Hello dowjones,
I think it's not the same thing: for a stripline structure for most of e.m.
codes you have formulas to estimate guided lambda, characteristic impedance (given the line width), line width (given the line impedance).
As an example, Ensemble offers this option... Of course you can't consider discontinuity effects but it's a good starting point.
In my case, the rectangular coaxial line is a too particular configuration; in fact I can't find in litterature formulas for the associated line impedance... So I'm obliged to build a configuration by a 3D code, mesh it very carefully, run the simulation and to hope the result to be accurated...
If I consider more lines with different rectangular sections, I can have strong discrepancies between simulated and measured components...
Hope you find some article or formula about configurations of rectangular inner conductors into squared outer conductors.
Regards
Lupin