## 3 Results and Discussion
According to Figure 16, the DG is around 9.999 in the vicinity of 4.3 GHz and across the radio altimeter band. The envelope correlation coefficient (ECC), which is a measure of how much two antennas affect each other's pattern, is practically waited to be less than 0.5 for MIMO [43]. Figure 16: Diversity gain (DG) and envelope correlation coefficient (ECC) simulation and measurement results of the MIMO antenna with its reflector surface with the representation of the surface current while one port of the antenna is fed (**a**). Mean effective gain (**b**). Total active reflection coefficient (**c**). Figure 16 also includes the ECC graph [44]. At 4.3 GHz, the ECC value is obtained as 0.0045, which is considerably less than 0.5. The contour representation of the surface current of the MIMO structure is also given in Figure 16. When any port is active, it is clearly seen that the surface current does not affect the other port. Mean effective gain (MEG) is the ratio of the average received power of the related port to the average power received by a reference antenna [45; 46]. MEG for the first and second ports were calculated and the results are given in Figure 16. The difference is in a linear scale, whose results correspond to less than \(-3\) in dB, and the dB variations of both of the ports are imposed into the figure. The total active reflection coefficient (TARC) was calculated over the average of the angle values from \(0^{\circ}\) to \(180^{\circ}\) with a \(10^{\circ}\) range (Figure 16) [47]. The measurement results show that the TARC is less than \(-10\) dB. The antenna was simulated by placing it on a life-size airplane's proper part, which is important to achieve the aim to send the waves to the ground properly [48], as seen in Figure 11. PEC was chosen as aircraft fuselage material in order to create the ideal environment. The length of the aircraft is greater than 70 m. A frequency of 4.304 GHz, which is the resonant frequency of the antenna connected to port 2, was chosen as the operating frequency in the simulation carried out with an integral solver. According to the simulation results studied on the aircraft, the directivity is 11.6 dBi, the gain is 10.3 dBi, realized gain is 10.3 dBi, angular width is \(61.6^{\circ}\), sidelobe level is \(-19.3\) dB, and radiation efficiency is \(-1.343\) dB. Radio altimeter antenna designs, whose results are confirmable from their own manuscripts, are listed in Table 5. Obviously, the size of the proposed structure could be reduced by reducing the distance between the opposing antennas. In doing so, care could be taken to keep the antenna characteristics within the radio altimeter requirements. For example, the separation could be reduced until the gain drops to 8 dBi. The choice of size reduction for such an antenna structure is in the hands of the designer. The proposed antenna stands out for airborne altimeter applications in terms of the features such as the conformity between simulation measurement results and providing the radio altimeter prerequisites. Considering that the largest size of commercial radio altimeter antennas per port is between 10 and 17 cm, it is also seen that the purpose of the proposed antenna in terms of space saving has been realized.