## 3 A Taxonomy for Planar FSS
### Meandered Cell  
Meander-based designs in FSSs leverage the idea of creating longer conductive paths within compact surfaces by folding straight conductive lines into multiple corners. This approach enables enhanced performance, such as size reduction and multiband behavior, while maintaining compactness and angle stability. Several designs have been proposed in the literature, and key examples are summarized below, with visual representations of some unit cells in Figure 9.  
Figure 8: Some examples of loop or ring unit cells: (**a**) nested circular loop [62]; (**b**) nested square loop [63]; (**c**) split ring [67]. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Design** & **Frequency (GHz)** & **Length (\(\lambda\))** & **Reference** \\ \hline Square loop & 2.4 & 0.16 & [59] \\ Square loop & 2.4 & 0.48 & [60] \\ Square loop aperture, with PIN diodes & 3.2 & 0.18 & [61] \\ Nested circular loop with recesses & 2.4/5.8 & 0.23 & [62] \\ Three concentric loops & 31.2/38 & 0.47 & [63] \\ Multilayer gridded-triple square loops & 4.7/10.5/15/21 & 0.08 & [64] \\ Meanders into the main ring & 6.3/8.3 & 0.16 & [65] \\ Square ring with meanders & 5 & 0.08 & [66] \\ Four-layer FSS based on split rings & 25.5/36.5 & 0.42 & [67] \\ Square loop with two folded arms & 3.5/5.2/10.2 & 0.12 & [68] \\ Two layer with different rings & 27.3/39.2 & 0.36 & [69] \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Summary of loop or ring unit cells considered. One notable design is a triple-layer FSS described in [70], which features a complex geometry with swastika-shaped convoluted arms in layer 1, interconnected to a second layer with a central portion etched away, and further connected to a third layer consisting of four open rectangles. This intricate structure achieves an extremely compact size of just 0.02 wavelengths. Another example is a self-complementary FSS, detailed in [71], which is centered at 4.6 GHz and exhibits very high angle stability, maintaining performance at angles up to 60\({}^{\circ}\). The unit cell size in this design is 0.10 wavelengths, highlighting the advantages of the self-complementary approach for achieving angle stability while retaining a compact form. The research of [72] presents a design that combines a cross dipole aperture with a meandered monopole aperture. This design resonates at 5.1 GHz and 8.8 GHz, with a compact size of 0.11 wavelengths. Even more compact is the FSS proposed in [73], which uses meander lines in a cross organization to achieve dual-band performance at 2.4 GHz and 3 GHz, with a unit cell measuring just 0.06 wavelengths. Further refinement of the meander design can result in multiband performance. In [74], the FSS design features two periodic metallic arrays separated by a thin dielectric substrate. The top array consists of four branched spiral triangles, while the bottom array contains gridded tortuous cross-dipoles. This design operates at three resonant bands--3.28 GHz, 5.2 GHz, and 5.4 GHz--with a size of 0.07 wavelengths. High angular stability is a key requirement for many FSS applications. In [75], a double-arc centrosymmetric cross loop with folded arcs demonstrates exceptional angle stability, with only a 0.08 GHz deviation at an incident angle of 85\({}^{\circ}\). The design resonates at 5.9 GHz and has a unit cell size of 0.16 wavelengths. Another design, introduced in [76], achieves high angular stability and compactness through a band-pass FSS printed on a single-layer substrate. The design consists of four symmetrical spiral patterns of metallic meander lines tuned at 3 GHz, with a unit cell size of just 0.04 wavelengths. This design underscores the potential of meander-based FSSs in achieving both compactness and high-performance stability. Table 6 provides a summary of these meander-based FSS designs, illustrating the benefits of using meander structures for compact, multi-band, and high-angle-stability FSS applications.