# Frequency Selective Surfaces: Design, Analysis, and Applications
## 10 Switchable and Tunable FSSs  
Electrically tuning the frequency response is an important feature of FSS design. For an adaptive screening of unwanted wireless transmissions, tunable FSSs are used [160, 161]. Such tunable and switchable structures are usually implemented by using semiconductor device loading, i.e., PIN diodes, varactors diodes, and Schottky diodes [162, 163, 164, 165]. DC bias is required for tuning the semiconductor devices, where the frequency response of the FSS is altered by controlling the bias voltage of the active elements. Thus, an appropriate metallic feed network should be added to the FSS to provide DC biasing to the active elements. Such a network adds to the complexity of the FSS structure and, in some cases, affects the original frequency response, especially under oblique incidences.  
Another method for FSS tuning/switching is by integrating micro-electro-mechanical (MEMs) switches into the FSS unit cells, where switching the MEMs elements alters physical paths on the unit cell geometry. MEMs devices can control the FSS resonance frequency by actuating two states switched electrostatically. Switching can be performed with high speed and low DC power consumption. However, MEMs devices require a complicated fabrication process that results in high implementation cost not suitable for low-cost systems [166, 167, 168]. An overview of different types of active FSSs alongside the comparison of their fractional bandwidth and tuning range is presented in Table 6.  
Other tuning methods such as liquid crystal manipulation, BST diodes, etc., have also been proposed; however, all of them suffer from high implementation complexity and biasing network requirements. In this work, tunable FSS are designed using a mechanical approach that removes the need for DC biasing. In addition, a novel embedded bias network has been used to conceive a varactor-based FSS that removes biasing interference issues and harmonic generation and minimizes lumped element or RF choice requirements.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**FSS Ref.** & **Type of AFSS** & **Switching/Tuning** & **FBW** & **Tuning** & **Polarization** \\ \hline
[70] & Switchable(Passband) & PIN diodes & 17 & & No \\ \hline
[169] & Tunable(Stopband) & Varactor diode & 12.5 & 32 & No \\ \hline
[43] & Tunable Absorber(Passband) & Varactor diode & 4 & 5 & No \\ \hline
[170] & Tunable(Stopband) & Varactor diode & 81.4 & 69.5 & No \\ \hline
[71] & Tunable & Varactor diode & 152.4 & 129 & Yes \\ \hline
[34] & Tunable(Stopband) & High-permittivity ceramics & 25.8 & 22.9 & No \\ \hline
[72] & Tunable Absorber & PIN diode & 91 & 31.6 & Yes \\ \hline
[33] & Tunable(Passband) & Chip capacitor & 5.9 & 30 & No \\ \hline
[83] & 3D Tunable AFSS(Stopband) & Graphene micro-ribbons & 5.5 & 10 & No \\ \hline
[92] & Tunable Absorber & PIN, Varactor diode & 113 & 47.3 & No \\ \hline
[171] & Tunable 3D AFSS(Passband) & Varactor diode & 14 & 65 & No \\ \hline
[172] & Tunable(Passband) & Varactor diode & 11.5 & 24.6 & No \\ \hline \hline \end{tabular}
\end{table}
Table 6: An overview of types of AFSS with the comparison of their FBW and tuning range (TRs).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Article** & **Frequency (GHz)** & **Thickness** & **Order** & **Bandwidth** \\ \hline
[153] & 21 & 0.273\(\lambda_{0}\) & second & 5\% \\
[156] & 10 & 0.033\(\lambda_{0}\) & second & 20\% \\
[157] & 10 & 0.067\(\lambda_{0}\) & second & 21\% \\
[158] & 16.5 & 0.22\(\lambda_{0}\) & second & 10\% \\
[159] & 8.5 & 0.257\(\lambda_{0}\) & third & 15\% \\ \hline \hline \end{tabular}
\end{table}
Table 5: Summary of some FSSs regarding overall thickness and fractional bandwidth.