# Design of an Optically Transparent Microwave Absorber Based on Coding Metasurface
## 2 Design and Simulation
Therefore, the perfect microwave absorptance is achieved by simultaneously reducing transmittance and reflectance to maximize the absorptance rate. Figure 1.0 Retrieved constitutive parameters, absorptance, reflectance, and transmittance. (**a**) Effective permittivity. (**b**) Effective permeability. (**c**) Effective impedance. (**d**) Absortance, reflectance, and transmittance characteristics. Figure 5d shows the simulated absorptance, reflectance, and transmittance of the proposed absorber. It has a good absorptance characteristic of over 90% in the 5.06-9.01 GHz band. In order to explain the influence of the upper resistive film on the absorptance performance of the proposed structure, the reflectivity of the absorber at different resistance values (\(R_{\text{S1}}\)) of the upper layer was investigated. As illustrated in Figure 6, as the resistance of the top layer increases, there is an optimum value of sheet resistance (\(R_{\text{S1}}=110\)\(\Omega\)/sq) to achieve the lowest reflection coefficient (\(-45.21\) dB). In this case, the absorptance of more than 90% was achieved in the frequency range of 5.06-9.01 GHz. To better understand the absorptance mechanism, the surface current distribution on the top layer at 7.72 GHz is shown in Figure 1.1 The surface current is mainly distributed on the surface of the rectangular unit, as opposed to the circular unit, where a smaller current distribution can be seen. When the current distribution on the surface increases, the ohmic loss on the resistive film also increases, thus converting more electromagnetic energy into heat, thereby the absorptance rate achieves its maximum value. Figure 1.2 Simulated absorptance of the proposed structure for various upper-layer surface resistance (\(R_{\text{S1}}\)) values. Figure 1.3 Simulated current distribution at the surface of the absorber structure at 7.72 GHz. As shown in Figure 8, the absorptance remains above 90% at polarization angles ranging from 0 to 90\({}^{\circ}\), while the absorptance band remains nearly unchanged, which illustrates the polarization insensitivity of the absorber. The simulation confirms the polarization insensitivity of the proposed optically transparent absorber. In addition to polarization stability, absorbers must have good angular stability in many applications. The simulated absorptance spectra at different oblique illumination incidences for the TE and TM modes show that the absorptance bands slowly and gradually shift to the higher frequency range (Figure 9). When irradiated in the TE mode, the absorptance remains above 90% at incidence angles ranging from 0 to 15\({}^{\circ}\), while the structure of the absorptance band remains almost unchanged. For the TM mode, the stability of the absorptance at different incidence angles is also similar to the TE mode data. Figure 1.4 Simulated absorptance of the proposed absorber versus frequency and polarization angle (\(\varphi\)). Figure 1.5 Simulated absorptance as a function of incidence angle \(\theta\) for: (**a**) TE, and (**b**) TM polarization. In general, the absorptance of the metamaterial absorber exceeds 80% for incident angles ranging from 0 to 45\({}^{\circ}\) over the entire operating bandwidth of both TM and TE modes. This result shows that the proposed design has a reasonably stable angular performance. The RCS of an ITO-coding metasurface with a cross-section of 240 mm \(\times\) 240 mm and the perfect electric conductor (PEC) of the same size are simulated and the results are shown in Figure 1.6 It can be seen that the proposed coding metasurface absorber scatters and spreads the incident electromagnetic wave energy in all directions when the frequency of the incident electromagnetic wave is 7.0 GHz. Furthermore, the energy scattered out in each direction is not strong, indicating that the absorber has good RCS scaling characteristics.