On Nov 30, 10:59 am, Jim Kelley wrote:
K7ITM wrote:
On Nov 29, 9:11 am, Jim Kelley wrote:
...

Over the range of a few octaves, propagation delay on the other hand
does not vary to any significant extent as a function of frequency.
Ostensibly, it should be equal to sqrt(LC) series L, shunt C.

Actually, Jim, I do expect it to have considerable frequency
dependence. I think you can find info about this in books that
address the design of travelling-wave tubes.

I can't think of an example of an active (or reactive) device which
doesn't have frequency dependent characteristics. To the extent that
indices of refraction are frequency dependent, propagation velocity
does in fact vary with frequency. If it didn't, we wouldn't see
rainbows. Dielectric constants do indeed have a frequency dependence.
But to first order, at radio frequencies, in amateur applications,
for the purposes of this discussion, and in my opinion, the effect is
less than considerable - particularly if we assume the L and C in
sqrt(LC) are correct at the frequency of interest. ;-)

73, Jim AC6XG

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