## 4 Conclusions  
In this article a novel SIW cavity backed circularly polarized antenna design concept has been proposed. The radiator of the proposed antenna is a circular-patch, embedded within the SIW rectangular cavity, where the SIW cavity is non-radiating. The circularly polarized radiation has been generated through the excitation of degenerated modes TM\({}_{110}\) within circular-patch, via pair of--orthogonally oriented metallic strips--connecting the periphery of circular-patch to the ground layer of the SIW cavity. The antenna has been designed at 33 GHz band over Rogers RT/Duroid 5880 substrate with dielectric constant of 2.2, through simulation in HFSS-2021 from Ansys corporation. The optimized antenna structure achieved the impedance bandwidth of 9.5% and the circular polarization wave is generated at 34 GHz. The sample of the designed antenna has been manufactured through single layer PCB process and the SIW-to-GCPW transition has been embedded within the antenna PCB: to facilitate the interconnection with test equipment. The conductor and the far-field tests have been conducted on the antenna sample, in anechoic chamber. The test results shows that the antenna have 3 dB axial-ratio bandwidth of 2.3% and the average peak gain of around 8.5 dBi, which concluded that the proposed design method is suitable for compact and the small size low-back lobe circularly-polarized antenna's basic radiator design--can be used in the phased array or in the high gain large antenna array design for satellite or the spaceborne communication.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Reference** & \begin{tabular}{c} **Frequency** \\ **(GHz)** \\ \end{tabular} & \begin{tabular}{c} **Antenna Size** \\ **mm** \\ \end{tabular} & \begin{tabular}{c} **Impedance BW** \\ **\%** \\ \end{tabular} & \begin{tabular}{c} **AR BW** \\ **\%** \\ \end{tabular} & \
\begin{tabular}{c} **Peak Gain** \\ **dBi** \\ \end{tabular} &
\begin{tabular}{c} **Number of** \\ **Substrate Layers** \\ \end{tabular} \\ \hline
[14] & \
\begin{tabular}{c} Dual-Band \\ 28, 38 \\ \end{tabular} &
\begin{tabular}{c} SIW Cavity Size at 28 GHz \\ 0.66 \(\times\) 0.63 \\ \end{tabular} & 7.3, 7.5 & 2.8, 2.6 & 8, 7.9 & 3 \\ \hline
[15] & 29 &
\begin{tabular}{c} SIW Cavity Size \\ 0.92 \(\times\) 0.92 \\ \end{tabular} & 24.37 & 26.14 & 10.53 & 2 \\ \hline
[16] & 8.15 &
\begin{tabular}{c} No Given Data \\ But Large \\ \end{tabular} & 6 & 3 & 6.16 & 1 \\ \hline
[17] & 32 &
\begin{tabular}{c} SIW Cavity Size \\ 0.94 \(\times\) 0.94 \\ \end{tabular} & 27.1 & 16.3 & 9 & 2 \\ \hline This Work & 34 & 0.59 \(\times\) 0.67 & 9.5 & 2.3 & 8.5 & 1 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Comparison with previously reported cp antenna elements.  
Figure 8: Normalized measured (at 33.35 GHz) and simulated (at 34 GHz) radiation pattern of the designed antenna; (**a**) in E-plane, (**b**) in H-plane.  
**Author Contributions:** Conceptualization, P.N.C. and K.X., methodology K.X., Validation X.Z. writing--original draft preparation, P.N.C. and K.X.; Resources, N.H.; Funding Acquisition, N.H. and T.H.; writing--review and editing, K.X., X.Z. and T.H. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This work was supported in part by the National Key Research and Development Program of China under Grant 2020 YFB1804800, the Key Research Project of Zhejiang Laboratory (No. 2020 LC0 AD01), and Natural Science Foundation of Zhejiang Province, China (Grant No. LY23F010006).  
**Data Availability Statement:** Not applicable.  
**Acknowledgments:** The authors would like to sincerely thank the team of Southeast University, Nanjing for allowing the use of test equipment and the anechoic chamber to conduct the far field tests.  
**Conflicts of Interest:** The authors declare no conflict of interest.