Uhm, right. This is over my head...
I was looking to find another impedance measurements for the said antenna, so I can check my measurements against. It looks to me unlikely to obtain such, so how about this:
use a few resistors after calibration and verify I am reading their resistance with 5% margin and +/-j10 reactance ?
I have done so with two thick film resistors, 24 ohm and 220 ohm; results are within the limits above. Call it good?