###### Abstract  
A compact Ka-band antenna array has been proposed to realize broadband and high gain for millimeter-wave applications. The antenna array is divided into a multilayer composed of a driven slot patch layer and a parasitic patch array layer, which is excited by a mixed CPW-Slot-Couple feeding network layer. According to characteristic mode analysis, a pair of narrow coupling slots are introduced in the driven patch to move the resonant frequency of characteristic mode 3 to the resonant frequency of characteristic mode 2 for enhanced bandwidth. In this article, a 1 to4 CPW-Slot-Couple feeding network for a 2 \(\times\) 2 driven slot patch array is implemented, and then each driven slot patch excites a 2 \(\times\) 2 parasitic patch array. Finally, a proposed 4 \(\times\) 4 \(\times\) 3 (row \(\times\) column \(\times\) layer) Ka-band antenna array is fabricated to verify the design concepts. 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. Due to its attractive properties of miniaturization, broadband, and high gain, the proposed antenna array could be applied to millimeter-wave wireless communication systems.  
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and the rectangular slot is used to reduce the TM\({}_{12}\) mode resonant frequency, so that the two modes are close to each other, and the final antenna frequency bandwidth is extended to 9% and the peak gain is 6.5 dBi. Although the above technique can make multiple modes radiate together, due to the difficulty of impedance matching, the antenna radiation efficiency is low, which limits its practical application.  
As for multilayer technology, its essence involves increasing the antenna profile height and forming multiple resonators to improve the radiation performance [10; 11; 12; 13]. Ref. [14] proposed an 8 \(\times\) 8 dual-polarized millimeter-wave antenna array, which adopts a two-layer SIW feeding network to excite multiple resonant modes to expand the frequency bandwidth; the measured results have a bandwidth of 14.6% (36.8-42.6 GHz) and a peak gain of 25.8 dBi. Ref. [15] proposed a 4 \(\times\) 4 CP patch antenna array, which adopts a double-layer substrate; due to the increase in profile height, the bandwidth is extended. The measured results show that the bandwidth is 16% (41.9-49.1 GHz) and the array gain is greater than 17 dBi. In addition, multilayer antennas can realize some specific radiation requirements, such as circular polarized, multi-polarized, multi-beam, etc. However, conventional multilayer antennas rely on a complex feeding network, resulting in low efficiency [16].  
In this paper, a novel multilayer patch antenna array based on a mixed CPW-Slot-Couple (CSC) feeding network to realize a wide bandwidth and high gain for Ka-band applications is presented, as shown in Figure 1. The proposed antenna array is divided into three layers, which are the feeding network layer, the driven slot patch antenna layer, and the parasitic patch array layer from the bottom up. According to characteristic mode analysis, a square patch with a pair of coupling slots is stacked together as a driven patch; then, a 1 to4 CSC feeding network for a 2 \(\times\) 2 driven slot patch array is implemented, and each driven slot patch excites a 2 \(\times\) 2 parasitic patch array. Finally, a proposed 4 \(\times\) 4 \(\times\) 3 (row \(\times\) column \(\times\) layer) antenna array is fabricated and measured to verify the design concepts. 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 was 20.1 dBi.