Antenna phase center

Well, if you analyze the transmission phase between the antenna input terminal and various +/1azimuths along a constant radius, then you have all the information you need to compute where the effective antenna phase center is.

When measuring an Antenna in the anechoic chamber, how should be positioned the antenna we want to measure, the DUT one? I read it has something to do with the antenna phase center, but it's not clear to me. About how to determine in HFSS the center position of the antenna, can you give more information.
Thanks

Simple antennas I have used, like quarter wavelength over a ground plane, chip antennas on a ground plane, etc have a somewhat moveable phase center, so you do want to be careful how they are measured. For instance, anything within 5 degrees elevation from the ground plane will be pretty funky. I have had the most luck with horn antennas, which usually have a nice stable phase center right at the horn aperture plane. Interestingly, quarterwave antennas usually show a phase center a little way up the antenna from the ground plane, instead of right at the ground plane like "theory" would suggest.