I'm trying to measure the S11 parameter of the NFC PCB antenna (actually PCB coil) by VNA.
Desired Smith chart pattern is like the below picture (probably with different values, because of different PCB antenna size), which is contained in application note document. However my Smith chart is not like it. Both measurements are done using Rohde & Schwarz ZVB20.
I checked probes and connections many times and calibration is done before tests. Also there was not any obvious noise source in the measurement environment.
Real part of my measurements was changing from 2 ~ 11 ohm. Has anyone got any idea why my Smith chart is like this? And why my measured values are not stable and changing permanently?