Two-Dimensional Beam-Scanning Broadband Circularly Polarized Transmitarray With Low Sidelobe Level

This article presents a broadband 2-D beam-scanning circularly polarized (CP) transmitarray with low sidelobe level (SLL) for millimeter-wave (mmW) applications. A polarization conversion unit cell, capable of receiving linearly polarized (LP) incidence and transmitting CP radiation, is first designed. Based on the geometric phase (GP) compensation strategy, a full 360° transmission phase with good CP radiation performance is achieved by rotating the transmitting element. Moreover, the quad-focal phase compensation scheme is implemented to construct the GP phase distribution required by 2-D beam scanning to minimize the scanning loss of the transmitarray. To lower the SLL during large-angle beam scanning, the multiobjective particle swarm optimization (MOPSO) algorithm is further employed to refine the phase distribution across the transmitting layer. For validation, a fully functional prototype of the proposed transmitarray illuminated by a movable LP horn is fabricated and measured. The measured results show a wide overlapped 3-dB gain and axial ratio (AR) bandwidth of 26% with a peak gain of 21.5 dBic. Besides, the measured SLL remains below −11.4 dB within a scan range of ±34° in two orthogonal planes. These preferred features make it a promising candidate for various mmW applications such as satellite communication systems.