So, I got nanovna V2 and calibration kit that comes with it. But I also have two more SMA 50 ohm terminators of the unknown specs/history.
Obviously, if I calibrate nanovna with any of those, other ones show different results. How to determine which one has best characteristics?
you can measure it's resistance with multimeter. Close to 50.00 Ohm is better.
You can calculate SWR in the following way:
SWR = max(50, R) / min(50, R)
For example, if you're measured 49.98 Ohm, it means that this dummy load has SWR = 50 / 49.89 = 1.0022 at DC.
This cannot guarantee that it's impedance will be the same 50 Ohm at high frequency. But at least you can choice better one for low frequency in such way...
If dummy load has very flat response from 10 kHz to 500-1000 MHz, then there is a reason to believe that your measurement with multimeter for DC will be very close to the value for high frequency.