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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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