Dear antenna pros, I've always taken it for grunted about the
300/75/50 ohms of TV ribbon, coax, etc. But how does one measure it?
My ohmsmeter doesn't budge. Is there some standard formula, like wrap
grandma 100 times, with the far end connected to a cheeseburger in her
mouth, the near end finally displaying the characteristic 300/75/50
whatever ohmses?

You don't measure it with most instruments you would have . It is a
calculated value from the size and spacing of the conductors and the
insulation between them. There are ways to measure it but most would not
have them.

Here is a place you can go for an explination.

http://www.epanorama.net/documents/w...impedance.html