# Frequency Selective Surfaces: A Review
## 4 Types of FSSs
### FSSs Based on Applications
#### 4.3.1 Microwave Absorbing FSSs  
In general, an FSS generates good reflection characteristics in a specific range of frequencies in its stopband, as shown in Figure 14a. Because for some specific applications in the stopband at the microwave range, such strong reflection should be reduced, defining a terminology as frequency selective absorber, also called microwave FSS absorbers illustrated in Figure 14b. The reflected signals are absorbed in the stopband usually by introducing some absorbing array elements in the FSS design. Therefore, instead of reflecting the incident EM waves back, microwave absorbers are chiefly used to absorb them. They are specifically useful in making military aircraft/targets undetectable from the radars. If \(\Gamma\) and T are the reflection and transmission coefficients, respectively, the absorption factor A can be calculated by (10). \[\text{A}=1-|\Gamma|^{2}-|\Gamma|^{2} \tag{10}\]  
Commonly, FSSs with reflect/transmit characteristics are integrated into walls of the buildings to secure the wireless local area network (WLAN). Purposely, they allow for mobile phone signals to pass while blocking the WLAN signals. However, serious stopband reflections from FSS plane take place and cause extra delay spread and multi-path fading in WLANs. Lossy FSSs as absorbers have recently been investigated [132; 133; 27]. Achieving a compact design with reduced space between the FSS and the resistive sheets, a stable response for higher angles \begin{table}
\begin{tabular}{c l l l} \hline
**FSS Ref.** & **3D FSS** & **Element Type** & **3 dB BW Range (GHz)** \\ \hline
[112] & Wide passband FSS & Two microstrip lines split by a horizontally & \multirow{2}{*}{5–10} \\  & & laid long and thin metallic rectangular bar & \\ \hline
[111] & Passband FSS with rapid roll-off based on & Subwavelength staggered vertical and & \\  & surface wave tunneling & horizontal metallic rectangular frames & \\ \hline
[127] & Miniaturized, thinner FSS with steep & Lumped capacitor on microstrip & \\  & quasi-elliptical band-stop filtering & stepped-impedance resonator & \\ \hline
[128] & Flat passband and sharp rejection & A symmetric unit cell in three dimensions & 12.4–16.3 \\  & response FSS & Periodic array of square waveguides & 12–24.7 \\ \hline
[129] & Wide and flat passband FSS & Two parallel U-shaped suspended strip lines, & \\  & & double printed on a thin substrate and & \\  & & located inside a square waveguide & \\ \hline
[130] & Novel miniaturized 3D FSS independent of & Square metallic waveguide with modified & \\  & polarizations \& incident angles & dumbwell slots in all waveguide walls & \\ \hline
[131] & Dual-polarized FSS with wide & Two arrays of vertically and horizontally & \\  & out-of-band rejection & placed double sided parallel strip lines & \\  & & connected together by a centered via and a & \\  & & number of inserted metallic plates & \\ \hline \end{tabular}
\end{table} 
Table 6: An overview of 3D FSSs reported in different researches. Figure 14: (**a**) FSS with good reflection characteristics (**b**) FSS with absorber characteristics. of incidence and both polarizations are the main challenges in absorber designs. Circuit analog absorbers have proved efficient in achieving stable frequency responses with oblique incidences and dual polarization [134]. A novel transmit/absorb FSS, which was designed in [135], provides security and/or isolation to systems at 5 GHz by absorbing signal but permits mobile phone signals to pass. It is noticed that surface impedance can be altered by etching a small circular opening at the centre of a conducting cross dipole, as shown in Figure 15a. The cross dipoles are packed in between two dielectric layers in order to give further frequency stability and power management [2]. Microwave FSS absorber is a class of functional FSS that is being used in stealth technology. Aircraft applications, such as helicopters, airplanes, airships (including blimps), gliders, and missiles require highly efficient broadband absorbers to diminish the likelihood of being detected.