I found that above 300 ohms or so the nano has trouble with accuracy. I used a GR-1606-A to measure an impedance and then compared that to the nano and as the Z got higher the accuracy diminished. Oristo explained the reason for this to me and it is because of the limitation of the resistive bridge. Even HP/Agilent/Keysight VNA's accuracy degrades at high impedances but not nearly as bad as the nano. They use directional couplers instead of resistive bridges (Wheatstone basically). One could use the nano with a directional coupler in S21 mode and do an open/short/load cal on it. The S21 mode returns the amplitude and phase angle of the reflected signal relative to the incident signal and from it you can calculate Z and all the other parameters. I have not worked on this yet but have used the HP8721A coupler in S21 as a great scalar return loss measuring system with about 50-60 dB of range and with excellent directivity.
In regard to measuring a typical axial lead resistor, many have shunt capacitances of 0.5 pF and up so this becomes significant at RF. Some are even spiral-wound.
