I'm not using CST for 13.56 MHz RFID coils analysis, some general comments nevertheless.
13.56 RFID is essentially a near field problem. The radiation impedance part of the coil's real impedance can be usually neglected, the far field is mainly interesting as an unwanted EMI.
The coil or "antenna" impedance can be derived by an AC magnetics analysis, without considering EM phenomena (coupling of a electrical and magnetical field).
The real part impedance is mainly representing coil resistance, skin effect and eddy currents.