# Compact Quad-Port MIMO Antenna with Ultra-Wideband and High Isolation
## 4 Result and Discussion  
### S-Parameters  
Due to the quasi-similarity of the four cells of the UWB MIMO antenna, the reflection and transmission coefficients with only one port were chosen for the study, and the simulation and test results were analyzed and compared as shown in Figure 10.  
In order to analyze and illustrate the decoupling effect of the MIMO antenna with the introduction of the strips, the S-parameters of the MIMO antenna at port 1 before the inclusion of the gap are also given, as shown in Figure 10a. The two green dashed lines in Figure 10a represent \(-\)10 dB and \(-\)15 dB, respectively, and are marked in the figure for a more visual observation of the operating area of the antenna (S\({}_{11}\leq-\)10 dB, S\({}_{21}\), S\({}_{31}\), S\({}_{41}\leq-\)15 dB). The green area is the \(-\)10 dB impedance bandwidth of the antenna: 1-14.78 GHz, which can cover the UWB communication range of 3.1-10.6 GHz with an absolute bandwidth of 13.78 GHz and a relative bandwidth of 174.65%. It can be seen that the isolations degree between port 1 and the other three ports is \(\geq\)17.31 dB, which meets the requirements of the MIMO antenna for isolation degree. When the antenna is operating at 14.9 GHz, the isolation between ports 1 and 3 has the lowest value of 17.31 dB, which indicates that the diversity of polarization modes can effectively improve the coupling of MIMO antennas, but the improvement results have some limitations.  
The simulated S-parameters of the MIMO antenna at port 1 after adding the decoupling slit are shown in Figure 10b, as can be seen from the reflection coefficient curve, the antenna operates at 1-17 GHz with an absolute bandwidth of 16 GHz and a relative bandwidth of 177.78%. The green dashed line in Figure 10b,c represent the maximum value of the transmission coefficients in the working area, which can be judged as the minimum value of the isolation. From the transmission coefficient, it can be seen that the isolation between the ports all exceeds 23.42 dB, and the isolation effect is improved, especially when the antenna works below 4 GHz and above 14 GHz, the isolation degree is improved obviously, and the simulated isolation degree between different ports is shown in Table 2.  
Figure 10c shows the measurement results after the MIMO antenna is introduced strips. The measured results show that the curve trends of the S-parameter measured  
\begin{table}
\begin{tabular}{c c c} \hline
**Isolation** & **Minimum/dB** & **Maximum/dB** \\ \hline Port 1 to 2 & 23.72 & 39.27 \\ Port 1 to 3 & 23.46 & 46.77 \\ Port 1 to 4 & 23.71 & 37.14 \\ \hline \end{tabular}
\end{table}
Table 2: The simulated isolation of the proposed UWB MIMO antenna.  
Figure 10: Reflection coefficient and transmission coefficient of UWB MIMO antenna at port 1: (a) simulation without introduced strips; (b) simulated results with strips; (c) measured results with strips.  
and simulated are basically matched, with a small offset due to manual operation within the error range. The test data show that the MIMO antenna meets: S\({}_{11}<\,-10\) dB and S\({}_{21},\)S\({}_{31},\)S\({}_{41}\,\leq\,-27.8\) dB in the operation band of 1-17 GHz. The measured S-parameter of the proposed antenna also indicates that this antenna is a UWB MIMO antenna with high isolation and good performance. The proposed MIMO antenna can be used not only in ultra-broadband communication (3.1-10.6 GHz) systems, but also can be used for satellite mobile communication in a lower frequency band (1-3.1 GHz) and for fixed communication in the higher frequency band (10.6-17 GHz).