Assessment of the Antenna-Equivalence Approach to Common-Mode Input Impedance Modeling

Analytical modeling of the common-mode input impedance of a motor along with its cable for various installation characteristics would allow designers to assess EMI levels and to evaluate in an early stage if adaptations are needed in their cable installation. Earlier work has shown that the input impedance over frequency of such a system is mostly dominated by the cable. A common assumption is that a cable can be approximated as a monopole antenna above a ground plane, which has an input impedance equivalent to that of a dipole with a correction factor. We compare the Hall´en and King & Middleton dipole models to a measurement setup which is designed to reproduce parastic effects from the installation, to assess the validity of the analytical model. We analyze these results for various distances between the cable and the groundplane. We show that large discrepancies occur due to paristics of the installation and the presence of the groundplane, but that for some applications such closed-form analytical models may suffice in assessing frequencies at which radiated emissions occur.