## 3 A Taxonomy for Planar FSS
### Loop or Ring  
Designs using square loops or rings are widely explored in FSS applications due to their flexibility in providing multiband functionality and compactness. Several key designs have been identified, as summarized below, with visual examples of unit cells shown in Figure 8.  
A square loop-based FSS design, with a size of 0.16 wavelengths, can be found in [59]. This design includes an equivalent circuit model to aid in performance analysis. A different traditional proposal, with a larger size of 0.48 wavelengths, is described in [60] and is designed to eliminate 2.4 GHz signals, specifically to prevent interference between WiFi networks in neighboring apartments.  
Active elements can further reduce the size of the unit cell. For example, in [61], a square loop-aperture FSS incorporates four PIN diodes placed at 90-degree intervals across the aperture, achieving a small size of 0.18 wavelengths while maintaining effective signal blocking.  
Dual-band performance is a key feature of many modern FSS designs, often achieved by nesting multiple concentric rings or loops. An example is the multi-stop-band FSS described in [62], which uses nested circular loops with recesses, having the inner loop divided into four pieces. This design is tuned to block both 2.4 GHz and 5.8 GHz, filtering common wireless local area network (WLAN) frequencies with a unit cell size of 0.23 wavelengths. A more complex, triple-concentric-loop design, with metal shorts in the outer ring, operates at higher frequencies (31.2 GHz and 38 GHz), with a size of 0.47 wavelengths [63].  
Advanced designs, like that in [64], provide multiband performance at four different frequencies (4.7 GHz, 10.5 GHz, 15 GHz, and 21 GHz). This design cascades three metallic  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Design** & **Frequency (GHz)** & **Length (\(\lambda\))** & **Reference** \\ \hline Triangular patches and meandered lines & 4.9/6.3 & 0.14 & [56] \\ Square loops combined with hexagonal rings & 13/24/25 & 0.18 & [57] \\ Active design & 8.3/10.7 & 0.16 & [58] \\ \hline \hline \end{tabular}
\end{table}
Table 4: Summary of patches with additional elements unit cells considered.  
Figure 7: Some examples of patches with additional elements unit cells: (**a**) triangular patches with meandered lines [56]; (**b**) hexagonal patch with loop structures [57]; (**c**) metdigital patch [58].  
layers, separated by thin dielectric substrates, with each external layer featuring a periodic array of gridded-triple square loops (G-TSLs), while the middle layer contains an array of double-square loops (DSLs). Despite its multiband capability, this FSS maintains a compact size of just 0.08 wavelengths.  
Meandered ring structures are a useful technique to introduce additional resonances. For instance, [65] presents a bow-type ring and a square ring FSS, tuned to 6.3 GHz and 8.3 GHz, with a size of 0.16 wavelengths. Another example, in [66], modifies a traditional square ring by adding meanders along each edge, achieving a reduced size of 0.08 wavelengths, designed for WiFi frequencies (5 GHz).  
Multilayer FSS designs are also popular for achieving dual-band or multiband performance. A four-layer FSS circular polarizer, detailed in [67], blocks signals at 25.5 GHz and 36.5 GHz with an electrical size of 0.42 wavelengths. Similarly, a single-layer FSS with square loops on the top and a folded-arm square loop on the back provides three exclusion bands (3.5 GHz, 5.2 GHz, and 10.2 GHz) with a compact size of 0.12 wavelengths [68].  
Stability against angle variations is critical in real-world applications, and this can be enhanced by using rings and resonators across multiple layers. An example is the two-layered FSS in [69], which combines a fractal square ring on the top layer with a cross resonator on the back layer. This design is tuned to 27.3 GHz and 39.2 GHz, with a size of 0.36 wavelengths at the lower frequency.  
Table 5 provides a summary of these designs, offering insights into their frequency bands and size optimizations.