I did some measurements on a variable capacitor in my junkbox. It was from an old MW receiver.
I measured the closed position on a calibrated DE-5000 LCR meter and it was 367 pF.
In the open position it measured 8.5 pF
I set my NanoVNA-H4 to a stimulus range of 1 to 10 MHz. I used a RG-58 cable with alligator clips on the end. This type of test jig is OK at lower HF frequencies but stray capacitance/inductance affects the results as you go much higher.
A SOL Calibration was done using the alligator clips and a 50 ohm carbon resistor (non inductive). When connecting the alligator clips to the variable capacitor make sure the ground alligator clip goes to the frame of the variable capacitor.
The NanoVNA gets the most accurate reading when |Z| is 50 ohms. In the fully closed position the capacitor measured 365 pF when the magnitude of Z was 50 ohms (bottom of Smith Chart). Only 2 pF difference compared to the DE-5000. Connecting the the NanoVNA to a PC and using the NanoVNA app the measured capacitance from 1 to 10 MHz was plotted. The measured capacitance ranged from 371 pF to 363 pF which is quite close.
Next the variable capacitor was set to the open position. It is not possible to measure at |Z| = 50 ohms because this occurs around 375 MHz.. However the NanoVNA-H4 still gives a reasonable estimate ranging from 8 to 8.3 pF over the 1 to 10 MHz. range which is close to the 8.5 pF measured on the LCR meter.
Experimental results are attached.
Roger
Roger Need
