## 2 Materials and Methods  
_Antenna Design and Its Operational Principle_  
The main motivation for the authors to work outside the EBG phase region for pattern shaping is that PEC surface reflection tends to make a diffuse pattern more compact when the antenna is near the surface, while PMC-type surface reflection tends to preserve or further disperse the diffuse pattern. An example of this foresight is shown in Figure 1. Figure 1a shows an antenna, whose pattern is diffused, operating at 4.3 GHz. In Figure 1b, this antenna is placed over an EBG structure with zero phase response at 4.3 GHz, which can be considered a PMC, and in Figure 1c, it is placed over a copper plate, which can be considered a PEC. The distance between the antenna-copper and antenna-EBG is the same. As can be seen from the figure, in the PEC simulation, the pattern is concentrated in a single lobe, and in the PMC simulation, the pattern is divided into three parts. The proposed MIMO structure consists of two opposing planar antennas and a reflective surface placed below them. While creating the structure of the antenna, the priority was to make it radiate both above and below the plane, thus creating a remarkable power reflection when the reflective surface was placed underneath. Namely, to utilize the EBG surface, the top planar antenna had to radiate on both surfaces of the plate so that the wave hitting the reflective surface could be collected and propagated at the top of the structure. This was made possible by adjusting the size of the planar antenna greater than 1.5 wavelengths. This step caused the top antenna pattern to split to be visible on both plate surfaces. Radio altimeter antennas are required to radiate horizontally polarized Hence, firstly, the top antenna is meant to be designed as a planar type. Secondly, the goal was to achieve area gain with a MIMO setup. For the radio altimeter's conical pattern, it was better to place the radiating parts on MIMO face-to-face, working as directors for each other to obtain a better conical shape. The MIMO antenna seen in Figure 2 has a width of 109.74 mm and a length of 146.5 mm (see Table 1). The thickness of the dielectric substrate layer is 1.524 mm. The permittivity value of the Isola dielectric material is \(\varepsilon_{r}=3\). Figure 1: An example of the effect of PEC and PMC surfaces on the pattern. The antenna (**a**), antenna and bottom EBG plate (**b**), antenna and bottom copper plate (**c**). Figure 2: Dimensions of antenna front face (**a**), and the backside (**b**). The copper patch structures placed at the backside of the plate can be called ground planes, which the antenna arm would take as a reference surface, and are squares with a side length of 11.57 mm. The entire surface on the backside was not chosen as the ground plane. Instead, partial support was provided on the ground plane with parasitic patch structures located next to the middle piece. When these parasitic patches are removed, the targeted antenna properties cannot be obtained. Although the structure in the upper copper layer presents the appearance of a dual structure, it forms a single arm of the antenna. Here, the enlargement and the square cuts placed at the extreme edges of those antenna arms are intended to increase the antenna gain. In addition, the outermost points of these arms were trimmed with the slanting cut; hence, the antenna beamwidth was narrowed in accordance with the design criteria of the radio altimeter antenna. The feed line given in Figure 3 with a close-up view was started directly from the laminate edge in order to not disturb the wave that will be reflected from the reflective surface, and it was designed with a thickness of 1.13 mm based on the t-shape divider. The arms and the supply line are combined together. Thus, the thickness of this line was gradually increased from 1.69 mm to 2.56 mm and finally reached its maximum size to increase the effective antenna aperture at the very far end.