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A square-aperture metallic waveguide antenna with bidirectional circularly polarized (Bi-CP) radiations of the same sense is proposed in this letter. Circular polarization was realized by two identical rectangular metal strips installed on one lateral side of the waveguide with a 45° inclination angle. The two rectangular metal strips were vertically separated by a distance of a quarter guided wavelength and horizontally intersected with an angle of 90°. They were excited with the same amplitude and phase by feeding the antenna in the middle of their diagonal line. A 90° phase difference between the two rectangular metal strips was introduced by the spatial distance of a quarter guided wavelength, and thus circular polarizations of the same sense in the two opposite radiating directions were generated. A quarter-wavelength impedance transformer was designed to help with the antenna's matching characteristic. The measured impedance bandwidth for |S11| ≤ -10 dB was 710 MHz (1.92 ~ 2.63 GHz), and the measured 3-dB axial-ratio bandwidth was 200 MHz (2.34 ~ 2.54 GHz). The measured bidirectional 3-dB axial-ratio beamwidth at 2.44 GHz was 62°, and the power variation was smaller than 3 dB within this angular range.