Re: Quarter Wavelength Frequency

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

Original poster: "Malcolm Watts" <m.j.watts-at-massey.ac.nz> 

On 24 Jul 2004, at 17:26, Tesla list wrote:

 > Original poster: FIFTYGUY-at-aol-dot-com
 >
 > In a message dated 7/24/04 1:41:58 PM Eastern Daylight Time,
 > tesla-at-pupman-dot-com writes:
 >
 >  > Bearing in mind that these are measured values, we do seem to have
 >  >  the real coils tending to a high velocity factor as h/d increases.
 >  > >  Is there anybody out there with a coil with h/d > 10 ???  If so,
 >  >  we want your measurements!
 >
 >      A real-world source for very high h/d coils comes to mind:
 >      spiral-core
 > spark-plug wires. Used to minimize EMI with low-resistance wire,
 > instead of using straight high-resistance wire. Easy to buy from
 > performance auto suppliers. And conveniently rated for high voltage
 > use.
 >      Also some uneducated thoughts:
 >      As the h/d increases, I'm guessing the vel factor and such don't
 >      approach
 > a straight wire because the fields are actually approaching right
 > angles to their "straight-wire" orientations. An infinitely long
 > zero-radius close-wound coil will have the magnetic field parallel to
 > wire's axis, not perpendicular to it as with the straight wire it
 > resembles.
 >      And in another post:
 >
 >      >To reach this straight wire velocity factor, our models and
 >      >calculations would have to allow the pitch to increase to
 >      >infinity as well as the h/d ratio.  This they do not do.
 >
 >      So as the pitch increases, you are "stretching out" the coil back
 >      into a
 > straight wire. But doesn't space winding decrease the "Q" of a coil,
 > and therefore make it less efficient as an inductor, which is what you
 > would expect from effectively unwinding one?
 >
 > -Phil LaBudde

I have a coil which I will measure tonight and post on tomorrow which
has a totally outlandish H/D ratio. I wound it a few years ago to
explore the outer reaches. Watch this space.

As far as space winding goes, the Q actually increases to a point as
the winding goes from closewound and then decreases again. I expect
the asymptote would define where reducing the proximity effect is
balanced by the decreasing inductance per unit length.

I once wound a series of coils on a 12" piece of PVC pipe to check
some of these things out and found that if one takes Wheeler's
inductance as gospel, Medhurst's value for Cself is independent of
the wire spacing and depends (as he said) only on the diameter and
H/D ratio of the coil. As we know, this is not a true picture as
Wheeler's formula applies only at lowish frequencies. In each case,
the winding height remained the same and only the number of turns was
varied. The coil was seen to be degenerating into a long wire antenna
as the turns approached 1 (and yes, the Q dropped dramatically at
this end of the expts).

Malcolm