# A Highly Integrated Millimeter-Wave Circularly Polarized Wide-Angle Scanning Antenna Unit
## 2 Design of a Circularly Polarized Wide-Angle Scanning Antenna Unit  
### Antenna Unit Structure Design  
A proposed antenna unit for circularly polarized wide-angle scanning phased array antennas in the millimeter-wave frequency range is depicted in Figure 1. The entire antenna unit utilizes SIW square waveguides and includes radiating square waveguides, circular polarizers, filters, and matching loads. The square SIW (depicted in yellow in Figure 1), which accommodates the circular polarizer (shown in orange in Figure 1), is situated in the upper portion of the antenna unit and is partitioned into two equal segments by the circular polarizer. Two flat SIWs (represented by blue and green sections in Figure 1) are positioned adjacently beneath the square SIW to supply the left and right segments of the square SIW, respectively. Various flat waveguide feeds enable distinct circular polarizations, facilitating the use of the same antenna configuration for both transmitting and receiving arrays. Moreover, matching loads are integrated at the terminations of the flat SIWs to mitigate cross-polarization effects, where no power feed is present. Figure 2 illustrates the structure of the radiating square waveguide incorporating the circular polarizer. The metal structure of the waveguide is shown on the right side of the figure. The square radiating waveguide is divided into two sections filled with dielectric material with a relative permittivity of 2.94 and a loss tangent of 0.0025. The  
Figure 1: Schematic diagram of the SIW square waveguide. dielectric substrate material, with a relative permittivity of 3.28 and a loss tangent of 0.0024, surrounds this structure. The circular polarizer is positioned centrally within the square waveguide, forming left and right circularly polarized antenna radiating units using the SIW structure on each side. In an array configuration, the metal blocks on both sides can function as the antenna's reflective metal plate. In order to achieve better performance, the dimensional parameters of the radiating square waveguide structure are optimized. First, we optimize each individual component and make it have a good performance, at which point we can obtain the appropriate range of values for each parameter. Then, we optimize the overall structure and determine the value of each parameter. The optimized dimensional parameters are shown in Table 1, where the radius of the metal cylinder is r and the length is l.  
To improve the antenna's ability to suppress out-of-band signals, a bandpass filter is incorporated into the feeding flat SIW of the antenna unit, illustrated in Figure 3a. This filtering capability is achieved by integrating metal vias into the SIW structure, ensuring seamless integration without the need for standalone filters. This approach enhances overall system integration and supports a more compact design. The field distribution of the SIW filter structure is shown in Figure 3b. It can be seen that the vias effectively limit the radiation of electromagnetic waves to the external space, which enables the SIW filters to be integrated with various passive and active devices without the need for additional connector conversion. To mitigate cross-polarization caused by reradiated waves, a matching load is integrated at the termination of the flat SIW without a power feed. This load absorbs electromagnetic waves reflected by the antenna. Figure 4 illustrates the structural configuration of the matching load. The buried resistor is depicted in red in the figure. The remaining blue part in the figure is the SIW structure. When electromagnetic waves enter \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**a** & **b** & **c** & **d** & **e** & **f** & **g** & **h** & **r** & **l** \\ \hline
7.00 & 12.65 & 5.90 & 5.95 & 8.95 & 0.80 & 0.55 & 1.62 & 0.24 & 4.62 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Dimensions of the radiating square waveguide structure (mm). Figure 3: (**a**) Model of the SIW filter structure.