## 3 A Taxonomy for Planar FSS
### Cross Cell  
Table 2 summarizes several FSS designs based on cross-cell configurations. A classic example of a square cross FSS designed for the 2.4 GHz band, typically used for WiFi transmissions, is presented in [34]. This band-stop FSS has a conductive cross printed within a square, with a maximum size of 0.32 wavelengths. A common evolution of the basic cross-cell design is the use of Jerusalem crosses instead of square crosses. For instance, a design tuned to 8 GHz and measuring 0.4 wavelengths is described in [35]. Additionally, electro-textile fabrics have been employed in a band-stop FSS with a Jerusalem cross pattern, such as the solution in [36], which operates at 30 GHz with a unit cell size of 0.68 wavelengths. Some modifications to Jerusalem crosses allow for dual-band performance, as seen in [37], which functions at 2.4 and 5.8 GHz and has a cell size of 0.26 wavelengths at 2.4 GHz. Another innovative approach involves a multilayer graphene-based FSS with a complementary cross-shaped pattern, incorporating layers of graphene, PVC, and FR4, for dual-band frequencies around 10 and 20 GHz, with a side length of 0.44 wavelengths [38]. Further variations include the use of concentric bow-tie dipoles in a cross configuration, centered at 2.45 GHz with a slightly larger electric dimension of 0.06 wavelengths [39]. At the higher frequency of 60 GHz, an FSS structure based on aperture-coupled resonators, \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Design** & **Frequency (GHz)** & **Length (\(\lambda\))** & **Reference** \\ \hline Square, with a conductive cross printed & 2.4 & 0.32 & [34] \\ Jerusalem cross & 8 & 0.4 & [35] \\ Textile Jerusalem cross & 30 & 0.68 & [36] \\ Dual-band Jerusalem cross & 2.4/5.8 & 0.26 & [37] \\ Multilayer graphene with cross design & 10/20 & 0.44 & [38] \\ Bow-tie cross & 2.45 & 0.06 & [39] \\ Aperture-coupled resonators with a single cross & 60 & 0.5 & [40] \\ Modified swastika cross & 5 & 0.11 & [41] \\ Two convoluted cross-slot layers & 7.9 & 0.08 & [42] \\ Tapered cross-slot & 10 & 0.53 & [43] \\ Resembling crooked cross & 2.54/3.54 & 0.09 & [44] \\ Second-order fractal cross & 0.9/1.9 & 0.18 & [45] \\ Fractal dual-band pass & 2.4/5/5.9/6 & 0.24 & [46] \\ Double metallic layers & 20.6/29.7 & 0.27 & [47] \\ Conducting and resistive cross dipoles & 5.6/5.7 & 0.31 & [48] \\ Square annular and Jerusalem slot & 28/37 & 0.56 & [49] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of cross unit cells considered. featuring a single cross on each side of the surface, has a unit cell size of 0.5 wavelengths [40]. Another design utilizing a modified swastika cross achieves stopband performance at 5 GHz for indoor shielding of WLAN signals, with a size of 0.11 wavelengths [41]. A more complex design consisting of two identical metallic convoluted cross-slot layers, connected by vertical metal vias, is proposed in [42], operating at 7.9 GHz with a compact size of 0.08 wavelength. Similar principles were applied in [43] to a tapered cross-slot design on either side of the substrate, surrounded by metallic vias and tuned to the 10 GHz band, with a unit cell size of 0.53 wavelengths. For designs requiring double-band attenuation, more intricate configurations are necessary. For example, a convoluted and branched crooked cross geometry provides dual resonant paths, operating at 2.54 and 3.54 GHz with a size of 0.09 wavelengths [44]. The fractal approach can also aid in achieving compact designs for lower frequencies, such as a second-order fractal cross dipole for cellular communications at 900 MHz and 1.9 GHz, with a unit cell size of 0.18 wavelengths [45]. Another fractal-based design on RC film, introduced in [46], supports up to four frequency bands for WLAN transmission enhancement, with the smallest dimension being 0.24 wavelengths at 2.4 GHz. At higher frequencies, a single-layer FSS operating at 20.6 and 29.7 GHz, consisting of metallic layers on both the top and bottom of the substrate, requires a size of 0.27 wavelengths [47].