Theoretical and Experimental Study of Effective Coupling Length for Transmission Lines Illuminated by HEMP

This paper presents theoretical and experimental study of effective coupling length (ECL) for transmission lines illuminated by a high-altitude electromagnetic pulse (HEMP). Based on the transmission line theory, the ECL of transmission lines is analyzed theoretically and numerically first, then the ECL is applied to simply the analysis of nonuniform fields coupling with a power line, and some conclusions are validated experimentally. The impacts of the incident angles, the line height, and the ground conductivity on the ECL are given. The results show that the ECL for transmission lines illuminated by the HEMP defined by the IEC standard can range from tens of meters to hundreds of meters. The results also imply that the coupled current of a load is mainly determined by the field coupling with the line section, which adjoin the load and has the same length as the ECL. This conclusion is also validated by the experiment. This study can be useful to simply the solution of HEMP or nonuniform fields coupling with long lines or large-scale networks, which may be rather tricky when computing the problem directly.