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Re: 8-9 RFI noise thoughts.
Hi Terry,
Your reply is fascinating. I will respond to some of the points you have
raised.
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.
Response:
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.
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
Response:
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
this.
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.
You wrote:
I used to have a long coil that had LEDs in series with the winding at
>every inch. It was fun to hit the various harmonics and set-up node and
>anti-node patterns on it. However, that device could not detect the phase
>along the coil.
Response:
This is a great demonstration tool. David (Tampa) may be able to sell one
of this to some one.
If the currents in the coil are locked together or linked presumable the
same current is following in each led. Then why where some on and some
off? I would think that this demonstrates that the currents in the coil are
not linked. And again how does this effect occur without propagation
delay along the coil.
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.
Response:
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.
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.
Response:
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)