To make things harder for the nanoVNA I pulled a couple of 220pF ceramic disk caps from an old Cobra 148GTL-DX CB radio that was probably made in 1981. I put them in parallel to make up 440pF.
These caps should have a higher Q than the previous Philips 470pF poly capacitor. See below for a comparison between the Agilent VNA and the nanoVNA. You can see that the nanoVNA is really struggling and (as expected) the data gets really noisy at 10MHz where the Q is just over 1000. This is a 'very' harsh test for both VNAs and also the cal kit and the test fixture but you can see that the pair of 220pF caps in parallel have a much improved Q compared to the poly 470pF cap.
I think it would be better to measure the Q a different way because I think there will be a fair bit of measurement uncertainty creeping in to the system when measuring higher Q capacitors like this but at least the Agilent VNA and the nanoVNA are performing roughly as expected across 5MHz to 50MHz for this type of capacitor.
