## 1 Introduction  
Microstrip patch antennas have been widely adopted in various applications such as mobile communications, radar systems, and global navigation satellite systems, considering their advantages of being lightweight, low-profile, and low-cost [1, 2, 3, 4]. These patch antennas consist of a radiator printed on a thin substrate, a feed pin, and a ground, so they typically have narrow bandwidth characteristics [5, 6]. Therefore, increasing the bandwidth of patch antennas has been a primary concern and is a very challenging goal that should be achieved for their use in a variety of applications. In general, methods to improve the bandwidth of a patch antenna include optimizing the radiator shape for high radiated power or optimizing the substrate material to lower the quality factor of the system [7, 8]. To date, there have been many studies conducted to optimize the radiator shape; for example, they include E-shaped patches [9], U-slotted patches [10], and L-shaped patches [11]. Since antenna bandwidth is proportional to the area of the conductive radiator, research on increasing the bandwidth using a stacked dual-patch antenna with a coupled-fed structure has been reported [12, 13]. Various methods for optimizing the substrate material have also been studied, and recently, increasing the bandwidth by bringing the permittivity of the substrate closer to 1 was attempted [5, 10]. By reducing the permittivity of the substrate, the quality factor of the system can be decreased efficiently, and the bandwidth can be increased [10]. To simultaneously improve the durability and bandwidth of the antenna,a quasi-air substrate structure can be fabricated using 3D printing methods that has the advantages of being easy to manufacture, inexpensive, and robust [14; 15; 16].  
In this paper, we propose a stacked dual-patch antenna with 3D printed thick quasi-air substrates and a cavity wall for wideband applications. In order to achieve the theoretical maximum bandwidth of the patch antenna, the quality factor of the system needs to be minimized. To realize this, the area of the conductive radiator should be enlarged, while the permittivity of the substrate within the patch must be reduced close to 1. To realize a patch antenna with this maximum bandwidth, a stacked dual-patch configuration is employed to obtain an extended conductive radiator area. This stacked dual-patch configuration consists of a directly fed lower patch and a coupled-fed upper patch, which can efficiently improve the antenna bandwidth. To further broaden the antenna bandwidth, thick quasi-air substrates with low permittivity close to 1 are employed between the two radiators and the ground plate. To simultaneously improve the durability and bandwidth of the antenna, the quasi-air substrate structure is fabricated using 3D printing methods because of their ease of manufacturing, low cost, and robustness. Square-pipe resin frames are built for the quasi-air substrate using the stereolithography apparatus (SLA) method [17] that can provide high-durability structures with low manufacturing tolerances. In addition, to increase the front-to-back ratio and suppress the leakage electric fields of the patch antenna, a metal cavity wall is placed around the square-pipe resin frames which is manufactured using the selective laser melting (SLM) method [18]. The stacked dual-patch antenna, the square-pipe resin frames, and the ground plate are then assembled with a metal cavity wall to form a complete antenna structure. To verify the antenna's feasibility, indicators of the antenna's performance such as reflection coefficients, bore-sight gains, and radiation patterns are measured in a full anechoic chamber. The results demonstrate that by using the proposed antenna structure, broadband characteristics close to the fundamental bandwidth limit of the patch antenna can be achieved, and at the same time, the durability of the antenna is improved for various applications.