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Design of a waveguide antenna with bidirectional circular polarizations of the same sense

A square aperture bidirectional waveguide antenna with circular polarization of the same sense in the two opposite radiating directions is proposed in this paper. Two identical rectangular metal strips vertically separated by a quarter guided-wavelength and horizontally with a 90° cross are installed on one side of the waveguide with a 45° inclination angle. The two metal strips are fed in the middle of their diagonal line by microstrip line of the same width. The feeding line is meandered so that a three-quarter impedance transformer is formed to help with the antenna's matching. The two vertically positioned metal strips are excited with the same phase and radiate bi-directionally with the same amplitude. The 90° phase difference is realized by the spatial distance of a quarter guided-wavelength, and therefore circular polarizations of the same sense in the two opposite radiating directions can be generated. The impedance bandwidth for |S11| <; -10 dB is 540 MHz (2.20 GHz ~ 2.74 GHz) and the bidirectional 3-dB axial ratio bandwidth is 200 MHz (2.34 GHz ~ 2.54 GHz). A minimum axial ratio of 0.27 dB and 0.56 dB is achieved at 2.45 GHz in two opposite radiating directions and the bidirectional 3-dB axial ratio beamwidths at 2.45 GHz are both 56°, which is suitable for 2.4-GHz WLAN communications.