## 4 Conclusions  
In this paper, a 3D-printed, GW-based, dual-band, dual-CP antenna array is presented. The K-band array is stacked above the Ka-band array to share the antenna aperture, resulting in an efficiently packaged, compact antenna structure. The RHCP and LHCP waves are excited in cylindrical cavities with chamfered edges. The use of GW not only results in high antenna efficiency at the K- and Ka-bands, but also decreases the sensitivity of the antenna performance to fabrication tolerances, which allowed us to leverage low-cost prototyping techniques. The antenna prototype consisting of 4 \(\times\) 4 radiating elements was fabricated using SLA 3D printing technology in conjunction with metallization and surface polishing processes, which greatly decreases the cost and weight compared to conventional CNC technology. The fabricated prototype exhibited gains of about 20 dB in both operating bands. Moreover, the design architecture can be seamlessly scaled to build a larger array without needing any additional layers. Owing to its high gain, high efficiency, scalability, low cost and lightweight characteristics, the proposed antenna design is a potential solution for commercial millimeter wave communication.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Reference** & **Band** & **Polarization** & **Gain (dB)** & **Efficiency (\%)** & **Technology** &
\begin{tabular}{c} **Aperture Size** \\ **(\(\mathbf{\Omega_{L}\times\lambda_{L}}\))** \\ \end{tabular} \\ \hline
[7] & X/Ku & Dual-CP & 17.5/18.3 & – & PCB & 28.4 \(\times\) 25.6 \\
[8] & K/Ka & Dual-CP & 13.2/11.5 & 78/87 & PCB and GW & 1.53 \(\times\) 1.53 \\
[9] & X/Ku & Dual-CP & 16.1/15.2 & – & PCB & 2.9 \(\times\) 1.7 \\
[19] & Ka & Single-CP & 22.0 & 98 & CNC and GW & 5.12 \(\times\) 5.12 \\
[20] & Ka & Dual-LP & 27.5 & 75 & CNC and GW & 10 \(\times\) 10 \\
[21] & Ka & Dual-CP & 27.0 & 75 & CNC and GW & 9.7 \(\times\) 9.7 \\
[22] & K/Ka & Dual-LP & 25.8/29.5 & 84/88 & CNC and GW & 6.3 \(\times\) 6.3 \\ This work & K/Ka & Dual-CP & 20.1/20.3 & 92.1/91.5 & SLA and GW & 2.6 \(\times\) 2.6 \\ \hline \hline \end{tabular} \(\lambda_{L}\) denotes the free-space wavelength referring to the lowest frequency of the corresponding antenna; LP denotes linear polarization.  
\end{table}
Table 4: Comparison with some relevant reported work.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Frequency (GHz)** & **20.25** & **20.5** & **20.75** & **28** & **28.5** & **29** \\ \hline Polarization & RHCP & RHCP & RHCP & LHCP & LHCP & LHCP \\ Sim. gain (dB) & 21.1 & 21.1 & 20.8 & 20.5 & 20.8 & 21.2 \\ Mea. gain (dB) & 20.0 & 20.1 & 19.8 & 19.9 & 20.3 & 20.2 \\ Efficiency (\%) & 92.1 & 91.5 & 88.9 & 91.3 & 91.5 & 89.1 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Gain and efficiency of the proposed antenna.  
**Author Contributions:** Conceptualization, S.W.; Methodology, S.W.; Project administration, J.L.; Software, S.W.; Supervision, J.L., S.Y. and K.X.; Writing--original draft, S.W.; Writing--review & editing, J.L., Y.C. and H.L. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This work was fund by the National Key R&D Program of China under Grant 2021 YFA0717500, the National Natural Science Foundation of China under Grant 62133013, and the Fundamental Research Funds for the Central Universities.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** All data are available from the author.  
**Conflicts of Interest:** The authors declare that they have no conflict of interest to report regarding the present study.