# Circularly Polarized Vivaldi Antennas Integrated with Septum-like Polarizer
## 2 Materials and Methods
### CP Vivaldi Antenna with Feeding Network  
In practical applications, a single port design is preferred for system compatibility. The CP Vivaldi antenna with a single port can be easily implemented by adding a simple T-junction power divider. Figure 7 shows the structure of the proposed CP Vivaldi antenna with a single port combined feeding network instead of a dual port. Note that, except for the feeding network, the other structures are the same as in Figures 2 and 3.  
Figure 8 shows the detailed structure of the feeding network. The 50 \(\Omega\) port is connected through a microstrip line with a characteristic impedance of 50 \(\Omega\) to two microstrip lines with a characteristic impedance of 100 \(\Omega\), and finally to the feeding position of the Vivaldi antennas with a local impedance of 100 \(\Omega\). A conducting via with a diameter of 1 mm is used to connect two orthogonal planes, where the influence of the conducting viaduct can be neglected due to the short path. Meanwhile, operation modes with RHCP or LHCP can be selected by setting the path difference of the left and right microstrip lines to be in-phase or out-of-phase. Figure 8a,b show the design for both RHCP and LHCP designs. The path length of the right microstrip line with a characteristic impedance of 100 \(\Omega\) is kept at 37.4 mm (=2\(\lambda_{\text{g}}\)), while the path length of the left microstrip line with a characteristic impedance of 100 \(\Omega\) is specified to 56.9 mm (=3\(\lambda_{\text{g}}\)) and 46.7 mm (=2.5\(\lambda_{\text{g}}\)) for the RHCP and LHCP designs, respectively.  
Figure 8: Structures of feeding networks for (**a**) RHCP and (**b**) LHCP designs.  
Figure 7: Proposed CP Vivaldi antenna combined with feeding network. (**a**) Perspective view, (**b**) top and (**c**) bottom view of the Vivaldi antenna in the \(xz\) plane, (**d**) top and (**e**) bottom view of the Vivaldi antenna in the \(yz\) plane.  
Figures 9 and 10 shows the reflection coefficient, AR, co- and cross-polarized gain, and radiation pattern of the CP Vivaldi with the feeding network. The simulated \(-\)10 dB bandwidth of the reflection coefficients S\({}_{11}\) is 1.15 GHz (=10.7%) for RHCP and 1.27 GHz (=11.4%) for LHCP, the simulated 3 dB AR bandwidth is 1.03 GHz (=9.2%) for RHCP and 1.36 GHz (=12.3%) for LHCP, the simulated peak gain is 6.2 dB for RHCP and 7.5 dB for LHCP, and the simulated \(-\)1 dB gain bandwidth is 1.46 GHz (=13.2%) for RHCP and 0.78 GHz (=7.2%) for LHCP, respectively. The discrepancies from the results of the CP Vivaldi antenna with dual ports are mainly due to the asymmetry of the T-junction power divider, microstrip line losses, and the curved microstrip line paths, etc.  
Figure 10: Co-polarized radiation patterns for the peak gain of the proposed CP Vivaldi antenna with feeding network: (**a**) LHCP mode and \(xz\) plane, (**b**) LHCP mode and \(yz\) plane, (**c**) RHCP mode and \(xz\) plane, and (**d**) RHCP mode and \(yz\) plane.  
Figure 9: Performance of the proposed CP Vivaldi antenna: (**a**) reflection coefficient, (**b**) axial ratio, (**c**) boresight gain for RHCP, and (**d**) boresight gain for LHCP.