John S wrote in :

....

If you are not concerned with trying to calibrate out the directivity of
the coupler (and if that is greater than the expected / tolerable Return
Loss, you don't need to do so), and you have convinced yourself that Vf
is independent of load impedance (as it will be if Zs=50+j0 and you use
short low loss line, or a large attenuator at the coupler to control
Zs), then the simple approach is to do the following.

I think I am the confused one. Do I even need to know the transmission
line characteristics if I am going to short the load end and set the
vector voltmeter for a phase reference of 180 degrees?

I am following the HP app note AN-77 and they do not mention a
transmission line. They say to short the load end of the coupler. I
need to get my antenna away from the test setup, so I add the
transmission line.

And you understand that the Gamma found is at the reference plane (the
plane of the calibrating s/c), and you can adjust it, or the calculated
impedance to another point on a known feedline using the well known
Telegrapher's Equation (http://www.vk1od.net/calc/tl/tllc.php solves
this problem for a range of popular lines), albeit subject to error due
to uncertainty about the known line.

(I did consider at one stage extending TLLC to allow specification of
mismatch in terms of Gamma, rectangular and polar, but no one ever asked
for it and I thought it not in demand. The complication is that finding
Z from Gamma needs to use the nominal Zo of the test equipment, not the
actual Zo of the lossy transmission line. I usually use a spreadsheet to
perform the calcs, Excel can handle complex numbers using the COMPLEX
and IM* functions either in the Analysis Tookpak in earlier versions, or
built in to the later versions.)

An important thing to keep in mind is that while the measurements you
make are of the TL in differential mode, it may be carrying significiant
common mode components which will affect the differential currents. In
making your measurements, if you change the common mode current path
from the normal system configuration, you are measuring a different
system and the results might not apply. There seems an unwarranted
assumption in most discussion of such measurement projects that there is
inisignificant common mode current.

Has this made any sense?

Perhaps it is my turn to ask.

Owen