Re: quarter wave

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Paul Nicholson" <paul-at-abelian.demon.co.uk> 

Replies to some comments...

Gerry wrote:
 > This sorta reminds me of the end effect on a propagating
 > light wave causing defraction.

Yes, the general process involves an incident field affecting
charges in some structure of dielectrics and conductors.  The field
from these joins in the general melee.  Poynting vectors are moved
around and the end result is that signal/energy propagation is
altered - usually in a frequency dependent way.

(I gave an example in the 'TC & Lightning' thread of my observation
  of a severe case occuring when signals pass through the earth's
  magnetosphere.)

This underlies many physical phenomena: diffraction, reflection,
scattering, guided waves (eg electricity), surface waves, dielectric
constants, rotation of polarisation, etc, etc, ...and dispersion in
solenoids.

Velocity factor can be defined for any structure - simply as the
time taken for a signal to propagate divided by the time it would
have taken if going at c.  For coax we get about 0.7, for wires
and sheets of various sizes it might be 0.90 - 0.98, and so on.
*Inside* conductors the effective EM velocity may be very slow -
barely above walking pace!  (The actual charge 'drift' velocities
are even slower!!)

For our coils we get about 0.001 if measuring along the axis, or
about 1.5 - 2.0 if measuring along the wire.

[The impedance changes as you approach the ends and so a
  travelling wave would see a gradual rather than a sudden sharp
  discontinuity]

 > Could this be the reason that some in the group recommend that
 > the top turns of the secondary be space wound to provide an
 > impedance transformation?

I don't know.  I suppose this is part of the more general question
of 'Is there anything to be gained by grading the pitch?'.

 > Every time I reread this, I pick up more nuances.

Thanks, I try to put in lots of those so that a post read a few
times will still have something to say.  The nuances should make
sense too...I hope!

 > Does the velocity factor depend on the number of turns for a
 > given H/D ratio?  Maybe your data says no but I'm not sure.

See my discussion of Ed's work in the thread "Wire length,
coil geometry, and velocity factor".

 > Interesting, the modes are only odd number of 1/4 (probably a
 > boundary condition).

Yes the boundary conditions (grounded base, open top) force odd
quarter waves.   They fix a voltage node at the base and a current
node at the top.  All the resonances must fit with those constraints.

Malcolm wrote:
 > I measured Fr ...

Thanks, I'll look at these results in that velocity factor thread.
--
Paul Nicholson
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