I recall an excercise in graduate EM class I took many years ago.
You draw the geometry and break it up into squares. Each square has
an impedance of 376.7 ohms in free space and you calculate the
charactistic impedance by connecting them in series or parallel. A
bit time consuming, but it works for arbitrary configurations.
The prof had a question on the mid-term where he made a meant to have
a coax with an off-center center conductor and we had to calculate
the impedance with this method. He made a mistake and the dimensions
given resulted in the center conductor being shorted to the outer
conductor. My answer of "zero ohms (by inspection)" only got me half-
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