## 2 Antenna Design  
### UWB Antenna Element  
The diagrammatic design of the proposed unit antenna element is shown in Figure 1. The unit cell consists of a microstrip line-fed octagonal-shaped resonating element and a rectangular ground plane. An HCSRR is implanted in the octagonal patch of the antenna for notching Wi-MAX (3.5 GHz) and WLAN (5.5 GHz) band frequencies. To obtain better impedance matching, a U-shaped slot is carved from the ground plane of the monopole antenna. The antenna is imprinted on the FR-4 dielectric material of 0.8 mm thickness and relative permittivity of 4.4. An ANSYS HFSS(r) tool is utilized for simulation, designing, and implementation of the designed antenna. The dimension details of the octagonal monopole element are provided in Table 1.  
The antenna design stages are demonstrated in Figure 2. Firstly, an octagonal-shaped radiator (stage-1), fed by a microstrip line of impedance 50 \(\Omega\) was considered, as displayed in Figure 2a. The radiator was characterized by high eccentricity, which is required for supporting multiple modes in UWB applications. The surface below the antenna radiating patch acted as an unbalanced impedance as it was not grounded. A small U-shaped slot was carved from the ground surface (beneath the microstrip feed line) for providing the required impedance matching among the patch and the feed line (stage-2), as demonstrated in Figure 2b. In Figure 2c, a hexagonal SRR was implanted in the antenna radiator to remove the interfering 5.5 GHz (WLAN) band from the UWB (stage-3). In the next stage, as shown in Figure 2d, an additional hexagonal SRR (opposite to the hexagonal SRR in stage-3) was loaded on the radiator to eliminate interfering 3.5 GHz (Wi-MAX) band signals (stage-4). The lengths of the implanted SRR (outer and inner) can be computed as [19].  
\[S_{o}=2\pi s_{1}-w_{5}\approx 0.52\lambda_{go} \tag{1}\]  
\[S_{i}=2\pi s_{3}-w_{4}\approx 0.52\lambda_{gi} \tag{2}\]  
\[\lambda_{g}=\frac{c}{f_{r}}\Bigg{(}\frac{1}{\sqrt{\varepsilon_{r,\,eff}}} \Bigg{)} \tag{3}\]  
\[\varepsilon_{r,\,eff}=\frac{\varepsilon_{r}+1}{2} \tag{4}\]  
\begin{table}
\begin{tabular}{c c c c} \hline
**Parameter** & **Dimension (mm)** & **Parameter** & **Dimension (mm)** \\ \hline \(L\) & 26 & \(w_{1}\) & 1 \\ \(W\) & 24 & \(s_{4}\) & 3.5 \\ \(a\) & 5.7 & \(w_{2}\) & 2.5 \\ \(b\) & 8 & \(w_{3}\) & 0.4 \\ \(f\) & 11.25 & \(w_{4}\) & 1 \\ \(s_{1}\) & 5 & \(w_{5}\) & 1 \\ \(s_{2}\) & 4.5 & \(L_{1}\) & 58 \\ \(s_{3}\) & 4 & \(w_{6}\) & 0.5 \\ \hline \end{tabular}
\end{table}
Table 1: Parameter details of the unit element and multiple-input-multiple-output (MIMO) antenna.  
Figure 1: Proposed octagonal-shaped monopole antenna: (**a**) schematic layout; (**b**) enlarged view of the hexagonal-shaped complementary split-ring resonator (HCSRR).  
where \(\lambda g\) and \(f_{r}\) denote the guided wavelength and notch-band central frequency, respectively, \(c\) denotes the speed of light in vacuum, and \(\varepsilon_{r\ell\!f}\) represents the effective relative permittivity. Figure 3 displays the S\({}_{11}\) characteristics of the design steps of the proposed UWB antenna element. A magnified view of the HCSRR is given in Figure 1b, which contains two concentric hexagonal split rings of different dimensions.  
Figure 3: Simulated reflection coefficients comparison of the evolution steps.  
Figure 2: Octagonal-shaped monopole antenna design steps: (**a**) stage-1; (**b**) stage-2; (**c**) stage-3; (**d**) stage-4.  
Figure 4a,b display the current behavior at notch frequencies 3.5 GHz and 5.5 GHz, correspondingly. From Figure 4a, it can be observed that the concentration of current was higher near the boundaries of the outer hexagonal split-ring, resulting in suppression of the Wi-MAX band. Similarly, Figure 4b shows that a high magnitude current flowed along the boundaries of the inner hexagonal split-ring, which resulted in the elimination of WLAN signals. Therefore, dual band-notched characteristics were achieved by loading an HCSRR in the octagonal radiating patch.