FDTD modeling of common-mode radiation from cables

Radiation from cables attached to printed circuit boards and shielding enclosures is among the primary concerns in meeting FCC Class A and B limits. The finite-difference time-domain (FDTD) method can be employed to model radiation from printed circuit boards and shielding enclosures with complex geometries, but difficulties in modeling wires and cables of arbitrary radii are encountered. Modeling the wire by setting the axial component of the electric field to zero in the FDTD method results in an effective wire radius that is determined by the mesh discretization. Neglecting the wire radius in applications, such as electromagnetic interference (EMI) or printed circuit board modeling, may result in gross errors because near-field quantities are typically sensitive to wire thickness. Taflove et al. (1988) have developed a subcellular FDTD algorithm for modeling wires that has been shown to work well for plane wave scattering. The method uses a quasistatic field approximation to model wires with a well defined radius independent of the mesh dimensions. The wire model is reviewed and investigated for application to common-mode radiation from cables attached to printed circuit boards, where the source is often a noise voltage at the connector. Also investigated is energy coupling to attached cables through enclosure apertures resulting in common-mode radiation from the cable. The input impedance for a center-fed dipole antenna, as well as a monopole connected to a conducting half-sheet, is computed with FDTD methods and compared to moment method input impedance results. A simulation of a shielding enclosure with an attached cable demonstrates the utility of FDTD analysis in modeling common-mode radiation.