So... in coaxial cable TEM is just "normal" electric circuit behavior and that's why it has no cutoff frequency? Despite coax's (or parallel plate's) gap size being centimeters it can pass 100 meter wavelength RF signals because the waves don't actually have to fit in the gap. They're from charges moving in conductors. And the TE/TM modes are very different mechanisms entirely where the space between the conductive cavities has to be big enough to support the electrical length of the frequency's TE or TM mode electromagnetic wave propagation? So tiny diameter coax in TEM mode can pass DC to some high GHz, it's actual size doesn't matter because it's just wires being wires (even if impedance is defined by the ratio of radii). But with a the TE_11 mode in a too big for the frequency coaxial cable, the absolute size *does* matter. And no centimeter size coaxial cable can pass a 100 meter size TE_11 mode wave (efficiently). But a giant 50 meter coaxial cable of the same impedance could.
