Near-Field Metasurface Coupling Pillbox Antenna for Steered Multiple Wavefronts Generation

In this letter, a compact low-profile antenna is proposed to generate multiple wavefronts with switchable propagation directions. The antenna comprises three parts, i.e., a pillbox beamforming network (BFN), a non-offset slot array, and a Huygens metasurface (HMS). The design concept can be regarded as mapping the 3-D transmitarray onto a planar structure. The defocus feeding is replaced by the pillbox BFN, and the flexible phase manipulation capability is achieved by the near-field coupling HMS. A bifocal phase compensation principle is adopted in the HMS to optimize the performance of the steered beams, resulting in low SLL and scanning loss. Moreover, the HMS also help correct the phase distribution of the slot array, achieving gain improvement and beam direction correction. To verify the feasibility, two prototypes are fabricated and measured. For the pencil-beam design, the input ports switching can excite five beams with the scanning range of ±32°. The crossover level and gain variation of the beams are less than -4.5 dB and 2.0 dB. After the vortex phase is superimposed on HMS, the second prototype can implement the orbital angular momentum (OAM) beams with different propagation directions.