# A MIMO Antenna with High Gain and Enhanced Isolation for WLAN Applications
## 2 Antenna Structure and Design  
Figure 1 illustrates three steps of the presented antenna design process. Antenna 1 investigated the optimization of the position and size of the two antenna elements according to the reflection coefficients. As shown in Figure 2a, the frequency bands of antenna 1 are approximately 1 GHz (2.7-3.7 GHz) and 1.7 GHz (6.9-8.6 GHz). The resonance points are 3.24 GHz and 7.6 GHz, respectively. Compared with antenna 1, antenna 2 has two inverted L-shaped stubs on the ground plane. As shown in Figure 2b, the S\({}_{21}\) parameter values of antenna 2 in the 5-7 GHz frequency band are almost 10 dB smaller than those of antenna 1. It can be seen that the L-shaped stubs greatly enhance the isolation performance at both operating frequency bands. The inverted L-shaped stubs play a role in the neutralizing line. The neutralization line structure guides the radiation field to gather along the diagonal direction of the neutralization line instead of cross-coupling effects. Antenna 3 is the proposed antenna of this paper. A small rectangular was cut out of the ground plane to reduce the S\({}_{11}\) values of antenna elements in the 2-3 GHz frequency band, and the resonance point of higher frequency band was shifted to the lower frequencies. It makes the radiation field of the operating frequency band concentrated on the two elements of antenna. In Figure 2a, the \(-\)10 dB operating frequency bands of the presented antenna are 2.16-3.05 GHz and 4.92-6.45 GHz. Figure 1: Structure of the designed MIMO antenna. (**a**) antenna 1, (**b**) antenna 2 and (**c**) antenna 3 (proposed). Figure 2: The simulated parameters of three antennas. (**a**) reflection coefficients, (**b**) isolation values. The top and bottom layers of the presented antenna are shown in Figure 3. It is fabricated on an FR4 substrate with a substrate height of 1.6 mm, loss tan8 of 0.02 and a relative permittivity \(\varepsilon_{\text{r}}\) of 4.4. Two radiating antenna elements are located on the top layer of the substrate. Each element includes an L-shaped short strip, a stepped folding strip and a rectangular feeding microstrip. The ground part is located on the bottom of the substrate. It is employed to improve the isolation of two elements. It is spliced together by two recessed metal elements, a semicircular strip and two opposite inverted-L-shaped strips. The two radiating antenna elements and ground plane are both symmetrical along the diagonal of the substrate. To optimize the dimension of the proposed antenna, different values of L1 and W9 have been investigated. As shown in Figure 4a, when the value of L1 is 1.5, 2 and 2.5 mm, the higher operating frequency band of the antenna only changes a little, but the lower frequency band changes a lot. Therefore, we can optimize the value of L1 to tune the lower operating frequency band. As shown in Figure 4b, the higher frequency band changes significantly when the value of W9 is changed. Therefore, we can optimize the value of W9 to tune the higher operating frequency band. Therefore, the operating frequency bands are 2.16-3.05 GHz and 4.92-6.45 GHz when the values of L1 and W9 are 2 mm and 4.5 mm, respectively. The final values of presented antenna dimensions are listed in Table 1.  
Figure 4: The simulated S parameters at different values of (**a**) L1 and (**b**) W9. Figure 3: Structure of the designed MIMO antenna. (**a**) top view, (**b**) bottom view. The designed antenna is based on linear polarization theory. The two ports adopt the same excitation mode. Figure 5 shows the current distribution of the proposed MIMO antenna when two ports are excited. At the frequency of 2.45 GHz, the main currents are focused on the stepped folding strip, the rectangular feeding microstrip and part of the inverted L-shaped strips on the ground plane. Therefore, the current intensity around the radius of R is small. At the frequency of 5.5 GHz, the main currents are focused on the corner of the stepped folding strips and both inverted L-shaped strips on the ground part.