## 5 Metasurface Antennas  
Metasurfaces have been successfully used to design and manufacture high gain holographic antennas [86; 87]. For holographic antennas, the surface wave is the major incident wave. The radiation for such antennas occurs when phase matching between forward and backward leaky wave occurs. Forward leaky wave arises from the grating which has a larger periodicity while the smaller period grating leads to a backward leaky wave. A source with elliptic grating is surrounded by both periodicities. Both leaky wave modes for such a grating are matched leading to directive radiation [88]. Based on this approach, circularly polarized leaky wave antennas with a 26 dB gain are presented in [86]. The grating effect was produced by modulating the surface impedance of the metasurface. A significant advantage of this approach is that instead of changing the antenna shape in order to design a specific response, the metasurface modulation (i.e., surface impedance) is engineered [89]. A metasurface antenna offering the possibility of both right- and left-hand circular polarization was presented in [90]. Both TE and TM surface waves were launched and the polarization decoupling of the two modes was utilized for their independent control. These modes were phase-matched and the modulated metasurface was rotationally symmetric. The antenna was fed by a circular waveguide and a corrugated hat was placed on top of the waveguide to suppress the space-wave radiation. Since phase-matching is critical for leaky-wave metasurface antennas, a bandwidth of such antennas is limited by the dispersion of the metasurface. The mismatch of phase leads to a decrease in the gain of structure. A bandwidth gain expression for these antennas is available in [91]. It was shown that this bandwidth can be increased by designing the modulated metasurface on a low permittivity dielectric and increasing the antenna size. It has also been shown that metasurface antennas can be designed for multi-beam operation [92]. Another use of metasurfaces for leaky wave antennas are Non-chiral bianisotropic metasurfaces called omega metasurfaces. When periodic leaky wave antennas are used to scan the broadside, Floquet mode coupling deteriorates its radiation performance. Non-chiral bianisotropic metasurfaces can be employed to solve this problem. Moreover, it has been discovered that metasurfaces of any period can be used to design such leaky wave antennas. There is no issue of Floquet mode coupling since only a single mode is excited [93]. The use of multi-layered [PERSON]'s metasurfaces to structure a bianisotropic metasurface has been proposed in [94]. In [95], multi-layered metasurfaces with sub-wavelength profile were used as partial reflective surfaces to design a high gain antenna with an enhanced bandwidth. In [96], [PERSON]'s metasurfaces have been proposed to convert a non-directive beam from an arbitrary source to a directive one. Such metasurfaces have been utilised to construct a low profile antenna with high aperture efficiency by employing them as a partially reflective surface fed by a cavity [97]. The aid of [PERSON]'s principle can be used to develop complementary metasurfaces. A comprehensive treatment of such metasurface design with an equivalent circuit has been presented in [98]. The complementary metasurfaces have been used to design antennas and transmission lines, and offer the possibility of tuneable antennas and circuits [99]. Metasurfaces can also improve the performance of horn antennas. In [100], a metasurface designed using a genetic algorithm has been used as an inner surface for a conical horn and the cross polar levels and the side lobe levels have been improved over the entire Ku band. A similar approach was applied to improve the performance of a hybrid mode square horn antenna using metasurfaces [101]. Metasurfaces have been proposed for medical body area networks (MBAN) as well, by improving the performance of a low-profile monopole antenna.