To test the imaginary high impedance measurement accuracy of the NanoVNA I decided to use a high quality precision capacitor. The best precision capacitor I had on hand for the job was a leaded silvered mica 30pF +/- 0.5pF type. I kept the connection leads to the BNC connector as short as possible.The results from these tests will also reflect the measurement accuracy for inductance which of course would be of opposite sine. As there is no such thing as a pure inductance because of wire resistance, distributed capacity and self resonance characteristics it is not practical to attempt to use an inductor for measuring the capability of the NanoVNA.
I chose to set the sweep to cover 1 MHz to 150 MHz. I limited the high end of the sweep to avoid influence from lead inductance which causes series resonance to occur around 320 MHz. The reactance range for the selected sweep width extends from 5462 Ohms at 1 MHz down to 28 Ohms at 150 MHz. The capacitor measured 29pF at 1 MHz, 31pF @ 50 MHz and 37pF at 150 MHz. Attached are screen captures and s1p files for the calibration steps and measurement. Accuracy at 1 MHz to 50 MHz was within about 3% with the Z 5.5K at the low end. Accuracy deteriorated to 10% at about 100 MHz then dropped off further to 20% at 150 MHz.
It is interesting that according to these test results the shunt measurement method seems also capable of accurately measuring a moderately high imaginary impedance with very little real component. Good news for most of my applications. It appears as if the shunt measurement method accuracy capability is not limited to just a few hundred Ohms for lower frequencies.