Modeling Coax Cable EMI Shielding Performance for Automotive AM Broadcast Band Applications

Automotive broadcast reception systems typically use common coax cable to convey RF energy from the antenna to the broadcast receiver. In special cases a modified low capacitance coax is used for cross vehicle runs to enhance performance in the AM band. Coax is specified by characteristic impedance, optical coverage, DC resistance per unit length and RF loss versus frequency per unit length. Optical coverage and DC resistance serve to provide a gauge for relative shielding performance to EMI. EMI currents on the shield translate to an impressed voltage at receiver via the transfer impedance of the coax. This paper examines the shielding effectiveness of coax, transfer impedance and the relationship to signal source and return path impedance to noise coupling. A 2D quasi-static solver application (QuickField) is used for the analysis.