*2.1. Single-Port Antenna*

An initial configuration of the proposed MIMO system with a single-port antenna is shown in Figure 2C and its total dimensions is 20 mm × 20 mm × 1.6 mm. The designevolution steps of the proposed antenna are shown in Figure 2. In Figure 3, the return-loss (*S*11) results obtained in each evolution step while designing the proposed MIMO antenna are shown.

Initially, an antenna with less than half ground plane and an L-shaped antenna element is designed, as shown in Figure 2A. It can be observed that the impedance bandwidth of 26% is achieved from 4.6 to 6 GHz (*S*<sup>11</sup> < −10 dB). In the next steps, the ground plane is modified (Figure 2B) and then further improved (Figure 2C) to radiate for the required frequency band. The S-parameter plot in Figure 3 shows that *S*<sup>11</sup> < −8 dB is achieved for the complete required frequency range from 3.5–5.4 GHz with impedance bandwidth of 43%.

**Figure 2.** Design-evolution process of the single-element antenna. (**A**) Step–1, (**B**) Step–2, (**C**) Step–3.

**Figure 3.** S–parameters for different configuration.

#### *2.2. Four-Port MIMO Antenna*

The four-port MIMO antenna is proposed from the single-port antenna design discussed in the preceding section. At the initial stage, four antennae are placed orthogonally to each other on the top layer of the FR4 substrate, as shown in Figure 4, and the total volume of the antenna is 40 mm × 40 mm × 1.6 mm. Each antenna element with its feeding port of width (w\_port) = 3 mm is placed at a distance of 7.6 mm from the corner end of the substrate. The inter-element spacing between the two antenna elements is 12 mm. The dimensions are properly adjusted in such a way to achieve good bandwidth covering the required frequency range and to obtain good isolation.

**Figure 4.** Design evolution of the ground plane structure for the proposed antenna.

The step-by-step configuration of the proposed antenna is shown in Figure 4. The simulated S-parameters plotted in Figure 5A clearly show that the return loss of −10 dB starts only from 4.5 GHz and from 4.2 GHz in steps 1 and 2, respectively. Similarly, both the step designs (STEP 1 and STEP 2) have high mutual coupling between the ports (Figure 5B). In order to achieve good isolation, the ground plane is modified as shown in Figure 4 (steps 2 and 3) by arranging a slot in the center of the antenna ground plane, connected to each other to form a common ground plane. It is also observed that, the currents almost penetrate between the nearby antenna elements in step 2 compared to current distribution in step 3. Good isolation and impedance bandwidth is achieved in the proposed design of step 3 (Figure 5B).

**Figure 5.** Frequency variation of S-parameters for different configuration: (**A**) *S*<sup>11</sup> [dB], and (**B**) *S*<sup>13</sup> [dB].
