According to theory the S21 shunt measurement is best for measuring low impedances. My experience on the NanoVNA-H4 has not been that good with this method. The CH0 and CH1 impedance should be 50 ohms for an accurate calculation and this is not the case with the NanoVNA-H and nanoVNA-H4. Adding attenuators to the test fixture helps improve things but my results were not as good as a S11 measurement.
Attached are some measurements done with S11 and S21 shunt on a 0805 SMD 1 ohm resistor. It measured 1.003 ohms on a 4 wire Kelvin DE-5000 LCR meter at 100 kHz.
Roger
Roger Need
