## 3 Results and Discussion
Figure 13: S11 simulation (**a**) and measurement (**b**) results when the distance between the antenna and the surface is 5 mm, 6 mm, and 7 mm. The measurement studies were conducted at 4.344 GHz for port 1 and 4.32 GHz for port 2. Note that the opposing antennas act like a director to each other and affect the propagation angle. The radiation properties of the antennas are gathered in Tables 3 and 4 according to \(\varphi\) = 90\({}^{\circ}\) cut angle. The radio altimeter features were obtained at \(\varphi\) = 90\({}^{\circ}\), hence the antenna was evaluated in this aspect. This is the same as turning the antenna \(\varphi\) = 90\({}^{\circ}\) while viewing it from the perspective of \(\varphi\) = 0\({}^{\circ}\). The simulated value of \(-\)19.1 dB side lobe level (SLL) is noteworthy and evidence of the successful focus of power, as intended, although the least measured result of \(-\)12.6057 dB appears to have been due to the laboratory conditions. Another result to pay attention to is angular width. The  
Figure 14: Simulation pattern without a reflector on the scale of directivity. Perspective view (**a**) with the operating frequency of f = 4.088 GHz while port 2 is fed, front view (**b**) and side view (**c**) with the operating frequency of f = 4.116 GHz while port 1 is fed. Side view (**d**), front view (**e**), and top view (**f**) of the antenna pattern simulated with the reflector on the scale of directivity while port 1 is fed. Figure 6.0 One-dimensional measurement and simulation results of the pattern for the cut angles of \(\varphi\) = 90\({}^{\circ}\) (**a**), and \(\varphi\) = 0\({}^{\circ}\) (**b**) derived with the reflector. recommended width for the radio altimeter is between 35\({}^{\circ}\) and 60\({}^{\circ}\), and the proposed antenna creates a half-power beamwidth (HPBW) very close to this value. The measured HPBW values are 60\({}^{\circ}\) and 70\({}^{\circ}\). As the measurements are performed at every 10\({}^{\circ}\), the coding performed by the authors caught these values. It can easily be seen that if the measurement angle hops were narrower, the code would also have caught an HPBW value close to 60\({}^{\circ}\) for port 1. According to simulation results, the gain increases from 5.34 dBi to 10.3 dBi for port 1 when the reflective surface is placed under these antennas. The measurements support this case with a measured gain of 10.1497 dBi achieved for port 1. There is a similar result for the other port. The measured gain can be evaluated as realized gain because it includes the losses. When the antenna operates without the reflector, its directivity is not very high due to having a pattern close to the omnidirectional pattern rather than directional, as was desired from the very beginning. This antenna has a minimum directivity of 5.46 dBi without the reflector. In our measurements, which were carried out with the surface at a distance of 6 mm, it is observed that the directivity value exceeded 11.5 dBi (see Table 4). The main purpose of the design with the reflective surface is clearly seen in these results. To obey the design criteria determined for the radio altimeter by ITU, the gain should be between 8 and 13 dBi [38]. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \(\varphi\) = 90\({}^{\circ}\) & \begin{tabular}{c} **Directivity** \\ **(dBi)** \\ \end{tabular}  & \begin{tabular}{c} **Realized** \\ **Gain** \\ \end{tabular}  & \begin{tabular}{c} **Front-to-Back** \\ **Ratio** \\ **(dB)** \\ \end{tabular}  & \begin{tabular}{c} **Half-Power** \\ **Beamwidth** \\ **(deg.)** \\ \end{tabular}  & \ \begin{tabular}{c} **Side Lobe** \\ **Level** \\ **(dB)** \\ \end{tabular}  \\ \cline{2-8} \multirow{4}{*}{
\begin{tabular}{c} **  
Due to the complexity of MIMO systems, their performance must be tested through some indicator data [39; 40; 41; 42]. The measure of how much the transmission power can be reduced while using two ports is given with the diversity gain (DG).