# T-Type Vertical Wall for Decoupling and Pattern Correction of Patch Antenna
## 1 Introduction  
The 5G/6G communication technology requires the corresponding devices to be developed in the miniaturization direction, thus promoting this trend in the multi-antenna system, such as the MIMO (Multiple-Input Multiple-Output), phased array. Then, decoupling techniques are necessary when they become closer. Generally, there are three ways to reduce the coupling between patch antennas: (1) Loading metamaterials or dielectrics above the patch [1, 2, 3]; (2) adding some novel structures and/or using the defect ground structures between them [4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]; and (3) through analysis of the field distribution rather than any other structure [21, 22]. For the first category, the authors adopted the high-profile structure that placed the loads above the patch to concentrate the radiated field [1, 2] or directly on the dielectric to cancel the surface wave for the isolation enhancement [3]. In Reference [1], the near-field resonator consisting of the split rings was proposed, and stronger coupling confines of the H-field generated an orthogonal coupling mode of the E-field to improve the isolation. The primary and secondary reflectors were used above the patch to generate the out-of-phase reflected wave with the coupling wave, which reduced the mutual coupling [2]. While the literature [3] shows that there was a common thick dielectric directly on the patch, the surface wave coupling can be canceled by controlling the space wave coupling through this block. The most commonly used method for isolation enhancement is by adding planar or three-dimensional structures between the patches. The metamaterial resonator is one of the most common methods to reduce the surface and coupling waves. The authors placed the metamaterial superstrate, where the modified complementary split ring resonators were, between the patches to reduce the surface and space waves [4]. Electromagnetic bandgap (EBG) [5] and the meander-line structures [6; 7; 8] were also employed for the same purpose. Additionally, the parallel coupled-line resonators were proposed in Reference [9] to generate the bandstop characteristics and to suppress the coupling. Three interdigital lines were indicated in Reference [10] to form the orthogonal polarization mode on the adjacent patch to enhance the isolation. Except for the mentioned surface wave decoupling techniques, the surface current reduction is another direction to enhance the isolation [8; 11; 12; 13; 14; 15]. The complete EBG structure separates the common ground plane of patch elements into two parts, which cut off the surface current [8], while the meander line [11] was curved or the fractal technique [12] was used to prevent the surface current. In Reference [13], a slot was proposed to cut the substrate and the ground for the surface wave and current. In Reference [14], the T-shaped and rectangular-ring-shaped defect ground structures were presented to produce the reverse current opposite to the ground coupling current. On the contrary, an etched rectangular slot on the ground plane, to form the band-notch filter with the strip on the top of the substrate to reduce the coupling effectively, was used [15]. The surface current, surface wave, and radiated wave could be blocked together using the vertical wall technique [16]. We also found that the planar filter consisted of a stub-loaded inverted-F radiator to suppress the near-field coupling [17], and the coupled strips close to the feed or the patches were shown to generate another current path for the reduction of mutual couplings [18; 19; 20]. The third direction to reduce the mutual coupling was the field analysis of antennas without adding any additional structure. The authors in Reference [21] realized a self-decoupling effect by placing the adjacent patch element into the weak-field area where the coupling strength could be controlled.