Roy Lewallen, W7EL wrote:
"As any text can tell you, the value of Z (ratio of V to I) varies along
a line which has a reflected waves (i.e., has a standing wave)."

Not exactly, maybe the apparent Z. Uniform line is assumed and it has a
Zo determined only by line structure. Zo is identical for a signal
traveling in either direction. However, a directional coupler must be
used to measure voltage and current traveling in one direction while
ignoring voltage and current traveling in the opposite direction. A Bird
wattmeter uses a directional coupler.

Voltage to current ratio on a line with reflections and standing waves
is of little practical value except for determinimg whether the capacity
of the line is exceeded or nearly so.

I would argue that a microwave oven uses the real power delivered to its
contents and not the standing wave as an article attached to another
poster`s comments stated.

Best regards, Richard Harrison, KB5WZI