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Slot antennas offer a number of advantages including low-profile, ease of fabrication, and ease of integration with electronics. Their main drawback, however, is that they are inherently bi-directional radiators. One common technique to redirect the back radiation forward is to place a conducting reflector at a fixed distance away from the antenna. This distance is usually chosen to be a quarter wavelength so that the reflected back radiation incurs an additional phase of 360° and thus adds in phase with the forward directed radiation. However, in this case the parallel-plate geometry permits the excitation of the dominant transverse electromagnetic mode, which drastically reduces the overall radiation efficiency. This work describes a solution to this problem and shows how to achieve uni-directional slot antenna elements by backing them with an electromagnetic band-gap surface, instead of the uniform conducting reflector. This structure, when operated in its band-gap, inhibits the propagation of electromagnetic waves over all possible azimuthal directions, in the region between the antenna ground plane and the periodic surface.