LOL... you could just make an air stripline and remove the top cover!

I have a related question... In quasi TEM transmission, waves in the two dielectrics try to travel at different velocities, does this mean that if the TL is long enough, you can have opposite phase in the fields within one dielectric and the other? If so, does this have impedance implications? (e.g. will the impedance be different at different points in the TL?)