At radiated frequencies, common mode(CM) current is the dominant radiation source for isolated power converters with long cables attached. Based on the existing radiation noise model, this paper proposes a novel method to measure CM current using a current transformer(CT), which has a higher transfer impedance and can measure the radiation noise in a broader frequency range than the traditional high-frequency(HF) clamp-on current probe. According to the established CT model, the corresponding design methods are proposed to improve the bandwidth and transfer impedance and the final CT that meets the requirements is determined after comparing different methods. The practicability and effectiveness of the designed CT are verified by comparing the measured CM current with the final anechoic chamber measurement results.