# Quad-Port Multiservice Diversity Antenna for Automotive Applications
## 2 Antenna Design  
### Antenna Element  
Figure 1 shows the layout of the proposed antenna element. The antenna is developed on the FR-4 substrate with relative permittivity of 4.4 and a thickness of 1.6 mm. A simple hexagonal monopole radiator is combined with a 50 \(\Omega\) feeding line and a modified ground plane to form the antenna element. The EM solver CST Microwave Studio(r) is used to perform simulations of the proposed antenna. The size of the antenna element is 19 mm \(\times\) 25 mm.  
The lower band-edge frequency (\(f_{l}\)) of the UWB monopole antenna is calculated as [29]  
\[f_{l}=\frac{7.2}{(l+r+p)\times k} \tag{1}\]  
where \(l\) and \(r\) are the height and width of the antenna, respectively, and \(p\) is the distance between the ground plane and the radiator. The value of the empirical constant (\(k\)) is evaluated as  
\[k=\sqrt[4]{\epsilon_{eff}} \tag{2}\]  
For the proposed antenna, Equation (1) is modified as  
\[f_{l}=\frac{7.2}{(0.295\pi[(a+b)]+p)\times k} \tag{3}\]  
where \(0.295\pi[(a+b)]\) corresponds to the term (1+\(r\)), and \(a\) and \(b\) represent the semi-length and semi-width of the radiator, respectively.  
Furthermore, resonant frequencies of 2.45 GHz and 1.5 GHz are obtained by adding G-shaped and E-shaped strips of length \(\lambda_{0}/2\), respectively. The wavelengths of G-shaped and E-shaped strips are calculated using the dimensions shown in Figure 1c,d. The dimensions of the G-shaped and E-shaped stubs are given in Table 1.  
Figure 1: Layout of the antenna element: (**a**) front view, (**b**) back view, (**c**) G-shaped stub, 2.4 GHz, (**d**) E-shaped stub, 1.5 GHz.  
The development stages of the proposed antenna element are shown in Figure 2. Figure 2a depicts a simple rectangular monopole antenna fed by a microstrip line of 50 \(\Omega\). The reflection coefficient characteristics of the evolution stages are shown in Figure 3. The rectangular monopole antenna exhibits poor impedance matching. In step 2, the edges of the radiator are truncated, and a defect in the ground plane is introduced for impedance matching. Step 3 involves etching the center metal of the radiator in order to reduce the physical size of the antenna. In step 4, an E-shaped strip is added to obtain resonance at 1.5 GHz, and a G-shaped strip is added (in step 5) to achieve additional resonance in the Bluetooth/Wi-Fi/RFID band (2.45 GHz). The addition of resonating stubs causes minor changes in impedance matching, which can be compensated by loading a patch in the ground plane, shown in step 6. Figure 4 shows the simulated and measured reflection coefficients of the proposed antenna element.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Dimension** & **Value (mm)** & **Dimension** & **Value (mm)** \\ \hline \(L1\) & 1.7 & S4 & 6.9 \\ \hline \(L2\) & 2.75 & S5 & 1.7 \\ \hline \(L3\) & 10.6 & S6 & 5.5 \\ \hline \(L4\) & 8.05 & S7 & 1.7 \\ \hline \(L5\) & 22.3 & S8 & 5.5 \\ \hline \(L6\) & 1.9 & S9 & 1.85 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Dimensions of the G-shaped and E-shaped stubs.  
Figure 2: Evolution stages of the antenna element: (**a**) step \(1\), (**b**) step 2, (**c**) step 3, (**d**) step 4, (**e**) step 5, (**f**) step 6.  
The antenna element offers an impedance bandwidth of 3.1 to 10.6 GHz and can be used for UWB automotive applications. The additional resonances (1.5 GHz and 2.45 GHz) can be employed for GPS and RFID/Bluetooth/Wi-Fi applications, respectively.