Free Space Calibration Procedure Dear Swadesh,
I would recommend that you perform a free space TRL calibration. The through is simply the distance between the two ports (antennas) without any sample. Reflect is a reflective plate in the sample holder. Line is the additional distance between the two ports depending on the measurement frequency - take the quarter wavelength of the mid-band frequency and move the second antenna by the corresponding length - example X-band - 9 GHz - 8 mm = line is through length + 8 mm.
Make sure you specify a sexless connector with a characteristic impedance of 377 Ohms.
Hope this helps.
Yassen