# Frequency Selective Surfaces: A Review
## 4 Types of FSSs
### FSS Based on Array Elements
#### 4.1.2 Convoluted or Meandered FSSs  
Some applications demand miniaturized FSS, especially in radomes where flexibility is a challenge for FSS designers. FSSs with relatively small electrical dimensions are enviable, so that the unit cell size is reduced without affecting the angular stability. It is proved in [50] that the convoluted square loop FSS structures can present better stability in polarization as compared to the simple element. Furthermore, in [62], it was studied that smaller unit cells are less distorted for a specified amount of curvature. Through convolution square loop, the element area can be minimized by a significant factor that may exceed an order of magnitude. The concept of convoluting the elements to reduce the unit size is a constructive approach to tighten the curved structures and evidently isolate the grating lobes effect from the fundamental resonance. However, manufacturing limitation falls on the way as a penalty to pack the resonating length to diminish the unit cell of a particular area. Miniaturized FSS with convoluted square loop array elements was initially coined in [62], and Figure 6a shows such a structure shape [50]. This FSS provides effective shielding in X-band with 3 dB fractional bandwidth (FBW) of approximately 48% enough to cover this band [50]. Miniaturized array elements with interwoven geometry, as shown in Figure 6b, have efficiently been used to provide selective frequency isolation in office windows and thus signal-to-noise ratio is improved, delivering great advancement in the reliability of mobile communications [63]. Similarly, to address the challenge of minimizing the unit cell, meandered elements are introduced in [64] to increase the resonant lengths, in which four symmetrical and spiral shapes of metallic FSS element are printed on one side of a dielectric substrate, as shown in Figure 6c. It is observed that these meandered lines are different from previous works as in this research; the element slots are nested with their neighborhood. The length span of resulting cross spiral slot is long enough to efficiently increase the equivalent inductance as well as the capacitance at the same time, allowing for this compact arrangement to demonstrate promising stability under oblique incident angles and polarizations. A similar concept of meandered lines was studied in [65; 66]. The conception of meandered array element not only trims down the unit cell size, but also gives a stable stopband frequency response \begin{table}
\begin{tabular}{c c c c} \hline \hline
**FSS Ref.** & **Layers** & **Patch/Aperture** & **Filter Type** \\ \hline
[51] & 2 & Interconnected non-complementary planes & Dual Band \\ \hline
[52] & 2 & Offset-circular ring patches & Single band \\ \hline
[53] & 1 & Centered square patch and connected L-shaped arms & Dual Band \\ \hline
[54] & 1 & Three capacitive and several inductive loops & Multi Band \\ \hline
[55] & 1 & Hexagonal-form with circular arrangement of & Single band \\  & & triangular loops & \\ \hline
[56] & 1 & Four sides loading aperture element into the inner side of the & Single band \\  & & square loop. & \\ \hline
[57] & 2 & Interconnected ring resonator element with two splits & Single band \\  & & arranged orthogonally in two layers & \\ \hline
[58] & 1 & Circular ring elements consisting of meander-slots with & Dual Band \\  & & multi turns & \\ \hline
[59] & 1 & Neighboring Y-hole elements rotated by 90\({}^{\circ}\) & Single band \\ \hline
[60] & 1 & Unit cell of single ring slot pair with similar three pairs & Single band \\  & & concentrically used & \\ \hline
[61] & 1 & Two circular loops of the nested structure with Recesses & Dual Band \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Different types of FSSs. (in case of loop type FSS) for higher oblique angles of incidence, and segregates the main resonant frequency from the grating lobes.