would have (a marvel of science). Your
>isolator should take care of this perfectly! I can put a 50 ohm resistor
>on its output and then a 1 ohm resistor to ground. That will only give a
>10mV signal to the coil but that should be enough. The coil would have to
>pump 1 amp through the resistor to mess with the generator which would be 1
>watt which is not possible, so all should be well. Does that sound ok?
>
>One big problem is measuring the output voltage. With the self capacitance
>of the coil in the 9.5pF range. Dropping a 16.6pF 10:1 Tektronix probe on
>the open load end would cause terrible loading and distortion. I think the
>one at work is 2.5pF at 100M ohm (tek P6109) but that my still mess with
>the measurements. I also worry that the wire to the probe will pass right
>through the self capacitance fields at the top of the coil but they may not
>be a big deal. Any thoughts on how to measure the top voltage without
>disturbing the thing would be appreciated.
>
>Your square wave test where one observes the returning square wave edges
>should me much fun!
>
>Cheers,
>
> Terry
>
>
>
>At 01:36 PM 04/16/2000 -0400, you wrote:
>snip...>
>>The best I can do is as follows. Use the isolator described below with
say
>>470ohm in place of the 4.7ohm. Starting at 50 ohms and increasing in say
30%
>>steps until you get close (or use a 1k pot), connect various resistors
>>across the output of the coil to ground until the output voltage is equal
to
>>the input voltage. At that point I think you have no reflections from at
>>least one of the ends.and hopefully its the terminated end. Replace the
>>divider at the input with a divider that has an output impedance equal to
>>the
>>value you found above. check the input and output voltages are the same if
>>not you may have to adjust the divider again. You only need to be within
>>say 10% if you only want to see the delay and not make accurate
>>measurements.
>>
>>Now your ready if you are using the resonant frequency of the coil you
will
>>be able to observer the 90deg phase shift. QED and conversion. (assuming
>>you don't invent a spurious reasons to account for this and the coax)
>>
>>You can now perform various experiments that you must have missed at
collage
>>like open circuit resonance, short
>>circuit resonance, effects of C or L and determine the dispersion i.e.
>>velocity against frequency. You can also switch back to a square wave,
drive
>>the coil from a high impedance source, say ten time the characteristic
>>impedance, and leave the other end open circuit you will be able to
observe
>>at the driven end the progressive step up of the square wave edges to the
>>source voltage as each reflection returns from the open circuit.
Similarly
>>if the end is shorted you will be able to observe the decaying transients
>>in voltage this time caused by reflections that are inverted at the short
>>circuit. Just like the primary transients when the gap closes.
>>You could also determine the characteristic impedance at different
>>frequencies.
>>
>>Note that the square wave must be at least ten time the resonant frequency
>>or there will not be time to observe the effects before the next edge
>>occurs. Note also that the edge may be smeared out because of the
dispersion
>>i.e. the harmonics will travel at different velocities.
>>
>>Some potentially important points:
>>
>>1) You don't appear to have a ground plain this could produce all sorts of
>>spurious responses that could be very confusing. Ideally you should do the
>>experiment over a large metal plate or aluminium foil. Preferably
although
>>not essential with the coil laying parallel to it. You could use the open
>>circuit primary. The generator ground, scope probe grounds and any
>>terminating resistors or short circuits should be connected to it
preferably
>>at a common point.
>>
>>2) You should have an isolator on the output of the generator at the point
>>it is connected to the coil. Particularly if your using coax to connect to
>>the generator and the generator has a 5 ohm output. You should also
>>terminate at the generator if its output impedance does not match the
cable.
>>Again use a 50ohm termination assuming 50ohm coax. This may be essential
if
>>you doing square wave experiments as the reflections in the cable will
>>degrade the edges of the signal and can upset the generator. Its
interesting
>>to speculate which point in your set up is equivalent to the point in
the
>>coax experiment where you measured the 90deg phase shift.
>>
>>3) Use the scope probes on X10 and a direct connection you only need 0.1
>>volt in or less depending on the ambient electrical noise.
>>
>>The isolator can be made in one of two ways depending on how you want to
>>drive the coil from a high impedance source or low. In the low impedance
>>case which you need to determine Tesla resonance (the coil is supposed to
be
>>shorted at one end) and assuming 50ohm coax, use a pie net work consisting
>>of a 50ohm(or 47) resistor across the generator and a 470ohm and 4.7 ohm
in
>>series across that. (for the coax experiment you my need to use 1ohm to
get
>>better results but it will make tuning sharp).The coil is connected to
the
>>junction of the 470 and 4.7 resistor. For a high impedance drive use a
>>50ohm (or 47) across the generator and a 10kohm resistor from it to the
>>coil.
>>
>>Note that for 50 ohm coax 4.7ohms is only 10% of the cable impedance so
>>it does not represent a good short. A better value would be 1ohm with a
>>series 100ohm. This should give you a good standing wave but the tuning
>>will be more critical
>>
>>I did say it was tricky but it should teach you a lot. Suddenly all those
>>spurious effects can be easily be explained. The funny ringing on the
spark
>>is produced by waves bouncing round the primary. A new layer of
complexity
>>and wonder will be open to you. Then all you will have to do is convince
>>the other non believers. Well I have done my bit.
>>.
>>Regards Bob
>>
>>
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