Consideration of the measurement environment of loop antennas is of great importance since loop antennas for low frequency magnetic fields are mostly used under near field conditions. To do this in the right and entire way, the whole measurement set-up has to be taken into account. For this application it would be of great advantage to use a contactless method. As classical calibration methods cannot satisfy these requirements, a new approach to measure the antenna factor of a shielded standard loop antenna has been derived. Combining the impedance method with the in situ calibration of an impedance clamp, it is possible to determine the antenna factor in a contactless way, while considering the measurement environment. In this paper the electrical characteristics of the shielded loop antenna are described first, in order to understand the electrical equivalent circuit which is needed to calculate the antenna factor. Afterwards, the new measurement procedure is applied and compared with the old one, followed by a comparison with the theoretical calculated antenna factor.