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Unidirectional, highly-efficient planar twin arc-slot antenna elements backed by a reflector are presented. The theoretical radiation efficiency is as high as 99.9%. This high efficiency is achieved because the proposed slot geometry reduces the coupling to the parallel-plate TEM mode effectively in all directions on the substrate plane. A corporate-fed as well as a compact, center-fed, twin arc-slot elements have been implemented and tested. The parallel-plate TEM substrate patterns for a representative twin arc-slot element have been measured and compared with those of a corresponding straight twin-slot element. It was verified that the parallel-plate TEM mode, coupled from the proposed twin arc-slot element is greatly reduced in all directions. The measured gain improvement when a back-reflector is added is found to be about 2.8 dB, further validating this high efficiency. In addition to the high radiation efficiency, the proposed twin arc-slot element is more compact compared to the corresponding, straight, full-wavelength twin-slot element, making it well-suited for two-dimensional antenna arrays.