# Frequency Selective Surfaces: A Review
## 4 Types of FSSs
### FSSs Based on Applications
#### 4.3.2 Active FSSs (AFSS)
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**FSS Ref.** & **AFSS** & **Element Type and Configuration** & \begin{tabular}{c} **Frequency** \\ **(Range)** \\ \end{tabular} & **Applications** \\ \hline \multirow{3}{*}{[169]} & Reconfigurable FSs with two states (stopband) & \begin{tabular}{c} Cross-dipole element \\ (Recomfigurable cross dipole \& \& \\ hash simply by changing the state of PIN diode) (1 layer) \\ \end{tabular} & 12.61 (12–13) GHz & Adaptive \& RFID antennas \\ \hline \multirow{3}{*}{[23]} & Single-layer single polarized AFSS (Pass band) & \begin{tabular}{c} Circular aperture with four PIN \\ diodes placed orthogonally \\ (1 layer) \\ \end{tabular} & 2.63 (2–3) GHz & \begin{tabular}{c} Intelligent spectrum \\ selective island \\ \end{tabular} \\ \hline \multirow{3}{*}{[168]} & Reconfigurable by orthogonal rotation, all-dielectric MM & \begin{tabular}{c} Cross-shaped high-permittivity \\ ceramic resonators \\ (No metal layer) \\ \end{tabular} & \begin{tabular}{c} (3.55–4.60 \& \& \\ (4.54–4.94) GHz \\ \end{tabular} & \begin{tabular}{c} RCS reduction, reconfigurable \\ antenna systems, high \\ temperature \& high power \\ environment. \\ \end{tabular} \\ \hline \multirow{3}{*}{[139]} & Tunable broadband & \begin{tabular}{c} Four quarter circles and plates \\ create the gaps, series PIN diodes \\ as variable resistor and inductors \\ place between two metallic plates \\ for isolation (2 layers) \\ \end{tabular} & (1.6–8) GHz & Stealth technology \& EM shielding \\ \hline \multirow{3}{*}{[162]} & Tunable MM based & \begin{tabular}{c} An array of metallic square loops, \\ a wire grid and lumped, variable \\ capacitors inserted in the gap \\ between the loops (2 layers) \\ \end{tabular} & 8.49–11.48 GHz & X band \\ \hline \multirow{3}{*}{[159]} & 3D tunable & \begin{tabular}{c} 2D array of vertical graphene \\ micro-ribbons, tunable by change \\ of chemical potential \\ \end{tabular} & \begin{tabular}{c} 21.787–24.088 THz \\ (BW = 1.2) \\ \end{tabular} & \begin{tabular}{c} Terahertz band \\ \end{tabular} \\ \hline \multirow{3}{*}{[164]} & Tunable AFSS absorber & \begin{tabular}{c} Pattern with gradational edges \\ and PIN diode as variable resistor, \\ Varactor diode as a variable capacitor (2 layers) \\ \end{tabular} & (1–5) GHz & Smart stealth systems \\ \hline \multirow{3}{*}{[170]} & Frequency tunable & \
\begin{tabular}{c} Metallic patterns, a liquid \\ metallopality Electric \\ Gallium-Indium (EGaln) injected \\ in a micro-fluidic channel, \\ engraved on Polymethyl \\ Metheacrylate \\ \end{tabular} & 10.96–10.61 GHz &
\begin{tabular}{c} Large area sensor applications \\ with micro-pumps \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 8: An overview of AFSS with their applications. Exploration of active or tunable 3D FSS structures has been in emergent attention since soldering a lumped circuit element into the microstrip transmission line is quite simple. As an example, the microstrip line in [173] was substituted by a microstrip stepped impedance resonator in every unit cell of the structure [127], lowering the main resonance frequency, and thus a miniaturized and low profile 3D FSS is realized. Additionally, employing a lumped capacitor reduces the primary spurious resonant frequency, subsequently widening the operating bandwidth. Interesting performance features can be predicted by implanting semiconductor devices, such as small transistor-based amplifiers, diodes, or mixers in a unit cell of a 3D FSS. Tunable and/or 3D FSS may find potential use in electronically steerable reflectors and radiators [118; 174]. From the time of its discovery, graphene and carbon-based nano-composites have been exemplified and studied to recognize the tunable MSs in a very wide band, but only a few researches on graphene based FSS are available.