## 3 Simulation, Experiment and Discussion  
In order to investigate the polarization conversion characteristics of the metasurface, we define \(r_{xx}=\left|\,E_{xt}\,\right|/\left|\,E_{yi}\,\right|\) and \(r_{yx}=\left|\,E_{yt}\,\right|/\left|\,E_{xi}\,\right|\) as the reflection amplitudes of the co-polarization and cross-polarization of \(x\)-polarized waves, respectively. Meanwhile, PCR = \(\left|\,r_{yx}\,\right|^{2}/(\left|\,r_{xx}\,\right|^{2}+\left|\,r_{yx}\,\right|^ {2})\) is defined to describe the polarization conversion efficiency. Figure 4a,b shows the corresponding simulated results. The summarized performances of the polarizer are presented in Table 1. Moreover, the PCR is above 87% at 14.37-24.75 GHz when the oblique incidence angle is 0-30\({}^{\circ}\), indicating that the vertically incident \(x\)-polarized waves are effectively reflected as \(y\)-polarized waves.  
To further validate our design, a sample of the reflective polarizer was fabricated and measured in a microwave anechoic chamber. The polarizer contains 40 \(\times\) 40 cells with a total size of 240 \(\times\) 240 mm\({}^{2}\). Figure 4c presents a partial photograph of the sample. The polarizer was placed on a test bed and normalized using metal plates of the same size. Two horn antennas were connected to the vector network analyzer (R&S ZNB40), one as the transmitting antenna and the other as the receiving antenna, as illustrated in Figure 4d. Figure 4a,b shows the test results. As can be seen from Figure 4a, the co-polarized and cross-polarized reflectivities coincide with the simulated results, but the drops at 20.82 GHz and 26.94 GHz are almost merged, which may be due to fabrication or experimental errors. The corresponding PCR stays above 95% at 15.3-27.5 GHz, proving that the polarizer designed in this paper has an excellent linear polarization capability.  
The surface current distributions at the resonance points of 15.45 GHz, 20.82 GHz and 26.94 GHz were monitored to explain the physical mechanism of the polarizer. The direction of the arrow in the figure indicates the direction of the surface current distribution.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Performance**} & **Operating** & **Co-Polarized** & **Cross-** & **Fractional** & **PCR** \\  & **Bandwidth** & **Reflectance** & **Reflectance** & **Bandwidth** & **PCR** \\ \hline values & 14.8–28.0 GHz & \textgreater{}0.9 & \textless{}0.2 & 61.7\% & \textgreater{}0.95 \\ \hline \hline \end{tabular}
\end{table}
Table 1: The performances of the polarizer.  
Figure 4: Simulated and experimental results for (**a**) reflectance, and (**b**) PCR, and images of (**c**) sample and (**d**) test environment and system.  
Figure 5a,b show the current distributions in the upper and lower layers at resonant frequencies of 15.45 GHz and 20.82 GHz, respectively. We can observe that the surface currents in the top and bottom layers are inversely parallel, which means that the magnetic resonance is generated at 15.4 GHz and 20.82 GHz. In contrast, the surface currents shown in Figure 5c are parallel to the same direction, indicating an electrical resonance at 26.94 GHz. Due to the presence of multiple resonances, an efficient linear polarization conversion is achieved in the ultra-wideband range.