# Frequency Selective Surfaces: A Review
## 3 Frequency Selective Surfaces (FSSs)
### History and Significant Advances of FSS in Recent Years  
FSSs have extensively been explored over the past five decades [2; 3] and the evolution process extends from simple geometries to the complex designs. Strict requirements of performance in advanced applications have stimulated considerable advancements in analytical techniques, computational power management, and manufacturing technology. Huge potential for military use has lead to intensive studies of such surfaces since mid-1960s. Although on record, the oldest patent came in 1919 to the well-known Marconi and Franklin. The earlier FSSs research focused on developing Cassegrain sub reflectors used in the parabolic antennas. During the Persian Gulf War, the Lockheed F-117 Nighthawk was openly launched by US air force, which increased the significance of stealth technology and FSSs. Although the adjustment of radar cross section is considered as the most thrilling function of this technology, FSS are employed in many other useful applications. These include dichroic sub-reflectors, radomes, RFIDs, lenses, and electromagnetic interference (EMI) protection. Mode matching methods were initially applied in waveguide problems for the behavior analysis of FSS, which gave rise to approach for equivalent circuit modeling [43]. This was partially based on the transmission line theory. However, it had a limited analysis capability to model the FSS geometry. More precise numerical techniques have been developed with the introduction of fast processing computers e.g., MoM, FEM, FDTD, and IEM/BEM. Verification process has now replaced the bolometer used in earlier experiments with modern Vector Network Analyzers with high capabilities of measurements for amplitude and phase of the scattered EM fields. Fabrication ease and cost effectiveness are essential factors that are investigated so far, since the precision in etching defines the performance of an FSS and it is usually required to be achieved over vast areas of such surface. Starting with a simple FSS or multiple cascaded FSS realizing a band-pass or band-stop characteristic, the slot and patch elements are complementary and are linked by Babinet principle. Generally, FSS applications presume those geometries which are periodic and represented by a unit cell repeating itself indefinitely [2]. However, several practical applications of non-periodicity exist, for example, in the use of curved surfaces, and infinite smooth surfaces, which may have randomness in geometry and its constituent properties [43]. Moreover, the selection of direction-dependent screening has been achieved by non-reciprocal FSS structures. Some initial uses of FSS lie in the design of radomes and sub-reflectors for reflector antennas and still extensively studied. Capabilities of functioning at lower frequencies and broader bandwidth have been facilitated by the lightweight, conductor-backed thin radar absorbing FSSs. Furthermore, since the \"metamaterial\" term was coined, there has been a stirring revitalization of interest in metamaterial EM structures, especially metamaterial inspired FSSs are investigated for compact filters, absorbers, and sensing applications. Recently, periodic structures having three-dimensional (3D) elements is being used with great interest as they allow the electric currents to flow with components at right angles to the surface and therefore, FSS geometries insensitive to the incidence angles may be designed [43]. Complex FSS structures are manufactured with advanced fabrication techniques by joining together consecutive layers of different shapes, such as tetrahedral Calthrop, is fabricated with a printer employing the fabrication process of fuse-filament. With the increase in satellite communication and pulse-powered devices, metallic FSSs have been proved incapable to operate the power/heat produced by such systems efficiently.