## 5 Conclusions  
In this paper, a novel compact Ka-band multilayer patch antenna array based on a mixed CPW-Slot-Couple feeding network is proposed. The array is divided into three metal layers, and from the bottom up, the first metal layer is a CSC feeding network for the second metal layer of the driven slot patch array. At the top is the parasitic layer, which is couple-fed by a driven layer to realize a stable radiation pattern. Then, a \(4\times 4\times 3\) (row \(\times\) column \(\times\) layer) multilayer antenna array is fabricated to verify the design concepts, exhibiting good agreement between the simulated and measured results. Finally, the measured results show that the frequency bandwidth of the antenna array is 25 GHz to 32 GHz, and the relative bandwidth is 24.5%. The peak gain is 20.1 dBi. Compared with other reported millimeter-wave antenna arrays, the proposed antenna has attractive properties of miniaturization, broadband, high gain, and easy fabrication and integration. This paper has several limitations, which should be improved in future research: First, the analysis of the effect of antenna directivity on gain is insufficient, which is inaccurate for the reflection coefficient and radiation efficiency. Radiation field analysis at specific frequencies should be considered. Secondly, multilayer dielectric loss and coupling feed loss are the main causes of low radiation efficiency; they should be taken into account in the theoretical model. Conceptualization, K.D. ; Methodology, K.D., N.Z. and R.S. ; Formal analysis, K.D., G.Y. and N.L. ; Investigation, K.D., N.Z. and Y.L. ; Resources, Y.L. ; Data curation, G.Y., Y.L. and N.L. ; Writing--original draft, K.D. ; Writing--review & editing, N.Z., G.Y., Y.L., R.S. and N.L. ; \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Frequency** & **25 GHz** & **26 GHz** & **27 GHz** & **28 GHz** & **29 GHz** & **30 GHz** & **31 GHz** & **32 GHz** \\ \hline Gain (dB) & 16.7 & 18.3 & 19.6 & 20.1 & 19.4 & 18.8 & 17.9 & 17.5 \\ Efficiency & 31\% & 42\% & 56\% & 58\% & 51\% & 47\% & 38\% & 35\% \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The measured gains of the antenna array. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Ref.** & **Technical Method** & **Size (\(\lambda\))** & **Array Scale** & \begin{tabular}{c} **Freq.** \\ **(GHz)** \\ \end{tabular} & \begin{tabular}{c} **BW** \\ **(\%)** \\ \end{tabular} & \
\begin{tabular}{c} **Peak Gain** \\ **(dB)** \\ \end{tabular} &
\begin{tabular}{c} **Efficiency** \\ **RAD. (\%)** \\ \end{tabular} \\ \hline
[19] & Parasitic patches + SIW & \(6.8\times 4.5\times 0.21\) & \(4\times 4\) & 28 & 18.6 & 18.2 & 65 \\
[20] & Rotary feed + SIW & \(4.8\times 4.8\times 0.49\) & \(4\times 4\) & 30 & 14.1 & 19.5 & 77 \\
[21] & Spiral antenna + SIW & \(2.4\times 2.0\times 0.4\) & \(4\times 4\) & 30 & 22 & 15.2 & 54.9 \\
[22] & Electromagnetic dipole & \(6.8\times 6.1\times 0.49\) & \(8\times 8\) & 30 & 18.2 & 26.1 & 75 \\
[23] & Slot antenna + CPW & \(12\times 12.7\times 0.14\) & \(16\times 16\) & 20 & 15.9 & 25.9 & – \\
[24] & Resonant Cavity & \(5.57\times 8.57\times 2.36\) & \(2\times 2\) & 25 & 15.15 & 7.69 & 62 \\
[25] & Short probe + SIW & \(3.6\times 3.6\times 0.508\) & 1 & 28 & 3.4 & 2.3 & 82 \\
[26] & Slot + SIW & \(4.48\times 4.48\times 2.575\) & \(4\times 4\) & 30 & 44.62 & 7.25 & – \\
[27] & SIGW + Air cavity-backed & \(3.8\times 3.8\times 2.361\) & \(2\times 2\) & 26 & 19 & 5.51 & 65 \\
[12] & ESIW + Slot & \(2.6\times 4\times 1.016\) & \(2\times 2\) & 28.25 & 2.4 & 12 & 85 \\ This paper & Multilayer antenna + CSC & \(4\times 4\times 0.1016\) & \(4\times 4\) & 30 & 24.5 & 20.1 & 58 \\ feeding network & & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Performance comparison between the proposed and other reported millimeter-wave antennas. Supervision, R.S. and N.L. ; Project administration, R.S. ; Funding acquisition, N.Z. All authors have read and agreed to the published version of the manuscript.