Richard Fry wrote:
"For a concept of what that length actually is in the real world, recall
that Bird Corp. and others supply directional wattmeters giving
reasonably accurate measurement of forward and reflected power --
leading to SWR value."

True, and these work with mismatched loads if you have enough 50-ohm
cable connecting the wattmeter.

The Bird Model 43 wattmeter is 5.125 inches (13 cm) wide. This is the
distance between its input and output connectors. This length of "high
precision 50 ohm coaxial air line designed for insertion between the
transmitter or load" requires either some more 50-ohm line or a matched
load to enforce Zo.

IF you were to insert the Model 43 into most 75-ohm transmission
systems, the precision 50-ohm meter line of 5.125 inches would not
likely enforce the 50-ohm V/I ratio and the meter reading would be in
error. At VHF, 1/2-wave of connecting line including the Model 43
wattmeter is ideal, allowing you to insert and withdraw the meter
without affecting the match.

Best regards, Richard Harrison, KB5WZI