Oh, the laws of physics don't preclude RF from getting in the ends of a
piece of coax, by the way.

There is no minimum cutoff frequency for the TEM mode in coaxial
waveguide.

There is in hollow waveguide with no center conductor.

You still need to be able to couple to the ends, and a floating center
conductor is not the best way to couple energy in. However, there's no
fundamental physical reason why currents *won't* flow on the center
conductor in an open-ended piece of coax.

Dan