The dual-frequency operation of a microstrip antenna consisting of two annular-rings in a stacked configuration is investigated. A rigorous analysis is performed using a dyadic Green's function formulation where the mixed boundary value problem is reduced to a set of coupled vector integral equations using the vector Hankel transform. The accuracy of the analysis has been confirmed by experimental measurements. It has been found that the two constitutive resonators of the stacked structure, which determine the dual-frequency behavior of the antenna, depend on the relative sizes of the circular-disks.
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