# A Low-Profile Wideband Linear-to-Circular Polarization Conversion Slot Antenna Using Metasurface
## 4 Measured Results Analysis  
To further verify the impedance, axial ratio, and radiation performance of the proposed design, the metasurface antenna was fabricated using a common printed circuit board (PCB) fabrication approach, as shown in Figure 7. Figure 8 illustrates the impedance bandwidth and 3 dB axial ratio bandwidth obtained by simulation and measurement in the broadband range. The simulated results showed that the S\({}_{11}\) bandwidth and 3 dB ARBW of the proposed metasurface antenna were 42.94% from 4.28 GHz to 6.62 GHz and 18.83% from 5.1 GHz to 6.16 GHz, respectively. The fabricated antenna had a \(-\)10 dB impedance bandwidth of 39.25% (4.28-6.37 GHz) and a 3 dB axial ratio bandwidth of 17.77% (5.18-6.19 GHz). The small variation between the measured and simulated results probably comes from fabrication tolerances and limitations of the measured environment.  
Figure 6: Axial ratio (AR) and S\({}_{11}\) characteristics of the proposed metasurface antenna with different parameters: (**a**) \(a\); (**b**) \(w\); (**c**) h\({}_{1}\); (**d**) p.  
Figure 7: Photographs of the fabricated metasurface antenna: (**a**) top view; (**b**) rear view.  
Figure 9 illustrates the gain and efficiency characteristics of the proposed antenna obtained by simulation and measurement. Over the bandwidth ranging from 5 GHz to 6 GHz, the measured gain and radiation efficiency fluctuated slightly around 6.8 dB and 88%, respectively. The slight differences between measured and simulated results may come from the increased actual dielectric constant of the substrate, the fabrication, or assemble tolerance of the antenna.  
Figure 10 shows the simulated and measured radiation patterns of the xoz-plane and yoz-plane at 5.5 GHz. It can be observed that the measured and simulated results basically agree with each other. The proposed antenna has a low LHCP in both the xoz-plane and the yoz-plane. The measured LHCP level was \(<\)-15 dB in the xoz-plane and \(<\)-10 dB in the yoz-plane. The measured RHCP level was approximately from \(-\)8 dB to 6 dB in the xoz-plane and approximately from \(-\)11 dB to 7 dB in the yoz-plane. The RHCP level was higher than the LHCP level, which means that the proposed antenna is a right-hand circular polarized antenna.  
Table 2 summarizes the key performance indicators of recently reported CP antennas based on metasurfaces for comparison. It can be seen that both the S\({}_{11}\) bandwidth and ARBW of the proposed CP metasurface antenna are much wider than those of the designs in [8; 9; 10; 11; 20]. Moreover, the size of the proposed antenna is much smaller in comparison. Furthermore, the size of the proposed antenna is a little larger than those of the designs in [5; 7; 12; 13; 14], but with a wider S\({}_{11}\) bandwidth and  
Figure 8: Simulated and measured characteristics of the proposed antenna: (**a**) S\({}_{11}\); (**b**) AR.  
Figure 9: Simulated and measured gain and efficiency of the proposed antenna.  
ARBW. Although the ARBWs in [15; 16] are a little wider than that of the proposed antenna, their S\({}_{11}\) bandwidths and overall sizes are worse than those of the proposed antenna.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
\begin{tabular}{c} large size, narrow S\({}_{11}\) bandwidth, \\ and wide ARBW \\ \end{tabular} \\ \hline
**Proposed** & \(0.65\times 0.65\times 0.06\) & 5.5 & 39.25 & 17.77 &
\begin{tabular}{c} small size, wide S\({}_{11}\) bandwidth, \\ and wide ARBW \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table}
Table 2: Performance comparison of recently reported CP antennas based on metasurfaces.  
Figure 10: Comparison of simulated and measured radiation patterns at 5.5 GHz: **(a)** xoz-plane; **(b)** yoz-plane.