Synthesis of SEM-derived equivalent circuits for energy-collecting structures

A systematic approach is developed for the synthesis of physically realizable equivalent circuits for antennas and scatterers with definable ports from their SEM (Singularity Expansion Method) description. The necessary SEM information includes the complex natural resonances (or "poles') of the structure and the associated modal currents ("natural modes') and normalization coefficients. From the poles and natural modes one can compute the coupling coefficients, which weight the given singularity's contribution to the response for a given incident field. The synthesis is performed as a superposition of circuit modules associated with conjugate pairs of poles. This approach leads to simple circuits for which explicit expressions for the element values in terms of the SEM parameters can be given. The entire equivalent network can be divided into two parts -- an immittance part describing the driving-point properties of the structure, and a source part describing its energy-collecting properties. The immittance part is intrinsic to the structure and is independent of the incident field. The source part, however, depends on the direction of arrival of the impinging plane wave and is driven by an automatic voltage source with the waveform corresponding to the time history of incident field. The developed equivalent circuit synthesis procedures are tested on a thin-wire loop and dipole antennas and on a spherical antenna. Results are presented showing good agreement between the SEM equivalent circuits' responses and responses obtained by other means.