# Design of a 3-Bit Circularly Polarized Reconfigurable Reflectarray
## 2 Materials and Methods
### Design of Circularly Polarized Reconfigurable Reflectarray Unit
The parameter of the circular polarization reflection unit is \(P=37\) mm, \(h_{1}=3\) mm, \(h_{2}=1.5\) mm, \(r_{1}=11.4\) mm, \(l=1.4\) mm, \(l_{1}=4.1\) mm, \(sl=0.15\) mm, \(wl=0.2\) mm, \(a_{2}=15^{\circ}\), \(a_{3}=223^{\circ}\), \(a_{4}=57^{\circ}\), \(a_{5}=217^{\circ}\), \(a_{6}=99^{\circ}\), \(a_{7}=319^{\circ}\), and \(a_{8}=143^{\circ}\), where a\({}_{2}\)-a\({}_{8}\) represents the angle between the loading direction of the PIN diodes 2-8 and the PIN diode 1. According to the analysis of the working principle of the unit, the theoretical value is a multiple of \(22.5^{\circ}\). The design idea of the reflecting unit is analyzed as follows: the radiation structure of the unit is rotated by \(22.5^{\circ}\) in turn, and then eight working states of increasing or decreasing reflection phase with an interval of \(45^{\circ}\) can be realized. Then, 8 reflection phase delay lines spaced \(22.5^{\circ}\) are placed around the circular patch, and 8 PIN diodes are connected to it. At the same time, only one PIN diode is switched on, and when the PIN diode is switched on at different positions, the unit structure is equivalent to rotation, so the phase shift effect can be obtained. Furthermore, in order to have enough space to add a bias circuit to each PIN diode and make the phase delay line more evenly distributed on the reflecting unit, the PIN diode and phase delay line are moved to a symmetrical position relative to the center of the circular patch, which is equivalent to rotating this part of the phase delay structure by \(180^{\circ}\) with no change in the phase-shifting properties. The PIN diodes in the circular polarization reconfigurable reflecting unit are all SMP 1340-040 LF. A three-dimensional model of the unit is established in HFSS, and the equivalent circuit model of the PIN diode, as shown in Figure 2, is introduced to it. The working characteristic diagram of the unit, as shown in Figure 5, can be obtained by changing the working state of the eight PIN diodes, and the eight states in the figure, respectively, indicate that the corresponding PIN diode in Figure 4 is in the on-state. The unit can realize 8 kinds of reflection phase adjustment with an interval of \(45^{\circ}\) in the frequency band of 3.37-3.66 GHz for both left and right circularly polarized incident electromagnetic waves. In addition, the reflection amplitude of the unit to the homodromous circular polarization is less than \(-1\) dB, which proves that it has a good low-loss characteristic. Figure 4: Model of the 3-bit circularly polarized reconfigurable reflecting unit: (**a**) overall structure and (**b**) lateral structure. In addition, this paper also analyzes the sensitivity of the unit to the oblique incidence angle. Figure 6 shows the changes in the unit's operating performance under two orthogonal incidence planes and different oblique incidence angles. It can be seen that, in the two incidence planes, the reflection phase of the unit under 8 operating states almost does not change within the incidence angle of 50\({}^{\circ}\). Although the reflection amplitude decreases, it is always within \(-2\) dB, so the stability of the unit to the incidence angle is very good.