those coupled drum core inductors

Why? And your second url doesn't exactly make the point in a way that is easy to follow.

The ferrite in the drums has permeability that is about a thousand times higher than that of free space, so the adjacent drum is going to suck in a whole lot of flux lines.

The transformer equations are

V1=L1.dI1/dt+M.dI2/dt

V2=M.dI1/dt+L2.dI2/dt

where M=K.sqrt(L1.L2)

That's somewhat surprising, indeed. Are the cores dumbbell-shaped? Does the coupling vary steeply with the spacing?

Jeroen Belleman

Stick a sig-gen across one and a 'scope across t'other, that will tell you how much coupling there is at any given spacing.

The inductors are covered by shrink tubing but seem to be tall skinny drum cores, dumbbells.

I've done that but didn't quantify coupling vs distance. I measured coupling and series inductance assuming two parts mounted as close to one another as makes sense, not quite mechanically interfering.

I used a third one as a mag field probe, to verify that the series pair has a magnetic null plane between the parts, and doesn't couple far-field very much.

The extra 6 mH helps my circuit a bit, without adding ESR. Cute trick.

I guess there could be other uses for this coupling.

I have made cheap high isolation voltage data coupling transformers that way.