A complex image method is presented for the analysis of a subwavelength circular aperture in a perfectly conducting screen of infinitesimal thickness illuminated by a plane wave. The method is based on the Bethe-Bouwkamp quasi static model of the aperture field and uses the spectral domain formulation as the point of departure. Closed-form expressions are obtained for the electromagnetic fields valid for all observation points. Sample numerical results demonstrate the accuracy and efficiency of the method for both normal and oblique illuminations, including an evanescent wave. In the latter case, the results show a circulating power flux and enhanced field confinement near the aperture.
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