Conducted disturbance measurements on the mains cable are seriously affected by the equipment under test port impedance of the artificial mains network (AMN). This impedance is called the AMN impedance. In order to measure the AMN impedance, an ac-coaxial adapter has to be used to connect the AMN to impedance measuring equipment. Accordingly, the adapter has to be accurately calibrated. This paper describes theoretical and experimental investigations on the characteristics of the conventional adapter calibration methods, i.e., open/short termination method, electrical length correction method, and Kriz's method. In addition, this paper proposes a novel calibration method named S-parameter method. Uncertainties are also evaluated for each calibration method. It is found that the open/short termination method, Kriz's method, and the S-parameter method yield almost the same corrected value of the AMN impedance.