## 4 Frequency Selective Surfaces Based on Metasurfaces  
A typical frequency selective surface (FSS) is a periodic structure having each individual element resonant at the resonance frequency. FSSs typically have periodicity equal to half the wavelength of the resonant frequency [62]. They have been frequently employed to provide spectral filtering in signal communications. Moreover, they are also used as diplexers, beam splitters and to make antenna radomes [63]. Metasurfaces can replace FSS due to the resonant nature of \"meta-atoms\". Since a metasurface's building blocks are sub-wavelength in nature, they also bring additional advantages in comparison to a traditional FSS. The sub-wavelength periodicity allows to pack a large number of unit cells in a constrained space which is highly useful for radomes with limited space [64]. The small size of the unit cell also allows metasurfaces to have a stable response with respect to the changes in the angle of incidence of the illuminating electromagnetic wave. Such a structure has been presented in [65], where a layer of patches is backed by a wire grid on the other side of dielectric. The capacitance of patches coupled with the inductance of the wire grid formed an LC resonant structure with the resonant frequency much smaller than the wavelength at the resonant frequency. In addition to structural design, lumped circuit elements have also been used to miniaturize the size of a unit cell of periodic metasurfaces [66]. By using capacitors, the size of the individual element presented in [65] has been successfully decreased [67]. Convoluted elements with a reduced unit cell size have been presented in [68]. Metamaterials are usually designed with the help of split ring resonators (SRRs) [69]. SRRs have sub-wavelength physical dimensions and their Babinet's complement has been used to design and tailor the properties of metasurfaces [70]. Non-reciprocal bianisotropic elements have been used to design \"one-way transparent\" metasurfaces [71]. Active spatial filters (FSSs) designed using PIN and varactor diodes with complementary SRRs as their building blocks have been successfully employed for tuning and dual-band operation [72]. Once a metasurface is fabricated, its response is fixed. Its reflection and transmission response cannot be changed. However, since metasurface response is based on localized control, reconfigurable metasurfaces are very much desired in order to use a single metasurface for a wide range of applications. This has been addressed by a variety of designs presenting tuneable/reconfigurable metasurfaces. There are three major ways in which a metasurface can be tuned (i) mechanical tuning (ii) material state change under a stimulus (iii) use of varactor diodes (in microwave domain) [73]. Mechanical tuning in all dielectric metasurfaces has been used in [74] and a shift in both resonances was recorded by rotating the individual elements. MEMS have also been applied for mechanical tuning in [75]. By shifting an engineered ground plane, beam steering from a reflector has also been achieved [76]. The capacitance of varactor diodes can be controlled by varying the voltage across them which make them an excellent candidate for electrically tuning the metasurfaces [77]. These diodes have been used for beam steering and switching the states(modes) of metasurfaces [78; 79]. A tuneable metasurface with 1125 varactor diodes in a planar array has used as a steerable reflector [78]. Metasurfaces with dispersive refractive index have been proposed for THz domain since the modulators at THz frequency usually work at extremely low (cryogenic) temperatures [80]. The use of multilayers to improve the response of an FSS has been well documented [62]. Multi-layered FSSs, formed by cascading two or more layers of the periodic structures and carefully aligning them, has led to an increase in the operational bandwidth of the structure and improve the filter's roll-off rate, thus making them available to be used for de-multiplexing in multiband radiometers [81].