# A Flower-Shaped Miniaturized UWB-MIMO Antenna with High Isolation
## 2 The Proposed Antenna System
### Design Evolution Stages of the MIMO Antenna  
To examine the implications of different MIMO antenna configurations, the 50 \(\Omega\) transmission line feed is utilized in combination with the fractional ground plane. The overall design procedure for the proposed UWB-MIMO antenna system is elaborated in Figure 3, and the MIMO system's working principle, using its reflection coefficient and transmission coefficient curves, is presented in Figures 4 and 5. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Dimension** & **Value (mm)** \\ \hline L & Length of MIMO antenna & 30 (0.84\(\lambda\)) \\ \hline W & Width of MIMO antenna & 18 (0.50\(\lambda\)) \\ \hline L\({}_{\text{d}}\) & Length of microstrip feed line & 5.416 (0.15\(\lambda\)) \\ \hline W\({}_{1}\) & Width of microstrip feed line & 1.8 (0.05\(\lambda\)) \\ \hline R\({}_{1}\) & Radius of ellipse 1 & 3.75 (0.10\(\lambda\)) \\ \hline R\({}_{2}\) & Radius of ellipse 2 & 5.25 (0.15\(\lambda\)) \\ \hline S & Width of rectangular metallic ground & 3 (0.084\(\lambda\)) \\ \hline L\({}_{\text{g}}\) & Vertical length of modified L-shaped ground branch & 14 (0.39\(\lambda\)) \\ \hline W\({}_{2}\) & Width of modified L-shaped ground branch & 2 (0.056\(\lambda\)) \\ \hline L\({}_{\text{f}}\) & Length of I-shaped ground stub & 10 (0.28\(\lambda\)) \\ \hline W\({}_{3}\) & Width of I-shaped ground stub & 1 (0.028\(\lambda\)) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Dimensions of the proposed MIMO antenna structure (f = 4 GHz). Figure 3: Evolution of the design process of UWB-MIMO system: (**a**) step 1, (**b**) step 2, (**c**) step 3, (**d**) step 4 (proposed MIMO system). The radiating element in step 1 (Figure 3a) is made up of two mutually perpendicular elliptical patches and a microstrip line, along with a full rectangular ground at the bottom. Notably, the radiating element in this case is identical to the one proposed in [17], which is utilized for 5G communication. Although the antenna designed in step1 can cover 4.58-12.85 GHz, its reflection coefficient is poor, and the best value for S11 is only \(-\)15.2 dB, as shown in Figure 4. Meanwhile, the isolation between the antennas in the covered frequency range is less than 19 dB, since there is no decoupling structure involved, as illustrated in Figure 5.  
Therefore, the radiating element is modified by merging three elliptically shaped patches in step 2, resembling a flower, but the ground structure is left unchanged. This antenna structure generates two resonant modes at 8.4 GHz and 12.8 GHz from its reflection coefficient, and the impedance-matching performance is improved. In the resonant modes, reflection coefficients are \(-\)43 and \(-\)21 dB, respectively, and the impedance bandwidth reaches 4.89-14.13 GHz. The transmission coefficient between the antennas, on the other hand, has not improved. Progressively, in step 3, a horizontal I-shaped stub is placed above the inverted L-shaped branch to produce a T-like branch on ground, which improves the impedance  
Figure 4: Comparison of reflection coefficients of 4 proposed UWB-MIMO antenna designs. Figure 5: Comparison of transmission coefficients of 4 proposed UWB-MIMO antenna designs. matching and isolation compared with step 2. The antenna is stimulated into five resonant modes (5.38, 5.8, 7.8, 9.6, and 13 GHz), as shown by its reflection coefficient results, thus suggesting that the new T-like branch functions as a resonator, hence extending the bandwidth so that it spans between 4.48 and 15.26 GHz. From these results, we can see that the antenna's bandwidth has been increased. Furthermore, the total reflection coefficient is lowered, indicating that the impedance-matching ability is improved. Due to the separation impact of the T-like branch on the antenna components, isolation is enhanced, reaching more than 14.2 dB. These findings suggest that the adoption of the T-like branch is important for boosting the bandwidth and isolation.