Dave wrote:
ah, but the sensor isn't in a 75 ohm environment, it is in a 50 ohm
environment inside the meter and sees a 50 ohm load.

Again, 40mm is not enough to establish a 50 ohm environment
when embedded within a 75 ohm environment. The Bird is simply
sampling voltage and current without knowing the reference.
If the Bird were recalibrated for 75 ohms, it would read the
correct values in a 75 ohm environment.
--
73, Cecil http://www.qsl.net/w5dxp