[deepsun]

What if it's something in between, say cross-section is not O-shaped, like in a hollow wire, but C-shaped or almost closed circle? Where one side is vacuum and another a dielectric.

Thinking about it I believe it would be one of three possibilities: 1 slow the signal to the slowest side, 2 increase circuit resistance, or 3 "smear" the wave, so a short sharp signal would arrive long and dull (increased reactance?).

[colanderman]

(Having pondered this a day) this is a good question and I do not know! I think this would be straightforward to perform a waveguide simulation of though.