K7ITM wrote:


OK, that leaves us with a difference of opinion, or a difference in
what we are describing. There was an article in "RF Design" maybe 15
years ago now by John Mezak, K2RDX, describing a helical transmission
line model for coils. At the time, he offered free software to
execute the calculations (which also, to me, offered a very practical
way to calculate coil parameters like inductance, effective shunt
capacitance, and first parallel and series self resonances). He later
charged a nominal fee for an improved version of the software, which I
have. For the "100 turn, 10 inch long, 2 inch diameter" coil wound
with 15AWG copper wire, using John's program, I see a variation of
about 2:1 in propagation velocity between 1MHz and 20MHz. Since the
first parallel self-resonant frequency is predicted to be around 8MHz,
it's perhaps not fair to look as high as 20MHz, but even between 1MHz
and 4MHz, I see about 25% change in predicted propagation velocity.

You may say that perhaps John messed all that up terribly, but I don't
think so...and there are other places you can find similar results.
There's an excellent inductance calculator on-line at
http://hamwaves.com/antennas/inductance.html, and though the absolute
value of its prediction of propagation velocity is about 5% different
than Mezak's, they both show very nearly the same percentage change
with frequency.

It might be worth having a bit closer look at, Jim. Perhaps it's just
that you're thinking of a different effect than what these two
programs (and the theory behind them) are modelling.

Cheers,
Tom

Hi Tom -

I suspect that for a given coil, depending on construction, L and/or C
may vary enough over several ocataves to resolve any apparent
'dispute' between my comments and the results provided by Mr. Mezak's
modelling program. I do not believe these effects are large enough to
be responsible for the differences being reported in phenomenon under
discussion.

I would be interested in knowing the results your program produces for
the 100 turn, 2" diameter, 10" long coil that Cecil is concerned
about, if you wouldn't mind sharing them.

Thanks and 73,

Jim, AC6XG