Manfred's plots of the 2000pF 'Hi Q' capacitor (on the EFHW transformer
thread) obviously had a problem somewhere with the test setup or maybe his
2000pF capacitor was faulty?
Everything is possible. But I do think that the main cause of the Q inaccuracy is the limits of the NanoVNA. It's just very hard to measure a very small resistance when it's in series with a very much larger reactance.
To rule out a faulty capacitor, and calibration problems, I just made a new measurement. The guinea pig this time is a 470pF, 1%, dipped silver mica capacitor made by Sangamo, new from the bag, never used. I let the NanoVNA warm up for one hour, then erased the old calibration, made a fresh calibration with averaging over 8 measurements, then connected my capacitor with very short leads and measured with long averaging. The resulting Q curve, and also the RX curves are attached. I measured from the low end to 30MHz, because much beyond that the reactance is so small that the job gets even harder for the NanoVNA.
As you can see, the Q curve is far too low again. Leaving aside the huge inaccuracies at the low frequency end, where the reactance of such a small capacitor shoots through the roof (or rather the floor, this being negative reactance), it's just not possible that such a mica capacitor has a Q in the range of 60 to 270 over the HF range. It must be MUCH higher.
The RX curves show clearly that the NanoVNA measures a higher resistance at lower frequencies, where the reactance gets high. This suggests that the NanoVNA cannot measure the phase angle of the reflection with enough precision. Which is no surprise, given how critical this phase measurement is: A phase of 90 degrees would be infinite Q, while a phase of 89.94 degrees is a Q of around 1000, and a phase of 89.42 is a Q of 100. To make a reasonably accurate measurement of capacitor Q, a VNA needs to be able to measure the phase of the reflection with an accuracy of at least one hundredth of a degree! I think that we cannot expect such performance from the NanoVNA, and not even from much more expensive VNAs.
What I would love to learn is some trick to get around this limitation, so I can plot a valid Q curve over frequency for a capacitor.
Manfred
