As an essential component in power applications, magnetic cores and their design play an important role in achieving high efficiency and high power density. Accurate measurement of the core loss is important for inductor and power converter optimization. Loss measurement depends on exactly determining the phase angle between the voltage and the current. However, measurement errors can be introduced due to the discrepancies in propagation delays in the voltage and current sensors. In addition, the propagation delay is frequency dependent, but has a large influence in the MHz range and above. Previously, several methods have been proposed to compensate for this discrepancy but they are time consuming and can result in large measurement errors. In this paper, a characterization method for core loss based on a vector network analyzer and an impedance converter is proposed to accurately measure the phase discrepancies between voltage and current sensors. The proposed method is experimentally verified up to 15 MHz with a three-coil test setup.