Magnetic loop antennas are commonly used for investigations on magnetic fields in a frequency range up to 30 MHz. As for all antennas the antenna factor is the result of a calibration procedure. Commonly used calibration routines like the standard field, standard antenna and three antenna method will be compared to a newer approach called impedance method, first introduced by Ishii [2]. Finally a new approach for contactless measurement of the antenna factor, the so-called in-situ impedance method, will be presented as well as a comparison of the resulting antenna factors of all calibration routines.