Gene, W4SZ wrote:
"The unknown of interest is generally a load or matching device, not the
improper coax."

Bird says:
"The Model 43 THRULINE Directional Wattmeter is a portable
insertion-type instrument which accurately measures forward or reflected
power in coaxial transmission lines under any load condition."

I accept the above description as I`ve seen confirmation under many
conditions.

Bird also says:
"Line section:
The line section is a high precision 50 ohm coaxial air line designed
for transmission line insertion between the transmitter and the antenna
or load. A socket is provided in each line section for the plug-in
element with the desired power rating and frequency range."

The width of the instrument including connectors is given as 4.25
inches. This makes the preciaion coax section longer than its width or
spacing. I conclude from this that it is capable of enforcing its 50
ohms.

Bird mentions cable lengths in regard to VHF measurements. They suggest
that insertion and removal of the 43 with random length cables attached
may affect the load match unless it is made exactly a total of
1/2-wavelength. In the 1/2-wave case, insertion and removal have no
effect on the load match. I`ve done this countless times and attest to
its effectiveness.

If there were other possible problems with Bird cable lengths, they
would have appeared during the past 50 years and been reported.

Best regards, Richard Harrison, KB5WZI