It is shown that, under some resonance conditions, a microstrip patch antenna can be designed to achieve the highest possible gain when covered with a superstrate at the proper distance in free space. The transmission line analogy and the cavity model are used to deduce the resonance conditions required to achieve the highest gain. The resonance conditions include the condition on spacing between the antenna's substrate and the superstrate and the thickness of the superstrate. The permeability and permittivity of the superstrate is determined based on these resonant lengths and the appropriate characteristic impedance of each layer in this multilayered structure. The results are verified using both analytical and numerical methods. The effect of anisotropy of the superstrate is numerically investigated. The design criteria proposed here will reduce the total profile of the radiating system by 50% when compared with previous trends.
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