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In this letter, a novel wideband 180° phase shifter with excellent phase and amplitude balance is presented and a new wideband millimeter-wave planar balun is proposed for the first time. The proposed balun consists of a wideband Wilkinson divider for power splitting followed by two microstrip-coplanar stripline (CPS)-microstrip transitions for broadband 180° phase shift. Good broadband phase and amplitude balance performances are achieved according to the inherent wideband characteristics of the Wilkinson divider and microstrip-CPS-microstrip transition and symmetry of the structure. A new ka-band planar balun has been fabricated and measured. Measured results show that the return loss, isolation, amplitude imbalance, and phase imbalance are better than 10 dB, 14.4 dB, 0.8 dB, and 3.1° , respectively, over the specified frequency range from 26.5 to 40 GHz.