## 3 Design of AcPAS
### Design of the Septum Polarizer  
The performance of the septum polarizer determines the CP performance of the antenna [42; 43]. It achieves circular polarization by inserting a stepped metal septum into a square waveguide. Figure 2 shows the ANSYS HFSS simulation model of a common structure of the septum polarizer, where Port 1 and Port 2 are input ports, which are rectangular waveguides excited by a coax, and Port 3 is the square waveguide output port.  
The TE\({}_{10}\) mode input from the rectangular waveguide port is converted into two orthogonal TE\({}_{10}\) and TE\({}_{01}\) modes in the square waveguide after passing through the  
Figure 1: Antenna structure of AcPAS.  
Figure 2: Septum polarizer.  
septum. For the TE\({}_{10}\) mode, the septum has no effect on its propagation constant. For the TE\({}_{01}\) mode, the septum is similar to a fin-line or ridge waveguide, which introduces phase delay. By adjusting the parameters of each step of the septum, the phase difference between the two orthogonal modes is made to be 90\({}^{\circ}\), thereby forming a CP wave at the output port. The field pattern structure of the polarizer is shown in Figure 3. As can be seen from the field pattern structure of the septum polarizer, feeding Port 1 or Port 2 individually can form left-hand circularly polarized (LHCP) waves or right-hand circularly polarized (RHCP) waves at the output ports, respectively. Therefore, the adoption of septum polarizer can achieve dual-CP. In addition, feeding Port 1 and Port 2 with equal amplitude and in-phase or reverse-phase will form linearly polarized (LP) waves at the output ports.  
The key technical specification in the design of the septum polarizer lies in its CP axial ratio (AR), which is mainly influenced by two factors: one is the amplitude difference between the two modes, and the other is the phase difference deviating from 90\({}^{\circ}\) between the two modes. From the perspective of the S-parameters, the septum polarizer can be regarded as a four-port network. Define the port parallel to the septum as Port 3 and the port perpendicular to the septum as Port 4. The AR caused by the amplitude difference is  
\[AR_{1}=20 lg|\frac{S_{13}}{S_{14}}|=10 lg(1-|S_{33}|^{2}) \tag{1}\]  
The AR produced by the phase difference deviating from 90\({}^{\circ}\) is  
\[AR_{2}=20 lg|\frac{S_{11}+S_{14}}{S_{13}-S_{14}}| \tag{2}\]  
where \(AR_{1}\) is the AR generated by the amplitude difference, \(AR_{2}\) is the AR produced by the phase difference deviating from 90\({}^{\circ}\), \(S_{13}\) is the transmission coefficient from Port 3 to Port 1, \(S_{14}\) is the transmission coefficient from Port 4 to Port 1, and \(S_{33}\) is the reflection coefficient at Port 3.  
Then, it can be considered that the AR of the circular polarizer is \(AR=AR_{1}+AR_{2}(dB)\). Therefore, to reduce the AR of the septum polarizer, it is necessary to optimize the design of the steps to make the amplitudes of the two modes as equal as possible and the phase difference as close to 90\({}^{\circ}\) as possible.  
Another key technical specification of the septum polarizer is the isolation between input ports, which is mainly caused by the standing wave of signals parallel to the septum. By optimizing the design of the steps, the standing wave of signals parallel to the septum  
Figure 3: Field pattern structure of the septum polarizer.  
can be reduced, thereby improving the isolation between the two senses of rotation of the septum polarizer.