Ian, G3SEK wrote:
"1. The Bird wattmeter does not in fact sense power."

The speedometer does not measure miles nor hours, but it can
satisfactorily indicate miles per hour. So, what?

"2. It only indicates power due to its calibration."

Without calibration most instruments only serve as crude indicators. The
usual Bird is calibrated for a 50-ohm line which enforces its Zo. Use
the common Bird in a different line and its calibration is off.

"3. But in this discussion, transmission line theory is under question."

I suggested using the Bird as an indicator of transmission line / load
match, period. This is based on balance in the directional coupler which
is usually good by design and factory adjustment. Many Bird users have
adjusted their lload matching networks for minimum reflected power
indication.

My allusion to the Bird was meant to show that it makes no difference if
you have two equal and opposite waves traveling in the reverse direction
on a line or no waves in that direction at all. A Bird wattmeter
operating properly should indicate zero reflected power.

Best regards, Richard Harrison, KB5WZI