Optimization of Partially Reflected Surfaces with Nonuniformly Sized and Spaced Inclusions for Enhanced Antenna Beam Shaping

Partially reflective surface (PRS) antennas are structures comprised of a Fabry-Perot cavity defined by a metasurface made of a series of conductive elements or inclusions at a resonant size and position relative to a feeder antenna and ground plane (G. Trentini, “Partially reflective sheet arrays,” IRE Trans. On Antennas and Propagation., vol. 4, no. 4, pp. 666–671, 1956). The presence of inclusions of identical size in a periodic (uniformly spaced) arrangement permits gain enhancement of the structure at broadside but does not permit optimal sidelobe levels or alteration of the radiation pattern. Alteration of these inclusions permits changes of beam direction and beam shape to attain or approach a prescribed pattern, gain, and/or sidelobe level. In particular, to optimize the side-lobe-level for a given beam width and/or to shape the far-field pattern, the concept of PRS structures made of non-uniformly spaced inclusions has been introduced (A. Hoorfar, J. Zhu, N. Engheta, Int. Conference on Electromagnetics in Advanced Applications, Torino, Italy, 2003; A. Hoorfar, C. Israel, IEEE APS-URSI Atalanta, GA, 2019). Other prior studies reported in the literature have focused on manipulating inclusion presence or absence on a fixed grid, or varying inclusion size on a fixed grid to achieve a prescribed phase-gradient across the PRS array for different applications, for example, to attain a tilted beam using variably sized unit-cell elements. For the latter geometry, approximate solutions for the PRS phase have been reported (Ratni, Badreddine, et al. IEEE AWPL pp. 1401–1404, 2017) which may serve as initialization to globally optimize more complex PRS structures for improved performance. Potential indeed exists for significantly enhanced performance and functionality using a metasurface geometry containing both nonuniform element (unit-cell) size and nonuniform interelement spacing to not only achieve offset (tilted) beam but perhaps also multiple beams.