# Highly Compact 4 \(\times\) 4 Flower-Shaped MIMO Antenna for Wideband Communications
## 4 4-Port MIMO Antenna Design  
The two introduced arrangements in the previous section are used to achieve the four-port MIMO antenna since antenna 2 is placed beside antenna 1 (side-by-side), while antennas 3 and 4 are placed 180\({}^{\circ}\) with antennas 1 and 2 (face-to-face) as demonstrated in Figure 7a. To enhance the coupling between the antenna elements, four pairs of stubs designed in the previous section are added on the top layer and the bottom layer of the substrate, as presented in Figure 7b. These stubs work as isolators between the four radiators to reject any radiation of power between them. The effect of the decoupling stubs on the antenna performance is illustrated in Figure 8. It can be noticed that the S\({}_{11}\) achieved the same outcomes in terms of impedance bandwidth, while the matching of the antenna is enhanced when the decoupling structures are used, as shown in Figure 8. Additionally, the matching was \(-17.5\) dB for the case without stubs at 6.7 GHz, and it turns to \(-37.5\) dB at 6.8 GHz when the stubs are added, which proves the additive impact of the decoupling stubs on the impedance matching outcomes. Figure 7: Layout of the 4-port MIMO antenna: (**a**) without decoupling stubs; (**b**) with decoupling stubs. The S\({}_{21}\) outcomes are also enhanced from \(-15\) dB without the decoupling structure to less than \(-20\) dB when the decoupling structures are utilized. The value of S\({}_{21}\) is below \(-20\) dB from 5.5 GHz till the end of the achieved band. Furthermore, S\({}_{31}\) and S\({}_{41}\) are improved from \(-17.5\) dB to less than \(-20\) dB after using the decoupling structures with a coverage of the whole obtained frequency band. So, these additional stubs have a significant effect on the impedance-matching performance of the suggested 4 \(\times\) 4 MIMO antenna. It is worth noting that the antenna occupies an area of 30 \(\times\) 30 mm\({}^{2}\) with a very small distance between radiators (5.7 mm for face-to-face radiators and 7.6 mm for side-by-side radiators), which confirms the high level of compactness of the suggested antenna and its eligibility to be embedded in small portable devices. The suggested MIMO antenna was experimentally fabricated on FR4 dielectric material of size 30 \(\times\) 30 mm\({}^{2}\). as photographed in Figure 9. Then, the model was experimentally measured using R&S ZVB 20 vector network analyzer (VNA) to test its impedance characteristics. Because of the antenna symmetry between ports, the results from one port were extracted, as illustrated in Figure 10. It is noticed that the simulation results show that the antenna is well matched over the entire band spectrum from 5.75 up to 11 GHz, as shown in Figure 10, while the measured results start from 5.8 GHz up to 10 GHz. There is a discrepancy between simulated and measured S\({}_{11}\) since there is a shift in resonance frequency of about 500 MHz (6.3 GHz measured and 6.8 GHz simulated), and this is due to the tolerance associated with fabrication and measurement. As well, the mutual coupling between ports has a value lower than \(-20\) dB for both the simulation and tested outcomes, as shown in Figure 6, with a relatively good trend between the two results. The S\({}_{21}\) outcomes are below \(-20\) dB, starting from 5.7 GHz till the end of the achieved band. In addition, the S\({}_{31}\) outcomes are totally below \(-20\) dB, except the measured result is above this level by 1 dB at 7 GHz only, and this tolerance may happen due to fabrication and measurement processes. Regarding the S\({}_{41}\) outcomes, both simulation and measurement outcomes are consistent with the same trend below \(-20\) dB. After all these outcomes that  
Figure 8: The S\({}_{-}\) parameters simulated outcomes of the MIMO antenna. compare the simulation and experimental work, it is evident that the suggested antenna is appropriate for diversified wideband MIMO applications due to its compact size, better matching, and improved isolation between the four ports.