Quite so - although it is seemingly enough at 50Hz to persuade them to sometimes use groups of multiple (usually 4) smaller conductors, rather than one large one, for overhead EHV transmission (well, at least, I've always assumed that is the reason, since it's what I was taught at school!). In any event, BAS seems to be thinking of using the outer wall of a waveguide as a single conductor, rather than using the waveguide as a waveguide. As I implied, a true 50Hz waveguide would surely be ludicrously large! (IIRC, the 'lower cutoff freqency' of a waveguide is half a wavelength!). The other mistake BAS seems to be making is to imply that one has 'right angle' corners in waveguides - which, if one has them, are, I think, extremely inefficient.The skin effect only starts to have an effect at very high frequencies.A waveguide carries electrical energy from A to B. Mentioning 50Hz merely reminds us that frequency is a factor to consider, and that at some frequencies cables might need to become hollow.
That's true. However, since the OP had asked whether one could have "90° corners", thought it would confuse him if I had said "yes, but you can't have a right-angled corner" - hence I wrote 'square'. I should have written 'zero radius bend'. However, I'm not convinced that your pedanticism is necessarily correct - cannot "square" be used as an adjective to mean "right-angled"?Being pedantic there is no such thing as a square corner. Nearest is a right angle corner. But a square has four corners.
Kind Regards, John