# Re: K Quiz

```Subject:      Re: K Quiz
Date:  Thu, 12 Jun 1997 11:55:35 -0700
From:  Skip Greiner <sgreiner-at-wwnet-dot-com>
Organization: Greiner, Ltd.
To:  Tesla List <tesla-at-pupman-dot-com>
References:  1

Tesla List wrote:
>
> Subject:      Re: K Quiz
>        Date:  Tue, 10 Jun 1997 11:41:13 +1200
>        From:  "Malcolm Watts" <MALCOLM-at-directorate.wnp.ac.nz>
> Organization:  Wellington Polytechnic, NZ
>          To:   tesla-at-pupman-dot-com
>
> Hello Mark, all,
>                  Following on from Mark's little quiz, I have the
> following query which might relate to something quite important
> performance-wise.....
>
> > Hello coilers,
> >         Here is a little food for thought.
> >      "K" is the term applied to the degree of magnetic coupling between
> > the
> > primary and secondary coils of a conventional tesla coil (or the primary
> > and
> > driver
> > coil in the magnifier configuration).  Typically, we aim for values
> > between
> > 0.1 and
> > 0.25 or so for a conventional tesla coil, and 0.4 or higher for
> > magnifier
> > primary/driver coil systems.  "M" is the amount of mutual inductance
> > between the
> > primary and secondary coil.  We define:
> > K = M / SQUARE_ROOT [ Lp x Ls ]
> > where Lp is the primary inductance, Ls is the secondary inductance,  M
> > is the
> > mutual inductance, and K is the coefficient of coupling between the two
> > coils.
> <snip of quiz>
>
>      k itself is relatively easy to measure and quantify. But as far
> as I know, the figure says nothing about _how_ the primary is
> coupling into the secondary. For example, I can score identical k's
> with a flat primary with the same outer dimension as that of the
> resonator when placed below the resonator as I can with a helical
> primary with the resonator sitting inside it (easy). Has anybody
> examined performance issues around the degree of coupling into say,
> just the bottom few turns vs a primary that loosely couples a long
> way up the secondary? I have a gut feeling that this may be a
> significant determinant of system performance given the normal high
> secondary Q's and the fact that all else being equal, performance
> for a given power input varies so widely in different systems.
>
> Malcolm

Hi Malcolm and All

I formerly used flat spiral coils. I now exclusively use cylindrical or
solenoid wound primaries. I have not taken the time and effort to
quantify my results but my gut feeling is that I get much better overall
performance with the cylindrical primaries. Usually the primaries are
about 4" to 6" larger in diameter than the secondaries and the
secondaries are set 2" to 4" above the top turn of the primary.

Skip

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