## 2 Design, Simulation, and Theoretical Analysis
The ground plane and the metasurface array form a Fabry-Perot-like cavity [51] as illustrated in Figure 5. The incoming electric field is expressed as E\({}_{\mathrm{iy}}\); E\({}_{\mathrm{r,rk,k}}\) and E\({}_{\mathrm{r,ryk}}\) (where k is a positive integer) are the kth reflected waves of the cross-polarization and the co-polarization, respectively. Repeated reflections exist for the incoming wave within the dielectric layer. The reflections of the cross-polarization are superimposed uniformly, thereby enhancing the cross-polarized reflection. In contrast, the interference of the co-polarized reflection wave is reduced, leading to reduction in the co-polarized reflection. Figure 4: Distributions of the surface currents on the top ((**a**1)–(**a**4)) and bottom ((**b**1)–(**b**4)) layers of the presented metasurface for the resonance frequencies of 10.67 GHz, 12.22 GHz, 16.18 GHz and 19.51 GHz, respectively. Figure 5: Schematic diagram of multiple reflections in the Fabry–Perot-like reflective polarization converter. The dielectric layer thickness for the reflecting metasurface is also a vital parameter that is closely related to the bandwidth and the PCR. Figure 6a,b show the PCR and the bandwidth for different dielectric layer thicknesses, respectively. As shown in Figure 6b, the PCR first increases and then decreases, while the bandwidth decreases gradually in the first frequency band as the dielectric layer thickness increases. In the second operating band, both the PCR and bandwidth initially increase and then decline with increasing dielectric layer thickness. When the dielectric layer thickness is 3 mm, the maximum PCR value in these two frequency bands is obtained, indicating that the dielectric layer thickness has a significant effect on the PCR. Figure 7 shows the reflection coefficients of the co-polarization \(r_{yy}\) for various dielectric layer thicknesses. According to Figure 7, the two resonance frequencies in the first operating band become closer to each other as the dielectric layer thickness increases, leading to a narrower bandwidth in the first band. In contrast, the distance between the two resonant frequencies in the second operating band increases first and then decreases with increasing dielectric layer thickness and the bandwidth is reduced after the initial increase. In addition, \(r_{yy}\) in the second operating band increases rapidly as the dielectric layer thickness increases, resulting in a reduction in the PCR. Therefore, when the PCR and the bandwidths of the two operating bands are considered, the dielectric layer thickness can be optimized reasonably well to achieve outstanding dual broadband performance. Figure 8 illustrates the relationship between the structure parameter \(\theta\) and the performance of the designed polarization converter. Figure 8a shows that the PCR maintains more than 95% in the whole regulation range. As shown in Figure 8b, there is a red shift in the center frequencies of the two bands. The bandwidth in the first band increases first and then decreases, while the bandwidth in the second band gradually increases with the increase of the \(\theta\) value. When \(\theta\) changes from 100\({}^{\circ}\) to 130\({}^{\circ}\), the tuning range of the central frequency in the first band is 12.41 to 9.78 GHz and the related relative bandwidth is 28.87-37.14%, while the central frequency of the second band ranges from 18.27 to 15.11 GHz and the relative bandwidth is 25.29-38.12%. Therefore, the designed polarization converter with high  
Figure 6: (**a**) PCR for various dielectric layer thicknesses. (**b**) PCR and bandwidth characteristics for the two frequency ranges with various dielectric layer thicknesses. Figure 7: Reflection coefficients of co-polarization for various dielectric layer thicknesses. polarization conversion efficiency can be adjusted in a wide range, which can greatly materialize the high flexibility and selection for controlling the polarization state of EM waves.