Roy Lewallen wrote in
:

Thanks Roy.

I was suspicious that proximity correction could ignore conductivity,
permeability, frequency and the actual diameter and distance.

Chipman paints a picture that it is pretty messy mathematically, and the
flow into Zo would be really messy.

What I do glean is that for D/d2, the error is small. That is a whole
lot better than the log formula which is poor for D/d10.

I have two immediate applications, one is a model for a 1:1 choke balun
(http://www.vk1od.net/balun/Guanella/G.1-1.htm) and the other is yet
another transmission line loss calculator, along the lines of my existing
calculator that is aware of about 100 standard transmission line types,
but this one allowing specification of an arbitrary two wire line
(http://www.vk1od.net/tl/twllc.htm).

For the balun cases, I am very interested in D/d2.

I think I will continue to use the acosh expression, and let it calculate
without limit on D/d, but qualify the results with a note that proximity
effect is not included in Zo, Zo is underestimated for D/d2 and the
error may be significant.

Roy, thanks for your time to research and type the notes up.

Owen