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In this paper, the directivity of microstrip patch antennas with double-layer superstrate is investigated. Two dielectric superstrate layers, each a quarter wavelength in thickness and separated by an air gap, are introduced above the microstrip patch, separated by another air gap. The parameters of these layers are used as key controllers of the directivity enhancement. Numerical results indicate that the directivity increases significantly even with moderate superstrate dielectric constants when the double-superstrate configuration (DSC) is used. To demonstrate how the layers affect the directivity, an infinite dielectric model is first used. Then the effect of the superstrate transverse dimensions is studied by means of a finite dielectric model. The directivity variations with respect to frequency are compared for different superstrate surface areas. This work leads to the conclusion that modification of the superstrate transverse dimensions can optimize both directivity and bandwidth. The experimental investigations of DSC are also provided to support the simulation results.