# Design of a 3-Bit Circularly Polarized Reconfigurable Reflectarray
## 3 Fabrication and Measurement Results  
In this study, the axial mode spiral antenna was selected as the feed source. The axial mode spiral antenna is an end-fire antenna with wider bandwidth and better rotation symmetry of the directional pattern. Compared with the horn antenna, its design method is simpler, and the processing cost is lower, so it is very suitable for a circularly polarized feed antenna. The dimensions of the spiral antenna are determined based on the required gain and bandwidth. After designing the antenna, the diameter of the aperture surface of the spiral antenna is 22.5 mm, and the final sample is shown in Figure 10a. Three-dimensional printing material was used as the support structure of the helix, and its performance was finally tested in the far-field test darkroom, as shown in Figure 10b. All samples in this work were measured by Ceyear 3672E Vector Network Analyzer. Figure 10c shows the comparison of simulation and measurement results of S-parameters and axis ratio of the circularly polarized spiral antenna. It can be seen that, in the frequency band of 2.5-4.5 GHz, the S\({}_{11}\) of the antenna is less than \(-10\) dB, the axis ratio is less than 3 dB, and the bandwidth is much higher than the requirements of the designed 3-bit circularly polarized, reconfigurable reflectarray. Figure 10d shows the far-field radiation pattern of the antenna. It can be seen that the antenna has good symmetry on both the xoz plane and yoz plane, the shape of the pattern on the two planes is close, the sidelobe level and cross-polarization level are low, and the maximum gain reaches 11.15 dB. The simulation results are in good agreement with the measurement results. The radiation pattern is fitted  
Figure 9: DC control circuit diagram of PIN diode. with the \(\cos^{\theta_{f}}\theta_{f}\) mathematical model, and the value of \(q_{f}\) is 4, which is suitable for use as a feed antenna. In order to evaluate the performance of the 3-bit circularly polarized reconfigurable reflectarray, as shown in Figure 8, a prototype was fabricated and tested. The sample of the reflectarray is shown in Figure 11. The reflectarray contains 64 units, and the overall size of the reflectarray is 333 mm \(\times\) 350 mm. Before the test, it is necessary to debug the DC control system of the reflectarray and check the components of the reflectarray. The debugging scenario is shown in Figure 11a. The working state of the unit can be judged by the luminous state of the LED (light-emitting diode). When the LED is bright, it means that the PIN diode connected to it is in a conduction state. The FPGA is mounted on the back of the antenna bracket during assembly to reduce the impact on the performance of the reflectarray. Subsequently, the designed reflectarray was measured in the far-field darkroom, and the test scene is shown in Figure 11b. Figure 10: Right-handed circularly polarized spiral antenna: (**a**) sample, (**b**) test scene, (**c**) reflection coefficient and axial ratio, and (**d**) radiation pattern. Firstly, the performance of the reflectarray when the beam direction is 0\({}^{\circ}\) was measured, and the comparison of the simulation result and the measurement results is shown in Figure 12a,b. It can be seen that the simulation results are in good agreement with the measurement results, the sidelobe level is less than \(-13\) dB in the two orthogonal planes, and the cross-polarization component is much smaller than the main polarization component. Figure 12c shows the measurement results of the antenna's axis ratio and gain. It can be seen that the maximum gain in the simulation results is 16.8 dB, and the aperture efficiency is 32.2%. The measured results show that the maximum gain of 16 dB can be achieved in the bandwidth of 3.43-3.71 GHz, and the aperture efficiency is 27%. In addition, the axis ratio is less than 3 dB, the gain loss is less than 3 dB, and the total working bandwidth is 8%.