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Re: New Spiderweb Primary/OK to use Flat spiral Formulae?
Original poster: "Jolyon Vater Cox by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jolyon-at-vatercox.freeserve.co.uk>
The spiderweb primary is a design of my own
based on antique spiderweb coils used in crystal radio sets; they use a
radially slotted former with uneven number of equally spaceed slots
upon which magnet wire is secured
passing over and under the board.
Traditionally, spiderwebs use insulated wire
but by replacing the slots with rows of holes instead, it is possible to
wind a spiderweb coil with bare wire (as I have done).
my ref on spiderweb coil design:-
my ref on official formula for spiderweb coil inductance:-
I hope to posting a picture of it on the mailing list soon
----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
Sent: Monday, October 14, 2002 2:14 AM
Subject: RE: New Spiderweb Primary/OK to use Flat spiral Formulae?
Original poster: "Ken Stevens by way of Terry Fritz <twftesla-at-qwest-dot-net>"
Forgive my naiveté, but what is a Spiderweb Primary?
Are there any pics of such?
From: Tesla list [mailto:tesla-at-pupman-dot-com]
Sent: Sunday, October 13, 2002 10:13 AM
Subject: New Spiderweb Primary/OK to use Flat spiral Formulae?
Original poster: "Jolyon Vater Cox by way of Terry Fritz
Are the rules concerning spiderweb coils really different to those for
Having just wound a 9-slot spiderweb primary for a mini-coil, I have
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
n the number of turns) gives remarkably accurate results.
The spiderweb is wound with 20 SWG tinned copper wire on 80 mil
sheet or "former"- because the 20 swg TCW is not insulated, the slots
divided into 22 holes 1/8" apart to keep the wire from short-circuiting
passes up and down through the former. There are 22 turns of wire in the
I suppose the thickness of the sheet (80mil) times the number of holes
times the number of slots (9) should rightfully be considered in the
calculation of the inductance -but is this really necessary when the
thickness is but a small proportion of the average radius?
The official formula for the inductance of a spiderweb is
presupposing that c -the width of the inductor- is less than 0.2 times
this is apparently true if the coil stack is high -the only way I can
this to be true with a spiderweb would be if the coil were wound on a
This is does not seem to be the case with my coil so I presume the
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
greater than 0.2 of a?