Network analyzers incorporate a concept called a "reference plane". This
is a theoretical point at which the measurement is actually made. It's
desirable to have this point be at the DUT connector. (In precision
and/or extremely high frequency measurements, the point within the
connector becomes important, and even a sex-change adapter can't be
tolerated between calibration and measurement.) Software in the network
analyzer is told where the reference plane is to be by means of a rather
involved calibration procedure, then the network's software corrects for
the phase shift and impedance magnitude transformation of the cable
between the reference plane and the analyzer itself. It effectively
makes the reference plane the point being measured, rather than the
analyzer input terminal.

When you make manual measurements, you have to do the correction
yourself. So what you need to know is the impedance and length of the
line between your point of measurement and the DUT. This can be
determined in the same way as it's done for some network analyzer
calibrations -- by measuring the impedance with the DUT replaced with a
short circuit, an open circuit, and a known load impedance, then solving
the resulting set of simultaneous equations. Once you know the impedance
and length of the cable between where your measurement is correct and
the DUT, you can calculate the actual DUT impedance from your measured
value. I do this routinely at HF, when I measure antenna impedance at
the input end of a transmission line. Accuracy is best when the
impedance being measured isn't far from the Z0 of the transmission line,
and the transmission line is short. The longer the line and the greater
the difference between line Z0 and DUT impedance, the greater the
sensitivity to measurement error in both the measured DUT impedance and
the line Z0 and length. A surprisingly small amount of line loss can
also skew the measurements quite badly if Z0 and DUT impedance are quite
different. If you need accurate results, you should do an error analysis
to see how far off your calculated result can be, given the estimated
accuracy of your measurement and calibration.

As I mentioned in my earlier posting, most people overestimate their
ability to make accurate RF measurements. It's not at all trivial. Be
sure to check your results frequently by measuring known load impedances
close to the values being measured. How do you find the values of those
"known" load impedances? Well, welcome to the world of metrology!

Roy Lewallen, W7EL