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I have also been looking to answer how accurate should the NanoVNA be at HF.
Most components I want to measure are wire ended so my test jig is a 60cm
RG316 with SMA on each end cut in half. Each cut end has 1cm of inner exposed
and an alligator clip put on. This is calibrated with open, short and a 50 ohm
made from 2 100ohm resistors with short wires soldered on. A 100pF silver mica
then gives the attached which indicates 105pF at 7MHz, 109pF at 14MHz and
126pF at 28MHz. I remove the alligator clips and repeat soldering the
connections (inconvenient but worth trying). This gives the same results.
Should the NanoVNA do better than this? Are there firmware versions which will
produce more accurate results? Is it my VNA hardware(made in China somewhere)
or test jig or test method?
I have found the VNA gives good results using S21 on filters but using a
single cable and S11, capacitors and inductors are difficult to measure. I
expect ferrite cored inductors to show a frequency variation but a 100pF
capacitor should still be 100pF at 30MHz. I've repeated calibration, tried
different leads but I don't have any confidence in the accuracy of reactance
measurement. From what I have read, I could manually add "fudge factors" to
the calibration and remove what looks like a systematic error but that is not
so easy for the mathematically challenged (like me).
My conclusion is that the accuracy depends on the accuracy of the standards
you have and the amount of effort you put into "improving" the calibration.
Some test jigs might be easier to get a good calibration but up to 30MHz the
test jig is not the most important factor.
I have done what Erik suggests and measured a 100 ohm resistor. It shows no
significant reactance at 30MHz. I don't have a record of it but certainly not
26pF.
73 Brian.

I have also been looking to answer how accurate should the NanoVNA be at
HF. Most components I want to measure are wire ended so my test jig is a
60cm RG316 with SMA on each end cut in half. Each cut end has 1cm of inner
exposed and an alligator clip put on. This is calibrated with open, short
and a 50 ohm made from 2 100ohm resistors with short wires soldered on. A
100pF silver mica then gives the attached which indicates 105pF at 7MHz,
109pF at 14MHz and 126pF at 28MHz. I remove the alligator clips and repeat
soldering the connections (inconvenient but worth trying). This gives the
same results. Should the NanoVNA do better than this? Are there firmware
versions which will produce more accurate results? Is it my VNA
hardware(made in China somewhere) or test jig or test method?
I have found the VNA gives good results using S21 on filters but using a
single cable and S11, capacitors and inductors are difficult to measure. I
expect ferrite cored inductors to show a frequency variation but a 100pF
capacitor should still be 100pF at 30MHz. I've repeated calibration, tried
different leads but I don't have any confidence in the accuracy of
reactance measurement. From what I have read, I could manually add "fudge
factors" to the calibration and remove what looks like a systematic error
but that is not so easy for the mathematically challenged (like me).
My conclusion is that the accuracy depends on the accuracy of the
standards you have and the amount of effort you put into "improving" the
calibration. Some test jigs might be easier to get a good calibration but
up to 30MHz the test jig is not the most important factor.
I have done what Erik suggests and measured a 100 ohm resistor. It shows
no significant reactance at 30MHz. I don't have a record of it but
certainly not 26pF.
73 Brian.

Hi again Jeff;

I believe I now sufficiently understand the technical aspects of the discussions in this thread to forego the wizardry behind the pursuit of high accuracy. it appears sufficiently sound.

On that happy note… I will state my one remaining question succinctly. Why the obsession over accuracy at the the two most unstable phase regions of highest Q and unreachable limits of infinity and zero?

A reasonable and credible answer will be a bounded tolerance of impedance or phase in those regions, and an estimate of the consequence of exceeding the tolerance boundaries.

I will reiterate… There is nothing wrong with how this is treated what is being done or the rationale behind the obsession. The only question is simply... Why?