The design of a planar wide-gain-bandwidth metasurface antenna at terahertz frequency have been presented. The proposed antenna comprised of a square patch metasurface and a planar feeding structure, both of which were etched on an electrically thin, high-permittivity GaAs substrate. The antenna with a single feeding structure showed a maximum broadside gain of 9.8 dBi, a radiation efficiency of 69%, and a 3-dB gain bandwidth of 16%. The antenna gain performance was significantly improved by exciting the antenna with an array of slit feeding and without changing the antenna size. The antenna with a multiple (five) feeding structure showed a gain of 15.5 dBi, with improved 3-dB gain bandwidth and radiation efficiency. This antenna achieved a huge size reduction while achieving a comparable gain with the lens-coupled antenna. Moreover, the antenna performed as Fabry-Perot antenna exhibiting, high gain and directivity at higher values of substrate thickness (H=160 µm and H=80 µm), while behaved as metasurface antenna with wide-gain bandwidth at a lower thickness of the substrate (H=40 µm and H=20 µm). The antenna gain can be linearly increased by increasing patch cells using multiple-feeding, and we can achieve useful antenna characteristics by a combination of patch number and different values of substrate thickness.
