# Designed Circularly Polarized Two-Port Microstrip MIMO Antenna for WLAN Applications
## 3 MIMO Antenna Circular Polarization Results  
From a single antenna, a square-shaped patch with a square-cut at the corners obtains a circular polarization. Later, the single antenna combines to make a MIMO-CP antenna, which has good S11, but bad S21, and an axial ratio. To improve the isolation and axial ratio, the slot is produce in the ground. The design combines both conventional techniques (square-shaped patch with a square-cut at the corners and ground slot) to shift the resonance frequency, improve AR, as well as isolation. In fact, the ground slot, having a width and length of the slot (c = 1.5 mm and g = 11.25), provides a longer effective path for the current flow, efficiently. The dual elements inspired by the ports are denoted as port-1 and port-2. The MIMO antenna was designed in two steps--a MIMO-CP antenna with no ground slot and a MIMO-CP antenna with the ground slot. First, we designed the antenna without the slot for the desired frequency. The proposed MIMO antenna with no slot in the ground shows a \(-10\) dB impedance band of 5.22-6.41 GHz, with more than 20 dB of return loss, and resonates at a 5.53 GHz frequency with 22 dB of isolation at resonant, as shown in Figure 2(a). From Figure 2(b), the antenna without a slot in the ground has an AR value greater than 3 dB, indicating that it is no longer in CP polarization. On the other hand, the proposed MIMO antenna with a slot shows a \(-10\) dB impedance band of 5.23-6.42 GHz, with more than a 38 dB return loss, as shown in Figure 3(a). In this case, the antenna resonates at 5.6 GHz frequency with 37 dB of isolation and an AR value of 1.51 dB, and a 3 dB AR band of 5.37-5.72 GHz. The AR is used to identify the feature of CP elements. The CP frequency ranges are from 5.37-5.72 GHz; the 3-dB AR bandwidth is 6.25%, as shown in Figure 3(b). The proposed MIMO-CP antenna covers the WLAN application. The proposed antenna peak gain is 6 dB at a resonant frequency. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Parameters** & **Dimensions (mm)** & **Parameters** & **Dimensions (mm)** & **Parameters** & **Dimensions (mm)** \\ \hline \(W\) & 50 & \(L\) & 22.5 & \(pl\) & 11 \\ \(ls\) & 3.1 & \(ha\) & 3.5 & \(d\) & 12.2 \\ \(c\) & 1.5 & \(h\) & 1.6 & \(g\) & 11.25 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Design parameter of the MIMO antenna circular polarization. Figure 2: (**a**) Corner square-cut shape of the MIMO antenna circular polarization front view; (**b**) MIMO antenna back view. Surface current distribution explains the performance analysis of isolating the structure over the desired frequency band. Figure 5a,b gives the MIMO-CP surface current distribution with and without a ground slot at resonant frequency achieved by exciting antenna 1 and terminating antenna 2 by the matched load, respectively. When the two patches are arranged on a ground with no slots, on the right side of the coupled antenna element, a high current is induced, while on the left side, a small current is produced when the left side antenna is excited. Figure 5a shows that without using of ground slot, some of the surface currents are coupled from antenna 1 to antenna 2. The proposed ground slot reduces the high mutual coupling between the antenna parts, which explains why mutual coupling occurs. In the case of a MIMO-CP antenna with a ground slot, the currents are especially focused at the corners of the right-side patch and there is no current on the left-side patch, as shown in Figure 5b. The magnetic field is trapped within the ground slot as a result of these phenomena, preventing the surface current from flowing further. As a result of using a slot, electromagnetic energy is prevented from propagating from antenna 1 to antenna 2, resulting in a high level of isolation. Finally, Figure 6 presents the current distributions that are used to determine the proposed antenna's CP generation. The simulated surface current, with a different port excitation at 5.6 GHz is taken for clarification.