# Two-Dimensional Beam Steering Based on Compact Programmable Coding Metasurface
## 3 Experiments and Discussions  
To verify the beam-steering capability of the proposed programmable metasurface, a prototype was fabricated using conventional printed circuit board (PCB) technology. The fabricated metasurface prototype is shown in Figure 6. The total size of the PCB is 6.6 mm \(\times\) 12.2 mm, and size of the compact metasurface is 6.4 mm \(\times\) 6.4 mm in the middle. The bias lines are connected to the designed control circuit board by a pin header.  
The beam deflection capability of the coding metasurface was measured in a standard anechoic chamber environment. As shown in Figure 7, the prototype was fixed at the center of the rotation stage. The transmitting horn antenna in front of the prototype was also fixed at the rotation stage, and the positions of the prototype and the transmitting antenna remained relatively fixed. In addition, the receiving horn was placed 3 m from the metasurface (see Appendix B). After applying the voltage that corresponded to different coding patterns, the EM wave with horizontal polarization was emitted by the horn antenna, deflected by the designed metasurface, and finally collected by the horn antenna at the receiving end, which was connected to the spectrum analyzer.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Scan Angle** & **Gain (dBi)** & **Half-Power Beam Width** & **Beam-Pointing Error** \\ \hline \(10^{\circ}\) & 17.3 & \(9.9^{\circ}\) & \(-1^{\circ}\) \\ \(20^{\circ}\) & 16.2 & \(9.7^{\circ}\) & \(0^{\circ}\) \\ \(30^{\circ}\) & 16.8 & \(10.7^{\circ}\)  
The measured distribution of the reflected beam for different coding patterns in the xoz plane at 28 GHz is shown in Figure 8. Because of the block of the transmitting horn, the steering beam is slightly deformed when the deflection angle is less than 20\({}^{\circ}\). With the increase in deflection angle, the gain of the metasurface decreases, and the half-power beam width increases, which is basically consistent with the design objective. Additionally, the left and right sides of the radiation patterns at some steering angles are not strictly symmetrical, which can be explained by the influence of the coaxial line on the reflected wave.  
To evaluate the broadband performance of the designed metasurface, the prototype was also measured at 29 GHz, and the results are demonstrated in Figure 9. The experimental far-field is close to the results at 28 GHz. Figure 10 shows the measured results of a 30\({}^{\circ}\) steering angle at three frequencies (28 GHz, 29 GHz, and 30 GHz). It can be seen that the gain and the direction of the beams remain in good consistency in the range of 28 GHz-30 GHz, while the side lobe level decreases with increasing frequency. The measured results confirm that the working frequency band of the metasurface can meet the design requirements.  
Figure 8: The measured results of beam steering at 28 GHz in the xoz plane. The measurements are under x\(-\)polarized incident wave.  
Figure 7: The Experimental setup for the characterization of the programmable metasurface performance.  
The beam-steering ability in the yoz plane was also measured. Figure 11 indicates the measured results of beam steering at 28 GHz in the yoz plane. Compared to the performance in the xoz plane, the beams in the yoz plane are obviously worse. In addition to the effect of specular reflection, the fan-shaped bias line is placed in the y-direction, which may affect the steering performance of the metasurface's yoz plane. Moreover, the coupling of the atoms in the y-direction is strong, so the interference between adjacent meta-atoms further reduces the performance of the metasurface.