## 3 Results and Discussion
Side view and front view of the antenna with its reflector on a simulated life-size aircraft [37]. Figure 10: S11 simulation and measurement results of the antenna without the reflector. S parameter simulation results of the antenna without the reflector are given on the right bottom corner. Low transmission values between the two antennas result in a high ability to operate independently and indicate a low interference for the two antennas on the MIMO. Again, S21 and S12 simulation values seen in Figure 12 are \(-35.37\) dB at 4.344 GHz and \(-36.19\) dB at 4.32 GHz in accordance with the reading, which was made at the operating frequencies of the measurement results. At the resonance frequencies obtained in the simulation, the transmission values can be given as \(-37.02\) dB for 4.304 GHz and \(-36.38\) dB for 4.316 GHz for a better comparison of the simulation and measurement results. It is understood that there is no significant difference between measurement and simulation results in terms of transmission results. In the simulation results, the antenna connected to the first port operates with \(-33.94\) dB S11 at 4.316 GHz, and the antenna connected to the second port with \(-29.38\) dB S11 at 4.304 GHz (see Figure 12). It is a preliminary indication that it can be used as a radio altimeter antenna before seeing the measurement result. The S parameter results obtained from the measurements of the whole structure are collected in Figure 12. According to the S11 and S22 reflection measurement results of the antenna and the surface, which are on top of each other with a 6 mm gap as seen in Figure 12, the antenna connected to port 1 has a \(-20.072\) dB value at 4.344 GHz, and the antenna connected to port 2 has a \(-27.44\) dB at 4.32 GHz. Considering the limits of the frequency band restricted with a \(-10\) dB reflection value, the measurement bandwidth seems suitable for the 4.2 GHz-4.4 GHz band region applicable to the radio altimeter. In Figure 13, the simulation values of S11 are compared with respect to the distance between the antenna and the surface varying as 5 mm, 6 mm, and 7 mm. The operating frequencies are 4.348 GHz, 4.316 GHz, and 4.284 GHz, respectively. Figure 12: S parameter simulation (**a**) and measurement (**b**) results when the distance between the antenna and the surface is 6 mm. It is seen that the operating frequency decreases as the distance increases. Similarly, for these distances, the operating frequencies are 4.36 GHz, 4.344 GHz, and 4.28 GHz in the measurement (see Figure 13). S11 values are measured as \(-\)22.87 dB, \(-\)20.072 dB, and \(-\)18.53 dB, in the same frequency order. This means that this structure behaves in such a way that the operating frequency decreases as the distance between them increases according to the measurement results as well. It is concluded that to operate precisely at the center frequency of 4.3 GHz, the distance between the antenna and the surface must be adjusted to a value between 6 mm and 7 mm. It can be easily said from the measurement experiences of the authors on this antenna that the entire surface must be separated by the mentioned distances from the antenna for these changes to occur. This means that the antenna radiation capabilities will not change unless the borders between the antenna and the surface flex in the vertical plane dramatically. Finding such borders like locks to keep them at the desired distance may be covered in a separate study. Figure 14 shows the radiation pattern with the reflective surface at the bottom when port 1 is fed. This pattern is proper for the radio altimeter pattern recommended to be conical. Also, this planar structure radiates with horizontal polarization, which is defined by ITU for radio altimeter [38]. A one-dimensional simulation pattern for each port of the antenna of the MIMO can be seen in Figure 15 when the cut angle is chosen as \(\varphi=90^{\circ}\) and \(\varphi=0^{\circ}\).