Why doesn't an open or short look like a dot on the Smith chart?
After performing a one-port or two-port calibration on
your network analyzer, what do you expect to see when you
measure the open standard from the calibration
kit? Some people expect to see a perfect dot at the edge
of the Smith chart. After all, a perfect open has a
reflection coefficient of one at an angle of zero degrees
at all frequencies. And since you are measuring the open
that you calibrated with, it should look perfect, right?
This sounds reasonable, but in fact the measurement of the
open will look like an arc instead of a dot. This is a
common source of confusion for many VNA users.
This measurement result is actually correct - there is
nothing wrong with the network analyzer system or the
calibration. You see, the cal kit's open standard is not
an ideal open. It has nonlinear fringing capacitance,
electrical length, and possibly some loss due to radiation.
All of these imperfections are included in the model of
the open as described in the calibration kit. Because
the goal of the calibration is to make your measurement
system give accurate results, and the open standard is
not ideal, it would be inaccurate for the calibrated
measurement system to report that it is an ideal open.
In most cal kits, this same argument applies to the
short standard as well. For mechanical construction
reasons, the actual short circuit is a small distance
away from the calibration plane, so the short standard
looks like a short circuit at the end of a very short
transmission line. When you measure it, the result is
also an arc on the Smith chart.