## 3 Results and Discussion
At this altitude, the starting frequencies of the top antenna remain in the resonance phase region and the pattern of the whole structure is diffused, similarly to the pattern of the top antenna (see Figure 8). The aforementioned half wavelength is 39.219 mm and belongs to the 3.822 GHz simulating frequency of the unit cell to acquire a 0\({}^{\circ}\) phase point for the reflective surface. In such a study, the 0\({}^{\circ}\) phase point is observed at 3.824 GHz. This can be seen in Figure 6. At a distance of 6 mm, the resonance region of the surface is on the left-hand side compared to the half wavelength response and occurs at lower frequencies. Thus, when the distance is 6 mm, both the two starting frequencies and the final frequency, 4.3 GHz, are out of the EBG resonance region (Figure 6). The final distance is 6 mm. Consequently, the necessary conditions for pattern editing are provided by paying attention to all these factors. The surface impedance variation embedded in Figure 6 occurs when the feed point is 6 mm away from the surface. It is seen that the impedance is greater than a thousand ohms at the resonance frequency, and around 166 ohms at the 4.3 GHz frequency. Such an impedance indicates that the 4.3 GHz region is neither like a PEC nor PMC, but much more like PEC. In Figure 9a, the distance between the unit cell and the feed is 39.219 mm, which is the half wavelength of the reflector's 0\({}^{\circ}\) phase resonance frequency, while in Figure 9b,  
Figure 8: The radiation pattern with its directivity scale, while the distance between the antenna and the reflective surface is 39.219 mm. The distance between the antenna and the surface is given with d.  
the distance is 6 mm. In Figure 9a, the surface current is negligible due to surface wave suppression. With the last distance of 6 mm between the unit cell and the feed as seen in Figure 9b, surface currents at 4.3 GHz are also observed since the surface impedance is lower. Figure 9c,d show the surface currents that occur when the whole structure is in operation. This figure is also another indication of the good isolation of the MIMO antenna. The stand-alone top antenna, which consists of two antennas facing each other, is given in Figure 2. The antennas operate at 4.088 GHz and 4.116 GHz frequencies according to the simulation results. In Figure 10, the simulation results of the antenna, when there is no bottom surface, can be seen. The simulated resonance frequencies are 4.116 GHz and 4.088 GHz for port 1 and port 2, respectively. Their S11 and S22 values are \(-\)20.381 dB and \(-\)19.04  
Since the antenna, that stands without the reflective bottom surface, does not have the same phase angles in simulation and measurement, it is not expected that the resultant phase graph, which is drawn for the results derived with the reflective surface, would be exactly the same. As can be seen in the figure, the phase characteristics are similar for simulation and measurement utilizing a 6 mm surface away from the antenna. It is explained in the ITU recommendation that the receiver unit has a bandpass filter and rejects to collect power from other sources outside the operating band [38]. At the transmitter, the signal is generated and transmitted by the voltage-controlled oscillator (VCO) between 4.2 GHz and 4.4 GHz. It is also stated in the recommendation that the radio altimeter system will not be affected unless the signal is at the edge of the band. When the reflective surface is under the antenna, the transmission value is \(-29.84\) dB at 4.3 GHz (Figure 12). The transmission value is measured as \(-29.41\) dB at 4.344 GHz, which is the operating frequency of the first port, and \(-29.08\) dB at 4.32 GHz, which is the operating frequency of the second port. These results indicate a low interference value for the two antennas on the MIMO. Figure 11: A comparison of measurement and simulation results of S11 phase angle values with the reflective surface underneath.