## 3 EM Simulation Results  
To optimize and verify the proposed design method, a full-wave simulation is carried out using COMSOL Multiphysics. As for the full-wave simulation set-up and analysis, a general periodic boundary condition (PBC) is used for the pixelated unit-cell based on a finite element method (FEM). To simulate the proposed structure with PBC, the plane wave incident port was set to the same size as the area of the unit cell. Also, to reduce the simulation time, the bottom side of the absorber was considered as the PEC. It was assumed that there are no transmitted electromagnetic waves through the perfect electric conductor (PEC) on the bottom of the absorber. As a result, the reflectance of the absorber is computed by the follow equation, A = \(1-\mathrm{R}-\mathrm{T}\), where 'A' is the absorption level, 'R' means the absolute value of reflection coefficient, and 'T' presents the absolute value of the transmission coefficient. The optimized pattern of the proposed metasurfaces is shown in Figure 4a. To verify the proposed metasurfaces and absorber design, 3D full-wave simulation with a numerical analysis is proposed. Figure 4a,b show the full-wave simulated reflectance results. Figure 4b shows the results of the reflectance obtained with a change in sheet resistance from 40 to 50 ohm/sq with an increment of 5 ohm/sq for a normal incidence. Here, the incident wave is TE polarized, and the electric and magnetic fields are perpendicular to the plane of incidence of y- and x-axes, respectively. The simulation results for all three cases show a reflectance less than \(-\)10 dB, corresponding to absorption of more than 90%, in the target frequency band from 2.2 to 18 GHz. Therefore, it can be expected that this design is robust against variation of sheet resistance due to manufacturing errors of the metasurfaces. Next, an absorption ratio for the sheet resistance of 40 ohm/sq was studied further with changes of polarization and incident angles. Figure 5a,b show the EM simulated reflectance dependent on the incident angle (\(\theta\)) equal to \(0^{\circ}\), \(7^{\circ}\), \(15^{\circ}\), \(30^{\circ}\), and \(45^{\circ}\) in the case of the TE and TM polarization, respectively. As shown in Figure 5a, in all cases except for the case of the \(45^{\circ}\) oblique incidence, the reflectance is lower than \(-\)10 dB at the frequencies  
Figure 3: Schematic of the proposed pixelated pattern. of 2.2-18 GHz. The results of reflectance for the TM polarization are observed in Figure 5b. Due to the symmetrical structure, the same results are obtained for the normal incidence between the TE and TM polarizations, while the absorption of 90% or more is satisfied from 0\({}^{\circ}\) to 45\({}^{\circ}\). Figure 6 presents the magnitudes of the electric-field (E-field) and magnetic-field (H-field) distribution with an xz-plane as the x-and y-component, respectively. The incident polarization is the same as the coordinate system in each figure. The plotted E-field is divided into three sections along with the \(z\)-axis. The first section corresponds to the area over the upper metasurface as a protective layer, in which electromagnetic waves over the air are incident. The second section is the area between the upper and lower metasurfaces as a dielectric, and the last section is the area between the lower metasurface and the reflector. For the lower frequency bands case as Figure 6a, the destructive interference is mainly created by the upper metasurface. This is because the path difference of the multi-reflected EM waves between the reflection due to the upper metasurface and the reflection caused by the floor reflector at 180 degrees with a certain height or more  
Figure 4: Schematics for optimized metasurfaces and EM simulation results of optimized metasurface absorber. (**a**) Left—upper metasurface pattern, right—lower metasurface pattern, (**b**) Simulated reflectance with respect to the sheet resistance at theta = 0 deg.