Wideband Equivalent Circuit for Multi-Aperture Multi-Resonant Waveguide Irises

Abstract

In this paper it is shown that the traditional shunt admittance represented by a rational function yields an accurate wideband equivalent circuit for multi-aperture waveguide irises. The electromagnetic analysis of the structures reveals two different types of resonant phenomena. The first type of resonance is associated with the cutoff frequency of the ${\rm TE}_{10}$ mode. The second type or antiresonace corresponds to short-circuit TEM modes. These antiresonances are generated when the asymmetry of the iris allows the excitation of electric fields with opposite phases at contiguous apertures. The proposed equivalent circuit is a shunt admittance represented by a rational function. This rational function is analytically obtained from the singular points (resonances and antiresonances) identified in the full-wave analysis. It is straightforward to obtain the circuit elements from the rational function by following the classical one-port synthesis leading to Foster’s or Cauers’s canonical realizations. One advantage of the proposed equivalent circuit is that it represents the electromagnetic behavior of complicated structures with a minimum number of circuit elements or degrees of freedom. Several examples, with different number of apertures and symmetry, are presented to illustrate the proposed general equivalent circuit. Finally, the proposed procedure is extended to the case of arbitrary shaped irises, such as those used in frequency-selective surfaces.