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A high-efficiency microstrip-fed Yagi-Uda antenna has been developed for millimeter-wave applications. The antenna is built on both sides of a Teflon substrate (epsivr = 2.2) which results in an integrated balun for the feed dipole. A 7-element design results in a measured gain of 9-11 dB at 22-26 GHz with a cross-polarization level of < - 16 dB. The antenna is matched to 50 Omega (microstrip feed). A mutual coupling of < - 20 dB is measured between two Yagi-Uda antennas with a center-to center spacing of 8.75 mm (0.7 lambda0 at 24 GHz), and a two-element array results in a measured gain of 11.5-13 dB at 22-25 GHz. The planar Yagi-Uda antenna results in high radiation efficiency ( > 90 % ) and is suitable for mm-wave radars and high data-rate communication systems.