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I've done some measures of S parameters with a VNA a long time ago. I'm not an expert om this, but I remember the procedure of calibration, which was using, for a single port, the three lumped elements open (infinite impedance,) short (0 impedance) and load.

Let's consider a coaxial cable which connects the VNA to your device under test. For the calibration procedure, we should use a component called "open" instead of simply leaving the coaxial cable floating (which from a circuital point of view is equal to open) because at high frequencies such a cable radiates. So, the coaxial cable radiates some power to the air, and this means that even if it is left open, it works as if it was loaded with a radiation resistance of the coaxial cable acting as an antenna.

For this reason, we must use the "open" component to get accurate results. The explanation I've written is that I've received at university.

How is the "open" component internally made? From the previous description, it should be something preventing the coaxial cable from radiating whilst having infinite circuital impedance.

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The calibration open for a VNA usually has several vital things

  • A shield over the open so there is no radiation loss. This may take the form of a solid cap, or could simply be an extended outer for at least three cable diameters. In the latter case, an evanescent wave travels into the outer, and is sufficiently attenuated before it gets to the open end that negligible power is lost.
  • A known length from reference plane to the end of the inner.
  • A known additional effective length to account for the discontinuity capacitance at the end of the inner conductor.

Depending on the connector series, the open may be little more than a radiation cap, relying on the analyser connector itself to define the electrical length.

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This is a picture of the parts in the calibration kit for my NanoVNA:

enter image description here

  • To the left is the 50 ohm terminator. The pin in the center is connected to the outer conductor through a 50 ohm resistor hidden in the cap.
  • In the center is the short circuit terminator. You can clearly see that the center pin is connected directly to the shell.
  • On the right is the open circuit terminator. As you can see, it has no pin.

The outer shell of the open circuit terminator is closed over the end of the cable, acting as a shield to the center conductor, but the center conductor is not connected at all.

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