Analysis of the Radiated Susceptibility of a Transmission Line under Near and Far-field Conditions

An analytical transmission line (TL) coupling model is used for evaluating the susceptibility of a DUT in the near-field of a noise source in comparison to a far-field excitation. For this purpose, appropriate field solutions of a lambda/2 dipole antenna are used in combination with the TL coupling equations of a short TL. The analytical model is validated by accurate 3D field simulations. A coupling ratio is defined to quantify the difference between the near and far-field coupling. The evaluation of the coupling ratio results in the conclusion that standard EMC-test may highly underestimate the near-field disturbance level of a noise source, in the meantime, testing in the near-field may overestimate the required protection measures with respect to the EMC test standard in the far-field. The size of the DUT also influences the coupling ratio.