# Triple-Band Notched Ultra-Wideband Microstrip MIMO Antenna with Bluetooth Band
## 3 Design of Ultra-Wideband MIMO Antenna  
Four proposed UWB antenna elements are perpendicularly placed to each other in order to design a MIMO structure, as shown in Figure 13a. In Figure 13a, the four UWB-MIMO antennas are placed with separate ground planes (i.e., non-connected ground planes), where the port isolation characteristics seem very high [16; 17]. The high isolation characteristics for the separated ground planes appear since there is no common coupling current passing through the ground panes (GPs). In practical systems, MIMO antennas with a separated ground plane topology are not preferable since the signal levels at all ground planes (i.e., zero volts) are not the same. UWB-MIMO antennas with a common ground plane configuration (i.e., connected ground planes) are utilized in order to increase the system reliability, as shown in Figure 13b. Here, a square metallic connection is tied to the four ground planes by four metallic strips. Table 2 shows the values of the parameters for the ground plane connection. Both UWB-MIMO antennas with the separate ground plane (i.e., non-connected GP) configuration and UWB-MIMO antennas with the common ground plane (i.e., connected GP) configuration are simulated using the CST microwave studio simulator. Figure 14a shows the four reflection coefficients (i.e., \(1\,S_{ii}\,|\) in dB, \(i\) is 1, 2, 3, and 4) at ports 1, 2, 3, and 4 of the UWB-MIMO antenna with the separate ground plane (i.e., non-connected GP) configuration. It is shown that the FCC-UWB spectrum with triple band notches was obtained in addition to the Bluetooth band. On the other hand, Figure 14b shows the four reflection coefficients (i.e., \(1\,S_{ii}\,|\) in dB, \(i\) = 1, 2, 3, and 4) at ports 1, 2, 3, and 4 of the UWB-MIMO antenna with the common ground plane (i.e., connected GP) configuration. It is noticed that the FCC-UWB spectrum with triple band notches was obtained but there is a mismatching characteristic at the Bluetooth band due to the coupling between the UWB antenna elements. \begin{table}
\begin{tabular}{l c c c c c} \hline \hline
**Parameter** & **Value (mm)** & **Parameter** & **Value (mm)** & **Parameter** & **Value (mm)** \\ \hline \(L_{\text{G3}}\) & 9.2 & \(L_{\text{H1}}\) & 8.5 & \(L_{\text{D1}}\) & 5 \\ \(L_{\text{G4}}\) & 5 & \(L_{\text{H2}}\) & 16 & \(L_{\text{D2}}\) & 6.1 \\ \(L  
In order to overcome the problem mentioned above, a defective ground structure (DGS) band stop filter (BSF) was proposed and designed at the Bluetooth band (i.e., 2.4-2.483 GHz). The two-port DGS-BSF was simulated in CST microwave studio, as shown in Figure 15. The DGS-BSF consists of two stages of two defected coiled slits on the ground plane underneath a 50-ohm microstrip transmission line. The length of each slit is 27.6 mm (0.41 \(\lambda_{g}\)) while the width of the slit is 0.2 mm. Figure 15 shows the reflection coefficients (i.e., \(\mid S_{11}\mid\) and \(\mid S_{22}\mid\) in dB) and the transmission coefficients (i.e., \(\mid S_{21}\mid\) and \(\mid S_{12}\mid\) in dB) for the DGS band stop filter (BSF). It is shown from the figure that there is a band-stop characteristic over the Bluetooth operating frequency (2.4-2.483 GHz). Four DGS-BSFs were placed and embedded between the four ground planes of four UWB antennas and connected together by an H-shaped strip ground plane, as shown in Figure 16. Table 2 shows the values of the parameters for the ground plane connection and BSF. In order to demonstrate the effect of the four DGS-BSF, the surface current distributions for both the UWB-MIMO antenna without DGS and the UWB-MIMO antenna with DGS at\(f\) = 2.44 GHz are shown in Figure 17. In Figure 17, as the first antenna is excited, the surface current is highly induced to the fourth antenna via the metallic connection between them. This induction causes a mismatch characteristic at the Bluetooth band. Meanwhile, in Figure 17, as the first antenna is excited, there is no surface current induced by any antenna elements in MIMO thanks to the fourth DGS-BSF between them.