Extraordinary transmission (ET) by a periodic conducting slot at normal incidence has potential applications in the optical and microwave frequencies. This paper presents a simple analytical model to account for the effects of oblique incidence of electromagnetic (EM) waves on a slotted periodic structure. The proposed analytical model, based on Floquet and circuit theory, provides physical insight to explain the reduction of ET frequency when the EM wave strikes the structure obliquely: ET occurs before the onset of the grating lobe. Higher evanescent Floquet modes contribute to the stored energy that represents fringing capacitance in terms of the incoming wave. Transmission bandwidth and material loading effects are also investigated. When applied on an artificial magnetic conductor (AMC), the model indicates that the thickness of the AMC can be dramatically reduced without any material loading. Results obtained by the analytical models are compared by full-wave simulations.Index Terms-Artificial magnetic conductor (AMC), equivalent circuit (EC), extraordinary transmission (ET), oblique incidence.

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