In article . com,
wrote:

On reconnecting everything, however, the
problem still persists, although the 50 ohm load is now showing as very
close to a dead short; virtually the same spot on the Smith chart
overlay as when the socket is shorted, in fact. Once again, the other
loads do likewise and they all check out fine as 50 ohms with a DVM.
Very strange! I wonder if there's some setting I may have got wrong
somewhere. :-/

Paul-

What would be the result of calibrating the system with an inductive
"short"? If it can be done, results of testing with a good 50 Ohm load
might be skewed.

Fred

Good thinking, Fred. That's theoretically possible, I guess. But I'm
carrying out these checks purposely at only around 5Mhz to minimise
such high frequency effects. You may well be on the right lines, though!