Capacitor

I want to measure a smd capacitor with a vna. Without any test fixture, just by contact with K connector of the vna.

How to visualize impedance?
How to get impedance & capacitance value from 1 port S parameter?

thank you

You need a 2 port setup on the VNA for doing capacitor evaluation. The same as the capacitor manufacturers, when they release the s2p files for their components.
And also you need a fixture for high frequency evaluation.

If the capacitance is not too low nor too high (roughly from 10 pF to 1 nF) and a high precision is not required, you can try to measure it setting the VNA at a frequency lower than 100 MHz (the lower the better). Then display the S11 Smith chart. Many VNAs gave the possibility to place a marker having a readout directly in LC units (uH or pF). Otherwise you will have a measurement Z = a + jb ohm. Measuring a capacitance "b" will be negative. Then simply apply:

C(pF) = 10^12/(2*pi*freq*|b|)

where "freq" is the frequency at which you placed the marker.
Before the measurement is better to check the internal calibration: leaving the connector open you should see the trace (a dot) on the right of the Smith chart on the orizzontal line. Shorting the connector you should see the trace on the left of the Smith chart on the orizzontal line. Good luck.

Ok, really helpfull

From 1 port S parameter measurement I got Z parameters and I computed:

C = -1/(2*pi*freq*imag(Z(1,1)))

I guess it's similar finally.

But I used capacitor value higher than 1 nF and it seems quite accurate for my needs.