Current through coils
Tom, the W8JI one, wrote, among other things,
"Capacitance from the coil to itself won't cause these problems. The
change in phase of current at each end of a coil would depend heavily
on stray C of the coil to the outside world as compared to reactance of
the coil, and it would also depend on less than perfect flux linkage
across the inductor."
I've lost track of exactly what "these problems" are, but I was
wondering about the "and it would also depend on less than perfect flux
linkage across the inductor" part. To help resolve that, I did a Spice
simulation; I modelled a transmission line with ten "L" sections
cascaded. Each was 1uH series, followed by 100pF shunt to ground. I
put a 100 ohm load on one end and fed the other end with a 2.5MHz sine
wave with 100 ohms source resistance. Sqrt(LC) is 10 nanoseconds per
section, so I expect 100 nanoseconds total delay, or 90 degrees at
2.5MHz. That's what I saw. Then I added unity coupling among all the
coils, and to keep the same net inductance, I decreased each inductor
to 100nH. The result was STILL very close to a 90 degree phase shift,
with a small loss in amplitude. In each case, the current in each
successive inductor shifts phase by about 1/10 the total. Although the
simulation is less than a perfect match to a completely distributed
system with perfect flux linkage (and just how you do that I'm not
quite sure anyway...), but it's close enough to convince me that
perfect flux linkage would not prevent behaviour like a transmission
line, given the requisite distributed capacitance.
(That was from a "transient" simulation, 10usec after startup so it
should be essentially steady-state; but I'll probably play with an AC
sweep of both cases as I find time.)
Cheers,
Tom
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