Analytical Synthesis of 2-D Flat-Top Beam and Its Application on 2-D Beam-Steerable Transmitarray Antenna in the 25-GHz Band

Two-dimensional (2D) flat-top beams are of great significance in wireless communication applications. This article presents the analytical synthesis of a 2D flat-top beam on a reflectarray (RA) antenna (i.e., a reflectarray + a feed source), and demonstrates its application in enabling a 2D beam-steerable transmitarray (TA) antenna in the 25 GHz band. A subordinate sequence, which contributes to the generation of a quasi-sawtooth-shaped beam, is first defined and then manipulated with two key parameters. By performing axially rotational phase symmetry operations on this sequence, quasi-sawtooth-shaped beams in all φ-cut planes are obtained, a 2D flat-top beam is therefore achieved at broadside by superimposing these quasi-sawtooth-shaped beams, with flexible beamwidth adjustment. Furthermore, its beam direction can be tilted by introducing an additional phase gradient. This tilted 2D flat-top beam can be utilized to enable a 2D beam-steerable TA antenna with a reduced profile, where an integrated phase-shifting and phase gradient (I-PSPG) surface is positioned above the RA antenna with a separation. By mechanically rotating the RA and I-PSPG surface with the feed source fixed, the beam of the developed TA antenna can be dynamically manipulated while keeping the polarizations of all beams the same. For demonstration, a RA antenna with a tilted 2D flat-top beam at 30° is implemented and then utilized to realize a 2D beam-steerable TA antenna, achieving a beam-scanning cone of 64° with approximately 1.9 dB gain roll-off. Prototypes of the developed RA and beam-steerable TA antennas are fabricated and characterized at 25 GHz, where the measured results align well with the simulated ones. The developed 2D beam-steerable TA antenna is attractive for low wind-load scenarios.