Roy, Tom, Richard and Richard,

Thanks.

I only asked half the question, I was also interested in a proximity
corrected calc for Zo.

The chart in Laport looks useful Richard. I have digitised it and tried a
number of curve fits. I think I will just use a cubic spline
interpolation to my digitised points where D/d2.5 and 1.1, and above
that range I will use 120acosh(D/d). Laport doesn't give an expression
from the graph, so I suspect that it is derived from experimental data.

Tom, thanks for the pointer to Reg's expression. I wish I knew where he
got it from, or something against which to validate it... but it is a
start. Interestingly, Reg doesn't seem to consider proximity effect in
calculating Zo (other than indirectly from the implied R element).

Roy, I note from Googling that a lot of work has been done in recent
years on skin effect in PCB tracks, ie small rectangular tracks over a
large ground plane, but haven't seen work on traditional two wire open
line.

I am playing with a model of a Ruthroff 1:4 balun. My model treats the
windings as a coiled lossy transmission line, so good estimation of Zo
and R are important to the results.

The ugly part of ferrite baluns is the ferrite. I have used published u'
and u'' vs frequency curves to calculate the choke impedance for a more
realistic estimate.

Comments appreciated.

Again, thanks to all for the hints.

Owen