An Ultra-Broadband Planar Monopole Antenna with Bidirectional Circular Polarization for Millimeter-Wave Applications

In this paper, an ultra-broadband planar monopole antenna with bidirectional circular polarization (CP) is proposed in the millimeter-wave (mm-wave) band. An ultra-wide CP bandwidth is realized through introducing a rectangular stub and a triangular patch into a hook-shaped monopole antenna and a circular slot, respectively. The grounded coplanar waveguide (GCPW) is chosen to feed the antenna via a short-tapered transmission line. Through these methods, the antenna achieves an impedance bandwidth (IBW) of 59.2%$(21.82-40.17 \mathrm{GHz})$, a 3 dB axial-ratio bandwidth (ARBW) of 63%$(20.89-40.08 \mathrm{GHz})$, and a peak gain of 6.5 dBic at 34 GHz. Besides, the antenna exhibits a low profile of $0.05 \lambda_{0}$ ($\lambda_{0}$ is the wavelength at 31 GHz in free space) by a single-layer structure. Because of its advantages, this antenna is a suitable choice among mm-wave antennas.