# Highly Compact 4 \(\times\) 4 Flower-Shaped MIMO Antenna for Wideband Communications
## 4 4-Port MIMO Antenna Design
In order to investigate the radiation characteristics of the suggested MIMO antenna with and without the decoupling stubs, the current distribution is carried out for that reason at two frequencies, 5.8 GHz and 9 GHz. To carry out the current distribution process in the proper way, the MIMO antenna is excited at port 1, while the other ports are terminated with 50 \(\Omega\) matched loads. It is clear from the current distribution in Figure 11 that a significant amount of current is distributed on port 2, and a tangible amount is shown on the two other ports for the case of no isolation, while a very small current is passing to the neighbor elements for the case of decoupling stubs. Furthermore, the majority of the  
Figure 10: The S\(-\)parameters outcomes (measured and simulated) of the MIMO antenna. Figure 9: Photograph of the fabricated 4 \(\times\) 4 MIMO antenna: (**a**) front view; (**b**) rear view. current is distributed on the radiating element (port 1), which confirms the low mutual coupling between radiators and low power radiation from port 1 to the adjacent elements since the isolation stubs act as absorbers between the radiators. So, the decoupling stubs on the top and the bottom sides succeeded in mitigating the power transfer between the four elements. In order to investigate the radiation characteristics of the suggested MIMO antenna, the antenna was mounted on the radiation characteristics setup inside the anechoic chamber, as shown in Figure 12, to test its radiation patterns in different planes and the realized gain. It can be noticed from Figure 12 that one port is excited, whereas the other ports are connected with matched loads of 50 \(\Omega\) characteristics impedance. In addition, a horn antenna is mounted in the opposite side of the setup in order to perform the radiation measurements. Figure 11: Current distribution of the suggested MIMO antenna with and without decoupling stubs at port 1: (**a**) at 5.8 GHz; (**b**) at 9 GHz. Figure 12: The measuring setup of the radiation patterns: (**a**) the proposed model; (**b**) the full system. The simulated and measured normalized radiation patterns in both planes are illustrated in Figure 8.0 It is clear that the antenna has an approximate omnidirectional pattern in one plane and a bi-directional pattern in the other plane at 5.8 GHz and 9 GHz, with a good match between the simulated and tested outcomes. The suggested MIMO antenna provides a radiation pattern similar to the pattern of a dipole antenna, which can be employed in various mobile applications due to the obtained radiation characteristics. The antenna realized gain is considered one of the important parameters to validate the radiation performance of the suggested antenna, and it was carried out within the achieved operating frequency band, as illustrated in Figure 8.1 The antenna has an average realized gain of around 4 dBi for the measured results since its minimum value is 2.5 dBi at 5.5 GHz and its maximum value is 4.75 dBi at 9.5 GHz. In addition, the average simulated realized gain is around 4.5 dBi with a minimum value of 3.1 dBi at 5.4 GHz and a maximum value of 5.25 dBi at 9.6 GHz. Furthermore, the two results are in good consistency with slight variation because of the fabrication and measurement tolerance. Figure 8.2 The normalized radiation patterns (in dB) at port 1: (**a**) at 5.8 GHz; (**b**) at 9 GHz. In order to evaluate the MIMO characteristics of the proposed four-port antenna, this evaluation can be accomplished through the investigation of different parameters such as envelop correlation coefficient (\(ECC\)), diversity gain (\(DG\)), and mean effective gain (MEG). The \(ECC\) measures the amount of correlation between antenna radiators and it can be extracted from radiation patterns as from Equation (1) [20; 21].