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Re: This phase shift stuff... - Plane Wave



At 02:11 PM 2/21/99 -0700, you wrote:

>Original Poster: Terry Fritz <twf-at-verinet-dot-com>
>
>Hi All,
>
>	Today I did the following experiment looking for phase shifts along my
>secondary coil.  

...

>I was worried that moving the antenna around the coil would affect its
>resonant frequency and thus its tuning which would cause phase shift.

I would too

>I could move the antenna toward the coil and the amplitude would increase
>without phase change being noticed.  Then there would be a point were the
>phase and amplitude were obviously being affected by the proximity of the
>basically grounded antenna plane.  However, I had more than enough room to
>make the measurements and still not affect tuning and I was able to get a
>very good idea of how close I could get.

I'll trust you on this, but it could be that decreased probe distance
increases capacitance, and results in a decreased phase shift; in other
words, your act of measurement could be canceling what your attempting to
measure.

To make a minimal impact measurement, you can use a technique used in
capactivie proximity detectors - drive a shield or guard electrode. For
instance the probe could be a piece of coax with the center 1mm out of the
shield conductor. Drive the shield from the signal generator, through a
variable attenuation and phase shift network. The idea is to adjust the
amplitude and phase of the driven shield to match the electric field around
the coil under test. Then there is no influence. You amplify the inner coax
conductor with a high - impedance charge amplifier (described in National
Semi's linear app handbook, used for very hi-Z piezo accelerometers, et.)
and compare it to the shield, and adjust the shield phase/amplitude until no
difference exists (the shield potential/phase = coil).

I believe a Langmuir probe for measuring plasma potential uses a similar
paradigm. Conform your probe potential to the local condition so it doesn't
affect it (and your measurement), then you know what it is.

>There was no gradual phase transition going on.  
>...
>So it did what I thought it would... No phase shift effects.  Computer models
>and calculations tell me there is a "little" phase shift (1.6 degrees) but
>that is to small for me to measure. 

This is what you expect from standing waves. Now how about terminating that
coil with various resistances? Will you not see traveling waves and reactive
phase shifts? I bet you can't wait to vary the resistance and reactance
according to time and the voltage, and currents & phase in the TC, to
simulate a real plasma, interactive dynamic environment ;-)

One thing about an impulse TC is that the energy bouncing around in the
resonator will conform to the whim's of the plasma, and the resonator
characteristics. That is, unlike a tube TC or signal generator, which throws
energy at a network, which may transmit or reflect it according to load
character, the single-shot charge in the TC resonator will conform and mold
itself to the discharge.