VNA calibration: "open" standard?

[dext0rb]

Hi all,

Been watching VNA videos on YouTube; hoping to own one someday. 

During one calibration step, the user connects an "open" standard to the VNA.
What makes this more ideal than just leaving the cable end unconnected?

My guess: entirely isolates the signal from the return, eliminating any capacitive coupling between tip and return.

Whats the real answer? Thanks!
 

[skipjackrc4]

When a connector is mated with another connector, the pin geometry changes slightly, which will alter its electrical parameters.  This is especially true when using microprobes instead of cable ends.

Also, the "open" standard of a cal kit will be supplied with calibration files that tell the VNA what the value of capacitance across the open is.  This is necessary because it's not a perfect open--there is still some reactance there.  If you were to calibrate assuming that the open was perfect, you'll get inaccurate results.  The same is true with a shorted load, except the inductance is more important.  Because cables change from one to another, you can't assume the value of capacitance will be constant between them unless you use a pre-measured standard.

[Bud]

Calibration standards are called so because they have known parameters. The Open standard does not eliminate fringing capacitance but rather has it well defined, and that known value is entered into the VNA so when the VNA measures some voltage during Open calibration step it knows this voltage is caused by that much of fringing capacitance.

[dext0rb]

Ahhh, makes sense now. Thanks!