# Re: More Coupling...

```Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>" <paul-at-abelian.demon.co.uk>

Barton B. Anderson <tesla123-at-pacbell-dot-net> wrote:

> Measured K on my flat primary for my 12.75" x 45" coil.
> The flat primary is 16.5"ID, 0.375"wire, 0.625" spacing (c-c)
> (or 0.375 edge to edge), and 11.6 turns in the case of using
> my 0.06uF cap setup.

> K = M / sqrt(Lp * Ls) = 627 / (107.13uH * 87.6mH) = 0.205 K

Assuming this is your 1000 turn, 18awg secondary, acmi predicts
you'll get that k-factor when the primary sits 4 cm above the
base of the secondary winding. If the primary is level with the
secondary base, the k is calculated to be 0.178.

I've made up a graph of k against primary height relative to the
secondary base

http://www.abelian.demon.co.uk/tmp/bart-k.gif

A small change in primary height seems to give quite a large change in
k as the primary starts to fall below the secondary.

p-s
(cm)    k
-3     0.156
-2     0.163
-1     0.171
0     0.178
1     0.186
2     0.194
3     0.201
4     0.208
5     0.215

Bart, I'd like to know the height of your primary wrt the secondary

Ed Phillips <evp-at-pacbell-dot-net> wrote:

> Grover (and BS C74) have occupied honored places on my book shelf
> for many years!  I'll look up Grover's tables at work tomorrow.
> I never really studied that part; just copied his formulae and
> tables for solenoids.

I only recently came across Grover's work, thanks to Mark Rzeszotarski
who drew my attention to it when I was struggling to map out the
mutual inductance of the secondary. They've been really useful and
(net of software errors!) quite accurate too.

Regards,
--
Paul Nicholson,
Manchester, UK.
--

```