hi:

i'm trying to find a way to get an accurate approximation (oxymoron?) of
mutual inductance between coils for different cases like tapped coil,
transformers, bifiliar/trifiliar/quad ... wound transformers, etc. with
different cores. so the mutual inductance can be used in analysis
without having to build the coil first to measure the M.

i was looking for the Neumann double line integral and all i could come
up with was

http://www.distinti.com/docs/ni.pdf

sec.6 of that doc looks promising, but i'm not sure it's the Neumann
integral. it allows the separation of the centers of the coils to be
changed, but it looks like that's only along the radial axis. so i don't
think you can use it for coupled coils offset along the long axis, like:

---/\/\/\/---/\/---

without modifying the equation.

it's also for single loop coils, though there's an adjustment for multi
turn coils.

another program i found which is said to be based on the Neumann
integral doesn't allow other permeabilities and the real ****er is that
it's answer for two single turn loops isn't the same as that of the
other program. could be because it's for tesla coils (it'll do solenoid
style windings) and outputs answers for toroid capacitance, so the whole
numerical method may be diluted. i don't know.

anyone have a solution for this? i'd like a link to something on the
Neuman double line integral, which i couldn't find. any other method
that works is ok, also.

thanks in advance,
mike