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This phase shift stuff... - Plane Wave
Hi All,
Today I did the following experiment looking for phase shifts along my
secondary coil.
I used my large coil (10.25 inch dia. 30 inch long 1000 turn space wound
#24 wire) WITHOUT a top terminal. The discussion seemed to point to this
being a good place to look.
I used my standard voltage (E-field) antenna that I discussed on my old
paper. It worked very well.
I feed a sine wave from a low impedance generator into the base of the
coil. This signal was also used to trigger the scope. As I varied the
frequency around the resonance point, the phase shifted as expected around
resonance. However, at any fixed frequency, the phase relationship between
the input signal and the antenna E-field signal remained very stable. I
adjusted the frequency so that the signals were in phase which is also
right at the maximum resonance point as indicated by the strongest fields
on the antenna. I was using a dual channel digital scope to see the
waveforms.
I was worried that moving the antenna around the coil would affect its
resonant frequency and thus its tuning which would cause phase shift.
However, I found that if I set the antenna to maximum sensitivity (2
mV/div) I could get far enough away to not affect tuning and still get a
good signal (I used the scope's digital averaging to reduce noise). 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 moved the antenna all around the coil and I could not see any phase shift
going on. I doubt I could see any less than a 5 degree shift but I sure
didn't see any 90 degree shift or anything like that. Not even close. The
antenna's signals always remained in phase with the generator no matter
where around the coil I looked. This agrees with everything I have seen in
the past but this effort was solely to TRY and see phase shift under the
best possible conditions to see it.
Next I tuned the coil from the Fo frequency to the 3Fo frequency. Under
this condition, the base and 2/3 point up the length of the coil are nodes
while the 1/3 point and top of the coil are antinodes. I could easily see
the nodes and antinodes as I moved up the length of the coil. They were
just as expected. However the phase did not change along the coil. Just
like a jump rope, the whole wave was either rising or falling but the whole
coil length was in unison. No phase variation. At the 2/3 point the phase
did shift 180 degrees as I expected but this transition occurred at the
nodal point. There was no gradual phase transition going on.
So it did what I thought it would. From my other work, I suspected this
would be the case but I had never actually checked for the effect in this
detailed way. The voltage along the secondary seems to rise and fall in
unison, just like a vibrating guitar string. Even the harmonic seems to
act just like a vibrating string. 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.
Antonio has done measurements like this to see the nodes and antinodes but
I am not sure he was watching the phase relationships? I was surprised at
his post that said:
>
>The phase goes from 0 to 90 degrees along the length of the coil.
>But as a vertical coil over a ground plane is a somewhat non-uniform
>transmission line, the phase may very in a somewhat nonlinear way.
>There is no essential difference in the mechanical analogs.
>
Antonio and I seem to have completely different results! I did not see ANY
phase shift let alone a 90 degree shift. Obviously, there is a little
correlation problem here! :-) "Real science" may be needed to find what
the discrepancy is. The effects are not subtle at all. Anyone else who
would care to try, please do! Antonio, if you have any ideas of what might
be wrong please let me know and we can work to resolve this.
Do remember that a simple wire on a scope probe can add phase shifts and
other unwanted effects. The probe looks like a 20pF cap in series with 1M
ohm (typically). So be sure to account or load the probe to prevent this
from messing with the measured phase. My plane wave paper talks of this
and explains the frequency response of my plane wave antenna. Of course,
there are a number of people out there who have reproduced my antenna
too... I also used 4 and 16 sample digital averaging on the scope to
reduce stray noise.
I really don't think now that one is going to see significant phase shifts
unless the length of the coil is so long that the propagation effects of
light start to have an effect. At 1 MHz, the 1/4 lambda distance is 246
feet so we have plenty of room!! I would still be very interested to hear
what the students of "FDTD (finite difference, time domain) electromagnetic
analysis" Jim Lux spoke of feel about all this? They sound like they
would be a very powerful source of knowledge on such matters…
Of course, suggestions, comments, ideas… are always welcome. I think the
secondary coils are starting to reveal their secrets! Apparently, they are
far simpler that we had ever imagined. In fact, I can't thank of anyway
their operation could BE simpler!! I won't miss the transmission line
equations for one second though :-)
Terry