# Frequency Selective Surfaces: A Review
## 4 Types of FSSs
### FSSs Based on Structure
#### 4.2.2 Multilayer FSSs  
A significant challenge in FSSs theory is the limitation of its operating bandwidth, particularly it is very difficult to improve the BW performance of single layer FSS. From Table 3, it can be deduced that even when the implementation of a fractal element on a single layer is employed, still the FBW is limited. However, meandered/convoluted and fractal technique has been able to reduce the unit cell size and also the profile of the FSS. In [29], we can observe very narrow band behavior of single layer FSS by using different fractal array elements, and the reference of insertion loss does not reach better than 20 dB. Separation of consecutive FSSs frequency bands is possible with wider transmission/reflection bands and also faster roll-off in some of the applications. However, the frequency response is declined gradually on both sides of the resonant frequency in the case of single layer FSSs, making them inappropriate for such critical applications. The problem can be solved essentially by using resonating elements of complex geometry or through multilayer FSSs construct [92]. The first choice typically results in multiband response and overall enhanced BW without improving it for single resonance and occasionally decreasing it in some cases. As a consequence, the second alternative is more suitable to increase the BW of single resonance. A broadband response in the desired frequency range is achievable by multilayered FSS structures due to the discontinuity provided by the layers. Higher order passband FSS can achieve better performance with faster roll-off and smooth in-band frequency response. The thickness of the substrate turns out to be approximately a quarter of a wavelength in implementing passband FSSs by use of resonant surfaces, which is deplorable in many smart applications nowadays. Alternatively, non-resonant array elements largely increase the FSS size. Therefore, a trade-off is ascertained depending on the type of application. The design parameters of multilayer FSSs can be optimizedthrough different optimization approaches, such as particle swarm optimization (PSO), genetic algorithm (GA), winning particle optimization (WPO), and artificial neural network (ANN). A low-profile second-order passband FSS was realized by using non-resonant conventional patch and mesh in [93] and a miniaturized higher order dual-band FSS in [94]. A second-order passband FSS with narrowband frequency response by using inductively coupled miniaturized array element is detailed in [95]. The design technique is based on dielectric spacers as the key resonating filter elements and is coupled together by means of shunt inductors, which are employed as two-dimensional wire grids with subwavelength periodicity. It is noted that, in this way, the FSS periodic structure does not utilize any metallic elements to achieve the desired resonating filter response. Nonetheless, using such a non-resonant configuration possibly affects in miniaturization the array elements. FSS designed by two compact resonating layers of our stepped-impedance transmission lines joined to an outer square ring and coupled by a non-resonant inductive cross dipole layer, as seen in Figure 10a, exhibits a second-order passband response [96]. Worthy to mention that conventional higher-order passband FSSs are usually designed for higher bands of frequencies, such as X, Ku, K, and Ka, due to their relatively bulky configurations. However, the proposed structure is exceptionally compact and low-profile having a functional frequency of about 3.8 GHz and applicable to very low-frequency applications. Furthermore, Figure 10b shows fractal Koch elements of different levels used in a double layer FSS [97] and Figure 10c gives the topology of a three layer FSS proposed in [98] based on fractal complementary elements. Table 4 demonstrates a comparison of different multilayer FSS presented in recent researches, and it is observed that achieving higher FBW is a challenge with smaller unit size and reduced profile.