Thanks for this info!
I finally find a demo version of Linpar from Artech House. I tried to
analyze the setup above, and as a result I got an impedance matrix.
However, I can't figure out how to get the differential impedance from
that matrix. It's probaly really trivial, but...
In another post I got the tip that I should try insertinga ground plane
halfway in the structure. I tried that also, and got the Z at 57 ohms.
It doesn't seem to be very accurate, does it?
Any comments from you SI experts out there?