# Frequency Selective Surfaces: A Review
## 4 Types of FSSs
### FSSs Based on Structure
#### 4.2.4 Three-Dimensional FSSs (3D FSSs)  
Generally, the required qualities of an FSS are wide passband, fast roll-off (sharp edges), and insensitivity to incident angles and polarizations of an incoming EM wave in applications, such as RCS reduction, terahertz sensing, EM compatibility, and telecommunication. Unfortunately, two-dimensional periodic elements (patch/apertures), planar FSSs (2D FSS) with the single layer or multilayer are inefficient to realize all of the above-mentioned features simultaneously [22]. Although such structures are easy to manufacture, but the inability to provide satisfactory performance, restricts these in many potential applications. Recently, a unique class of FSSs, distinguished from traditional FSS, has been introduced as 3D FSSs to beat the limitations of 2D FSS [22; 109]. Equivalent LC resonance circuit, waveguide cut-off, or multilayer coupling technique is mostly used to deduce the filtering characteristics of 2D FSS. Same methods apply to the existing 3D FSS too. However, additional structures/cavities between planar layers of 2D FSS are introduced instead of the conventional multilayer structure to create 3D FSS design. The inter-structure additions can be periodic arrays of microstrip line board, metal plate bits, vias (holes), or some resonating metal lines. These 3D configurations grant superfluous choice of design methods, and therefore, adaptable for advanced performance characteristics of in-band response stability, sharper cut-off, superior flexibility, and wider out-of-band rejection. Theory of cascading the cavity filters implies that by cascading the Substrate Integrated Waveguide (SIW) cavities in an FSS, selective performance can be amazing as compared to single cavity FSS. Figure 11: The Configuration of fractal Antenna-Filter-Antenna (AFA) based FSS with top and side view [74]. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**FSS Ref.** & \begin{tabular}{c} **Conducting** \\ **Layers** \\ \end{tabular} & \begin{tabular}{c} **f\({}_{0}\) (GHz)** \\ \end{tabular} & \begin{tabular}{c} **Array Element** \\ **Size** \\ \end{tabular} & \begin{tabular}{c} **Total** \\ **Thickness** \\ \end{tabular} & \begin{tabular}{c} **FBW \%** \\ \end{tabular} & \
\begin{tabular}{c} **Order** \\ \end{tabular} &
\begin{tabular}{c} **Patch and Aperture Elements** \\ \end{tabular} \\ \hline
[75] & 5 & 10 & 0.25\(\lambda\) & 0.267\(\lambda\) & 37 & 3 &
\begin{tabular}{c} Minkowski fractal patch with \\ slot ground \\ \end{tabular} \\ \hline
[74] & 3 & 5.1 & 0.2\(\lambda\) & 0.07\(\lambda\) & 19.6 & 2 &
\begin{tabular}{c} Minkowski fractal patch with \\ slot ground \\ \end{tabular} \\ \hline
[107] & 3 & 36 & 0.632\(\lambda\) & 0.116\(\lambda\) & 8.2 (1 dB) & – &
\begin{tabular}{c} Hexagonal patch and CPW \\ quarter wave resonator (U slot) \\ \end{tabular} \\ \hline
[98] & 3 & 10 & 0.25\(\lambda\) & 0.23\(\lambda\) & 60 & 3 & Fractal patch with slotted ground \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Comparison of Antenna-Filter-Antenna fractal FSS structures. Therefore, conventional FSS with grid slot is united with cascaded SIW cavity structure, thereby improving the selectivity response and giving faster roll off in sidebands [110]. Given the lumped element parameters along with structure geometry, the frequency responses of an FSS with an arbitrary number of cascading SIW cavities (1-8) can be obtained, and it is noticed from this reference that the transmission properties reach closer to the ideal passband transmission response. Additionally, the BW and insertion loss are not sensitive to the cavity count  
A novel design concept of guiding and confining the surface wave propagation through the transmission tunnels, by subwavelength metallic staggered rectangular frames, as shown in Figure 12a, has been utilized for spatial filtering as compared to earlier investigated planar surface structured waveguides [111].