# Frequency Selective Surfaces: A Review
## 4 Types of FSSs
### FSSs Based on Applications
#### 4.3.2 Active FSSs (AFSS)  
Generally, once conventional passive FSSs (consisting of conducting patches and/or apertures on a dielectric substrate) are built, they turn to be inflexible in performance due to their permanent physical characteristics after manufacturing. Although their design and fabrication are relatively easy, yet, certain applications require amendment in their frequency response. Nowadays, a solution to such a challenge is Active Frequency Selective Structure (AFSS), which can potentially control the EM performance by dynamically fine tuning the exterior excitations, for example, DC voltage range, light strength, or the field energy [151; 152; 153]. As compared to the invariable frequency characteristics of passive FSS, the EM properties of AFSS, such as resonant frequency and polarization, are reconfigurable during its operation. The addition of active elements in the FSS structure provides this reconfiguration feature. Commonly used active elements in microwave AFSS include On/Off switches, PIN diodes \begin{table}
\begin{tabular}{c l l} \hline \hline
**FSS Ref.** & \multicolumn{2}{c}{**Type of Microwave Absorber**} & \multicolumn{2}{c}{**Element Type**} \\ \hline
[140] & Wide Incidence angle insensitive dual polarized two & Symmetric geometry of eight-circular sectors and \\  & layers MM absorber & Bottom ground plane \\ \hline
[141] & Miniaturized FSS absorber & Minkowski fractal loops \\ \hline
[142] & Resistive FSS with Ferromagnetic nan-ofilms based on & A square patch of capacitive impedance. \\  & high-impedance surfaces & \\ \hline
[143] & Lightweight, ultra-broadband, wide incidence angle & Resistive patches on top, middle Polyurethane foam \\  & resistive FSS absorber based on topology optimization & substrate \& metal ground on bottom \\ \hline
[144] & Ultra-thin absorber with cascaded high impedance \& FSS & Pixelized metallic surface placed above the high \\ \hline
[145] & Polarization dependent FSS absorber with multiple & C shaped slot in a circular patch, ground bottom and \\  & absorption peaks & polyimide film as dielectric substrate \\ \hline
[146] & FSS absorber with low profile & Capacitive patches and inductive mesh \\ \hline
[147] & Compact broadband FSS absorber & Resitively loaded quadruple hexagonal loops \\ \hline
[148] & Ultra compact partly transmissible absorptive FSS at & Via-based structure with staggered patterns on front and \\  & 5-GHz Wi-Fi band, to solve EMI/FRI problems in & back metal layer while back metal layer has air vents in \\  & mixed-signal systems. & the ground plane \\ \hline
[149] & Thin broadband FSS embedded absorber & Double layer composite coating of higher iteration of \\  & Sierpinski gasket patches \\ \hline
[150] & High absorptive FSS embedded absorber & Fractal FSS with top metal and bottom Aluminum \\  & metal plate \\ \hline \hline \end{tabular}
\end{table} 
Table 7: An overview of microwave FSS absorbers. Figure 16: (**a**) Trend graph showing effect of altering the bias voltage on resonance frequency. (**b**) Unit cell geometry (**c**) The topology of tunable FSS absorber. (heavily doped P and N regions split by a broad intrinsic region), Varactor diodes, Varicap, Schottky diode, and some others. The response of the FSS changes when the state of such active device (e.g., PIN diode) switches from ON to OFF or vice versa, which depends on the external DC bias source. Broadly, two classes of electrically controlled AFSSs are defined including the switchable AFSSs, and the tunable AFSSs [154]. The first kind of AFSS toggles its response at the desired frequency band, between the transmission and reflection characteristics [155], whereas the second type of AFSS can shift the resonant frequency in a certain frequency band [152]. A switchable AFSS based absorber/reflector has recently been reported in [156], where it works as an absorber in absorptive condition, while it acts as a reflector in opposite state.