With any analyzer it is necessary to take into account the coax feeding the antenna. The actual antenna feed impedance are modified at the far end of the coax by the characteristic impedance of the coax.
Usually some sort of calibration using a load/open-circuit/short-circuit at the end of the cable should be performed, and then this cable piece can be used as a calibration reference for future measurements using that particular cable.
If a calibration at the far end of the coax cannot be performed, then cut the length of coax to be an exact multiple of 1/2 wave (allowing for the velocity factor of the cable). By doing this whatever impedance is present at one end of the cable should be more or less the same at the other end.
