Thanks Tom,

I did mean to note in my last post that Laport's graph of D/r vs Zo with
proximity correction is frequency independent.

I thought the effect would be frequency dependent. Perhaps not sufficient
to be concerned about, perhaps he overlooked it?

I did ask Reg about the R calc a long time ago, and he dismissed the
discussion with an explanation that it was from notes in his engineering
notebook from a long time ago, and then went on the tell me the value of
his book of personal 'recipes', but he didn't share it.

When he did publish the source of a few of his progs, it didn't occur to
me to look at them for possibly useful hints.

I think the model I have now is probably adequately accurate to question
some of the golden rules of baluns. At the end of the day, the
uncertainty in characterising ferrites at a frequency, at a temperature,
at a flux level, and on a given day is the main worry. I don't think I
need to get into Bessel functions for the actual current distribution,
and trying to derive corrected values for R and Zo from first principles.

Owen