New Spiderweb Primary/OK to use Flat spiral Formulae?

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

Original poster: "Jolyon Vater Cox by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jolyon-at-vatercox.freeserve.co.uk>


Are the rules concerning spiderweb coils really different to those for flat
spirals?

Having just wound a 9-slot spiderweb primary for a mini-coil, I have found
calculating the length of wire needed to wind the coil using the formula wl = a
* 2 * pi * n (where wl is wire length, a is the average radius of the coil and
n the number of turns) gives remarkably accurate results.

The spiderweb is wound with 20 SWG tinned copper wire on 80 mil polystyrene
sheet or "former"- because the 20 swg TCW is not insulated, the slots are each
divided into 22 holes 1/8" apart to keep the wire from short-circuiting as it
passes up and down through the former. There are 22 turns of wire in the entire
coil.

I suppose the thickness of the sheet (80mil) times the number of holes (22)
times the number of slots (9) should rightfully be considered in the
calculation of the inductance -but is this really necessary when the board
thickness is but a small proportion of the average radius?

The official formula for the inductance of a spiderweb is L=a^2*n^2/(135a+433c)
presupposing that c -the width of the inductor- is less than 0.2 times a- the
average radius;

this is apparently true if the coil stack is high -the only way I can see for
this to be true with a spiderweb would be if the coil were wound on a thick
former.

This is does not seem to be the case with my coil so I presume the official
formual does not apply well in this instance.

Therefore wouldn t the ordinary flat spiral inductance formula L=
a^2*n^2/(8a+11C) be better used to describe the L of a spiderweb where c is
greater than 0.2 of a?