In this paper, the effect of both uniaxial anisotropy in the substrate and air gap layer on the resonant frequency and bandwidth of circular microstrip patch are investigated. The problem is rigorously formulated based on the spectral domain technic in conjunction with Galerkin approach for computing the resonant frequency, half-power bandwidth, and radiation field of a tunable circular patch antenna which is printed on isotropic or uniaxial anisotropic substrate. The TM set of modes issued from the magnetic wall cavity model theory are used to expand the unknown currents on the patch. Resonant frequency shift due to uniaxial anisotropy is firstly investigated for different anisotropy ratio values of substrate. Then, the effect of inclusion of air gap layer inserted between anisotropic substrate and ground plane on the resonance characteristics is also investigated. The results obtained from this approach are in very good agreement with the experimental results available in the literature.
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