## 3 Results and Discussion  
Since the antennas are placed opposingly, they will affect each other like a director and change the pattern during the operation. Looking at the pattern of the top antenna given in Figure 14c, it is seen that the pattern is separated outward from both surfaces of the plate. This is because it was aimed to put a reflective surface at the backside of the antenna from the very beginning. It is desired to collect the energy on the upper side by reflecting the part of the pattern facing the reflective surface. The reflected energy merges with the energy that is already at the top. The radio altimeter antenna requires wide beamwidth, high gain, and a conical pattern as defined in aviation standards. The effective aperture should increase to achieve these goals by increasing the antenna dimensions. However, the growth of sizes has an effect on the pattern. When the antenna length reaches more than one and a half wavelengths, the pattern ceases to split more than two parts. Thus, an antenna with a diffuse pattern emerges. Therefore, the need to gather the messy radiation pattern of the antenna occurs. To reach both of these goals at the same time, a couple of technical tricks were developed. These steps were taken with the foresight that a PMC reflector, which is defined as the ideal artificial magnetic conductor with its zero-degree reflection phase, preserves the antenna pattern. Accordingly, the whole resonance region of the reflective surface is avoided to prevent the messy pattern from being preserved. The first step is to set the operating frequency of the stand-alone top antenna (top section of the whole structure) lower than the final target frequency, 4.3 GHz. These frequencies (4.088 GHz and 4.116 GHz) will be referred to as the starting frequencies of the antenna in the rest of the manuscript. This step also results in a slight increase in antenna gain. Figure 7: Manufacturing of the antenna (**a**), and the reflective surface (**b**) by engraving. Top view of the whole structure (**c**). Measurement of the antenna without the reflector (**d**), and with the reflector (**e**). Pattern measurement setup (**f**). Because the frequency increases as the distance to the surface decreases, the radio altimeter band is reached. We propose that at the beginning, the starting frequencies of the top antenna should be chosen away from the zero-degree EBG reflection region and be close to the ends. We recommend performing this so that the target frequency, 4.3 GHz, remains outside of the phase curve but in the immediate vicinity after the top antenna and the surface are brought closer together and the frequency of the entire structure increased to the target frequency. At this stage, we want to be as close to the response curve as possible, as the phase values will decrease as we move away from the phase curve. Thus, the second step is to design a unit cell so that its chosen frequency, in which the reflection phase angle of the surface is adjusted to 0\({}^{\circ}\), is smaller than the operating frequencies of the top antenna to ensure that the starting frequencies are away from the zero-degree reflection region. Hence, the designed surface provides a reflection phase response of \(-170.47^{\circ}\) and \(-176.77^{\circ}\) at a half-wave distance to the feed, at the operating frequencies of the top antenna, 4.088 GHz and 4.116 GHz, respectively. In order to ensure the starting top antenna frequencies to be very close to the end of the response graph, gaps were opened on the unit cell and the phase response graph was slightly shifted to the left (see Figure 6). With the proposed solution, the need to change the unit cell and initial antenna dimensions is eliminated. Reducing the size of the top antenna and increasing the starting frequencies could be another solution here, but it would cause the antenna gain to decrease. After these designs, in the first step, the distance between the top antenna and the reflective surface was kept at half a wavelength.