*3.1. Design Procedure*

This section presents a precise design evaluation of the proposed antenna. Figure 2a gives the four structures during the design process. The first structure is a single rectangular plane with a small open-ended L-shaped slot. It can be seen from Figure 2c that an obvious resonant mode around 4.3 GHz of Ant. 1 and Ant. 2 is obtained. However, the port impedance matching needs to be optimized. As shown in Figure 2b, the simulated normalized port impedance curve is far away from the center point of the Smith chart. Another small rectangular slot and ground clearance (145 × 3.5 mm2) are cut from the antenna element and grounding plane in the second structure. A T-shaped strip is introduced to diminish the mutual coupling between Ant. 1 and Ant. 2. It can be distinctly observed from Figure 2c that two resonant modes (around 3.6 GHz and 5.5 GHz) are excited. Little frequency offset between S11 and S22 occurs because of the two elements' different locations. The mutual coupling S12 of the second structure is 10 dB and 13 dB in the lower and higher bands, respectively.

**Figure 2.** (**a**) Design evolution, (**b**) simulated Smith chart of S11, (**c**) simulated S11 and S22, (**d**) simulated S12 and S23.

Furthermore, a significant enhancement in the normalized impedance matching condition is acquired, which can be observed in Figure 2b. In the third structure, a small rectangular slot is cut from the antenna element of the second structure. An extra two C-shaped strips are added to the upper terminals of the aforementioned T-shaped decoupling branch. The simulated S22 obtains a good matching condition at 3.53 GHz. The

lowest isolation of 8 dB occurs at 3.53 GHz, and little isolation promotion at 5.5 GHz is realized. The final antenna element structure is produced by cutting a small rectangular slot in the feedline side of the lowest rectangular strip and reconnecting it on the other side. At the same time, another newly introduced horizontal T-shaped branch is connected to the existing decoupling branch as the third structure. The final design of an antenna element and DB1 are generated in Figure 2a. The simulated S11 and S22 can entirely contain the target bands, and the isolation S21 is also lifted to 15 dB and 18 dB across 3.4–3.6 GHz and 4.8–5 GHz, respectively. Table 1 lists the simulated normalized impedance at each design stage at 3.5 GHz and 4.9 GHz. The final structure obtains better impedance matching performance than the former three design stages.


**Table 1.** The simulated normalized impedance at two resonant frequencies of each design stage.
