Ed,
What I am trying to
understand is how one relates the marked value of a component to what nanovna
shows.
The value marked on a component is supposed to be valid at a very low frequency. As soon as the frequency goes up, parasitic effects become increasingly important, so the actual value of the part varies with frequency. Also, of course, there is always a tolerance. With good capacitors and air-core inductors this tolerance might be only 5%, but with ferrite-cored inductors it can easily be 40%, and with some sorts of ceramic capacitors it can be even larger.
So, rule #1 is to measure on the lowest frequency possible, and rule #2 is to never forget that the marked value is subject to a tolerance.
But then the characteristics of the measuring instrument come into play. The NanoVNA is natively a 50? instrument. It should produce the best accuracy when measuring impedances reasonably close to 50?. When the impedance gets close to zero, or into the kiloohm range, the accuracy of the NanoVNA drops. So, if you are measuring small values of capacitance or inductance, measuring at the lowest frequency the NanovNA supports might produce poor measurement accuracy.
So, rule #3 is to measure at a frequency where the impedance of your part is at least close to the order of magnitude of 50?.
In practice that means that you should look at what frequency the part has a reactance of 50?, and then measure at a frequency a few times lower. If the measured value is reasonably constant over the range between those two frequencies, then probably you have a valid measurement. If instead it varies all over the place, it probably means that there are too high parasitics even in that frequency range.
Of course I'm assuming that you properly calibrated the NanoVNA, putting the short and the load exactly at the same place where you then put the part to be measured.
In my experience it's best to use the shortest possible connection between the NanoVNA and the part under test. It seems that correct measurement of difficult impedances through a long piece of coax cable is harder for the NanoVNA, even when carefully calibrated through that long cable.
Always keep in mind that if you do all this and get a consistent result, then you are getting the low frequency value of the part you are testing, and that at higher frequency its actual value will change. At a high enough frequency a capacitor becomes a short circuit, further up it becomes an inductor. And what's an inductor at low frequency will become an open circuit at some high frequency, and a capacitor beyond that. Generally both inductors and capacitors will rise in value, when you start going up in frequency starting from a low frequency. But core materials tend to decrease their permeability beyond some frequency, and this effect can win over the other in some cases, so you might see cored inductors whose value goes down rather than up, when increasing the frequency. These are all real effects, not measurement errors! In RF work you often need to measure each part at the frequency you will be using it, rather than trusting the value printed on it, which is valid only at low frequencies.
