Owen wrote:

"The myth that measurements with a Bird 43 of conditions on the Thruline
section are invalid unless it has some minimum length of 50 ohm line on
both sides of itself is busted."

Bird says the wattmeter can be placed anywhere in the line. That
precludes a requirement to have minimum lengths of 50-ohm cable on both
sides of the wattmeter. Particular total lengths of line woould be a
function of wavelenth.

The Model 43 Thruline Wattmeter accurately measures forward or reverse
power in transmission lines under any load condition.

Regardless of the load impedance, the forward and reflected powers are
forced by construction of the coax to conform to Zo. Line volts divided
by line amps in either direction has an absolute value of 50 ohms.

Bird plug-in elements are designed for insertion into a precision
coaxial rigid air line which is a part of the Model 43. Elements are
available in a wide variety of frequency ranges and maximum power
levels..

Cancellation of response to one direction of power flow while responding
to the to the other is accomplished by careful balance of the current
and voltage samples within an element. The samples which are in-phase
add. The samples for the other direction are out-of-phase and cancel.

To make sure cancellation is complete in the undesired direction, Zo
must be as specified in the design. Accuracy can`t be expected in the
wrong Zo environment.

Best regards, Richard Harrison, KB5WZI