NanoVNA – how accurate does the LOAD need to be – part 2? answered the question in the context of an instrument that assumes the LOAD is exactly 50+j0Ω.
As mentioned in the article, there are more sophisticated models of the imperfection of nominal SHORT, OPEN and LOAD calibration parts (or ‘standards’), but the NanoVNA-H4 does not currently implement any of them.
A simple improvement is to accurately measure the DC resistance of the LOAD part, and use that resistance to calculate the expected LOAD response, and to use that in calculating the error terms that will be used in correction of raw measurements.
Precision Loads are expensive
LOADs with specified high ReturnLoss is an expensive option. Try to find a LOAD with ReturnLoss>40dB (VSWR<1.02) for less than the price of the entire NanoVNA.
Roll your own
Another option is to buy some 0.1% 100Ω SMD resistors (50 for $7 on Aliexpress) and make a LOAD device that is suitable for calibration, but it is challenging to make such a lot that is good above say 100MHz.
Select for DC resistance
An option I have used is to buy a few inexpensive 6GHz Chinese terminations (~$6 ea) and measure their DC resistance, selecting the best as ‘good’ terminations. The best of a bunch of three I purchased had Rcd=49.085 which equates to RL50=54dB.
If the NanoVNA-H4 supported calibration based on DC resistance of the LOAD…
Some suppliers offer inexpensive calibration kits with measured DC resistance, eg SDR-KITS.
Some users may have a 4W ohmmeter that is capable of high accuracy measurement of loads, preferably to 0.1% uncertainty or better.
If the DC resistance was used during calibration / correction, then the instrument Directivity at frequencies below about 100MHz would be limited mainly by the instrument noise floor.
I have suggested such a facility to Dislord for NanoVNA-D so that its accuracy as a stand alone VNA can be improved.
See Dislord’s NanoVNA-D firmware v 1.2.35 includes a facility to apply a correction based on the DC resistance of the LOAD for implementation details.