John Popelish wrote:
The question, I think is whether large, air core coils act like a single
inductance (with some stray capacitance) that has essentially the same
current throughout, or is a series of inductances with distributed stray
capacitance) that is capable of having different current at different
points, a la a transmission line. And the answer must be that it
depends on the conditions. At some frequencies, it is indistinguishable
from a lumped inductance, but at other frequencies, it is clearly
distinguishable. You have to be aware of the boundary case.

Dr. Corum says the boundary is 15 degrees, or 0.04 wavelength.
Another place in his class notes he says that if 1/6 of a wavelength
of wire is used to make the coil, the lumped-circuit model will NOT
work. My 75m bugcatcher coil is more than 1/6 of a wavelength of
wire.

But a continuous coil is not a series of discrete lumped inductances
with discrete capacitances between them to ground, but a continuous
thing. In that regard, it bears a lot of similarity to a transmission
line. But it has flux coupling between nearby turns, so it also has
inductive properties different from a simple transmission line. Which
effect dominates depends on frequency.

Dr. Corum has a test equation to see if his velocity factor equation
applies. The test is: 5*N*D^2/lamda(0) = 1 where N is number of turns,
D is the diameter of the coil, and lamda(0) is the self-resonant
frequency. If this equation is satisfied, then equation (32) applies
for velocity factor. For my 75m bugcatcher coil, the test number is
0.4 = 1 and the velocity factor equation yields 0.0175. That's certainly
a slow wave device.

But its propagation speed will be slower than it would be if the wire
were straight. don't know if that qualifies it for a "slow wave" line
or not.

A velocity factor of 0.0175 for a 75m bugcatcher seems to qualify.
--
73, Cecil http://www.qsl.net/w5dxp