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A metamaterial unit cell with a low refractive index over a wide frequency band is proposed and designed. The effective material parameters of the unit cell are extracted, and the unit cell forms a planar three-layer metamaterial structure used as a superstrate for broadside gain enhancement of a patch antenna at 10 GHz. The proposed superstrate is optimized along with the antenna to enhance its beam-focusing ability, taking into account the oblique wave incidence from the radiation source. Both simulation and measurement of the antenna with the optimized superstrate show that this configuration is able to achieve a broadside gain 70%-80% of the maximum gain from the ideal effective radiation surface.