# A MIMO Antenna with High Gain and Enhanced Isolation for WLAN Applications
## 3 Results and Discussion  
As shown in Figure 7a, the \(-10\) dB impedance bandwidths of the measured results are 0.65 GHz (2.25-2.9 GHz) and 0.975 GHz (5.05-6.025 GHz), which can cover IEEE 802.11 a/b/g/n/p/ac/ax (2.4-2.497 GHz and 5.15-5.85 GHz) frequency bands. Figure 7b depicts the measured and simulated S21 at both frequency bands. The values of S21 are smaller than \(-19.5\) dB and \(-19.3\) dB at the lower and the higher operating frequency bands, respectively.  
At the frequencies of 2.45 GHz and 5.5 GHz, the measured radiation patterns of the fabricated antenna are illustrated in Figure 8. It can be seen from the distribution circle in Figure 8 that the superposition gain and polarization effects of the radiation field generated by the same two ports are excellent. The co-polarization is illustrated by a black solid line, the cross-polarization by a red dotted line. The antenna has a wide beam width at the desired bands, which is suitable for WLAN applications.  
Figure 8: Radiation shapes of the fabricated antenna: (**a**) E-plane at 2.45 GHz, (**b**) H-plane at 2.45 GHz, (**c**) E-plane at 5.5 GHz, and (**d**) H-plane at 5.5 GHz.  
Figure 7: Simulated and measured S-parameters of presented antenna. (**a**) \(S_{11}\); (**b**) \(S_{21}\).  
The envelope correlation coefficient (ECC) is an important factor used to assess the level of correlation between the communication channels [12; 13]. In a MIMO system, the ECC indicates how multiple antennas are independent in the radial direction. To some extent, it can explain the correlation between radiation elements. The calculated ECC of the MIMO antenna are presented in Figure 9a. The ECC parameters of the fabricated antenna can be obtained by Equation (1):  
\[ECC=\frac{\left|S_{11}^{*}S_{12}^{\prime}+S_{21}^{*}S_{22}^{\prime}\right|^{2}}{ \Big{(}1-\Big{(}\left|S_{11}\right|^{2}+\left|S_{21}\right|^{2}\Big{)}\Big{)} \Big{(}1-\Big{(}\left|S_{22}\right|^{2}+\left|S_{12}\right|^{2}\Big{)}\Big{)}} \tag{1}\]  
where \(S^{*}{}_{11}\) and \(S^{*}{}_{21}\) are the imaginary components of S-parameters \(S_{11}\) and \(S_{21}\), respectively, and \(S^{\prime}{}_{12}\) and \(S^{\prime}{}_{22}\) are the real components of S-parameters \(S_{12}\) and \(S_{22}\), respectively. This shows that the calculated ECC values are very low in both operating frequency bands. The maximum value of ECC at 2.45 GHz and 5.5 GHz is 0.026.  
The total active reflection coefficient (TARC) [2] can be used to judge the impact on impedance bandwidth when adjacent antennas work at the same time. The TARC is an important index for reflecting the performance of the MIMO antenna. The TARC parameters of the fabricated antenna can be obtained by Equation (2):  
\[TARC=\sqrt{\frac{\left(S_{11}+S_{12}\right)^{2}+\left(S_{21}+S_{22}\right)^{2} }{2}} \tag{2}\]  
The calculated TARC values are presented in Figure 9a. It is observed that the values of TARC in both operating frequency bands are less than \(-15\) dB, which indicates that the MIMO system is stable.  
The measured results of peak gain and efficiency are demonstrated in Figure 9b. The measured efficiencies are larger than 61.4% and 65.6% at the lower and higher operating frequency bands, respectively. The measured peak gains are 2.6 dBi and 3.8 dBi at the lower and higher operating frequency bands, respectively.  
In Table 2, the summarized parameters include the operating frequency band, peak gain, minimum isolation and size of the designed antenna. Compared with recently published dual-band MIMO antennas, the presented antenna shows a significant improvement on peak gain. With similar occupied size, the presented antenna shows better isolation and ECCs. With similar peak gain, the presented antenna shows a smaller occupied size. The designed antenna has a better comprehensive performance.  
Figure 9: (a) Calculated ECC of the fabricated antenna and (b) measured peak gain and efficiency.