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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.


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