Cecil Moore wrote:
Consider the following:

Source---50 ohm feedline---+---1/2WL 150 ohm---50 ohm load

Isn't the 50 ohms that causes rho=0 on the 50 ohm feedline
simply the V/I ratio at point '+'?

The nature of things a point '+' are undefined, so I can't address
that. But according to the way you defined the problem, the
characteristic impedance of the 50 ohm feedline is 50 ohms. That sets
the V/I ratio. The impedance is determined by the distributed
capacitances and inductances of the transmission line - not by the
voltage you put across it. Is there some other way I'm supposed to look
at it?

73, Jim AC6XG