Performance Improvement of Mechanically Beam-Steerable Transmitarray Antennas by Using Offset Unifocal Phase Symmetry

— This paper presents the concept of offset unifocal phase symmetry to improve the performance (gain, gain roll-off, sidelobe level, etc.) of mechanically beam-steerable transmitarray (TA) antennas. The phase shifts are initially determined from an offset feed source to make the corresponding TA antenna radiate a tilted beam with high gain. The phase shifts are asymmetric and can be broadly divided into dominant and ordinary phase shifts with respect to the offset feed source. By mirroring the dominant phase shifts to make the phase shifts symmetric, it can enable symmetric radiation beams for symmetric offset feed sources and improve phase error. When the offset feed source is moving inward to steer the main beam, the phase error of the phase-shifting surface with respect to the feed source starts to occur, while the electric-field spillover/ illumination from the feed source is improved, which can maintain the gains of scanning beams. To verify the validity of the concept, the performance of a beam-steerable TA antenna enabled by the offset unifocal phase symmetry has been simulated and compared with the counterparts of unifocal and bifocal beam-steerable TA antennas. The measured results agree well with the simulated ones, revealing that the beam-steerable TA antenna enabled by the offset unifocal phase symmetry can maintain realized gains of scanning beams, suppress sidelobe levels, and reduce gain roll-off within the beam-scanning coverage. The offset unifocal phase symmetry, in principle, is a generalized approach and applicable to reflectarray antennas to improve the beam-steerable performance as well.