Richard Fry wrote:
"If the Model 43 is unable to make an accurate masuremnt of this, is
that not due to reasons other than not having the right V/I ratio in its
line section?"

Many details of desisn, construction, and application must be chosen and
executed right to get accuracy, but the line impedance is essential.

Bird can adjust the current sample to exactly equal the voltage sample,
both taken from the transmission line at any point. But it must work
with a fixed voltage to current ratio. Bird chose 50 onms.

For a directional meter, it`s necessary to respond to one direction
while rejecting the other. When power is applied to a line, the
resulting current is is in phase with the volts. On reflection, the
volts and amps in the reflected wave are 180 degrees out of phase. The
phase difference of the reflected wave is used by Bird to distinguish it
from the incident wave.

By selecting and adjusting for equal samples of volts and amps in the
forward wave, their total is 2X that of either sample. But, the samples
from the reflected wave, being equal but out of phase, cancel.

To get the value of the reflected power samples, it is only necessary to
reverse the polarity of one of the samples. They are now in phase and
the forward power samples are now out of phase and cancel.

If some other voltage to current ratio is used for the power samples
than that of the design, the samples won`t be exactly equal and
cancellation of the undesired direction does not work.

Best regards, Richard Harrison, KB5WZI