60-GHz Planar Circular-Polarized Filtering Antenna and Its Integrated Design With Folded Reflectarray

The circular-polarized (CP) filtering antenna design based on the radiation-null-introducing technique emphasizes introducing specific filtering structures on the radiator or feeding structure. However, imperfect filtering structures are prone to generate radiation leakages toward non-boresight directions at the stopband, resulting in degraded filtering characteristics in the peak realized gain response. This article introduces two new CP filtering antennas operating at 60 GHz: one involves a single antenna element, and the other involves a folded reflectarray antenna, both presenting an outstanding peak realized gain filtering response. A unique folded double-layer substrate-integrated-waveguide (SIW) feeding structure incorporated with a self-energy-bounding CP radiator is proposed for the antenna element. No filtering structure is needed in the radiator. The feeding structure, independent of the resonator cavity, inherently minimizes radiation leakages and can produce three radiation nulls, guaranteeing the desired filtering characteristic. The proposed CP filtering antenna element is then integrated with a folded metasurface to create a new CP filtering folded reflectarray antenna. Both antennas feature high selectivity, ample stopband rejection, stable CP radiation in upper millimeter-wave frequencies, and easy integration, making them suitable for 60 GHz communications and sensing systems applications.