# Design of a 3-Bit Circularly Polarized Reconfigurable Reflectarray
## 2 Materials and Methods
### Design of Circularly Polarized Reconfigurable Reflectarray Unit  
The phase compensation method used in this study was the rotating unit structure loaded with the phase delay line. When the direction of loading phase delay line differs 180\({}^{\circ}\) from the reflection phase of another orthogonal direction, the reflection phase control of circular polarization can be realized by rotating the element structure. Figure 3a shows the structure of the homodromous circularly polarized reconfigurable reflectarray unit, whose radiation structure consists of a circular metal patch, a phase delay line, and a PIN diode. The length of the delay line is used to adjust the reflection phase of the unit [36], and the width of the delay line is used to adjust the impedance matching between the delay line and the circular metal patch [37]. When the PIN diode is switched on, if the length of the delay line is about a quarter wavelength, a reasonable design of the delay line width can make the unit reflect the circularly polarized electromagnetic wave [38]. First, the unit's reflection on x and y polarized incident electromagnetic waves is simulated, and the results are shown in Figure 3a. It can be seen that the amplitude of Sxx and Syy is close, the reflection loss is small, and the reflection phase difference is about 180\({}^{\circ}\) within a certain bandwidth. Subsequently, the circularly polarized incident wave is simulated, and the results are shown in Figure 3b, where \(|\)SRR\(|\) or \(|\)SLL\(|\) indicates the homodromous reflection amplitude of right-handed or left-handed circularly polarized wave, respectively; and \(|\)SLR\(|\) or \(|\)SRL\(|\) indicates the antidromous reflection amplitude of right-handed or left-handed circularly polarized wave. The two minima of \(|\)SLR\(|\) or \(|\)SRL\(|\) are due to the fact that the phase difference between the Sxx and Syy is closest to 180\({}^{\circ}\). In addition, according to the bandwidth determination method of the circularly polarized reconfigurable reflectarray unit, when the value of \(|\)SLR\(|\) or \(|\)SRL\(|\) is lower than \(|\)SRR\(|\) or \(|\)SLL\(|\) by more than 15 dB, it can be considered that the axial ratio of reflected electromagnetic waves is less than 3 dB. Therefore, the unit can achieve the homodromous circularly polarized reflection performance within a certain bandwidth. Based on the homodromous circularly polarized reconfigurable reflectarray unit introduced above, the 3-bit circularly polarized reconfigurable reflectarray unit was designed by the method of rotating element structure and is shown in Figure 4. The reflection unit consists of three metal layers and two dielectric substrates. The functions of the metal layer from top to bottom are as follows: receive and radiate electromag  
Figure 3: The circularly polarized reflecting unit. (**a**) Simulation results of S parameters. (**b**) Reflection characteristics of circular polarization. Figure 2: The equivalent circuit models of PIN diode (1–6 GHz): (**a**) on-state and (**b**) off-state. DC bias circuit wiring, where the material of the upper dielectric substrate is F4B, with a dielectric constant of 2.65 and loss tangent of 0.002; and the material of the lower substrate is FR4, with a dielectric constant of 4.6 and loss tangent of 0.02. The reflection unit is loaded with 8 phase delay lines and 8 PIN diodes, the negative electrode of each diode is connected to the circular patch, and the positive electrode is connected to 8 phase delay lines with the same length and shape, respectively. The DC bias circuit of the PIN diode is composed of a microstrip spiral inductor, a metalized through-hole connected to the bottom DC line and a DC wiring on the bottom.