# An Ultra-Wideband Vivaldi Antenna System for Long-Distance Electromagnetic Detection
## 2 Antenna Configuration Design and Performance
### Ultra-Wideband Narrow Beam Antenna  
To ensure the detection accuracy of the detection system, the receiving antenna should be featured by owing ultra-wideband, narrow beam and high gain performance. Therefore, the beam width of the antenna unit was required to be reduced. For Vivaldi antennae, the main solutions for reducing the antenna beam width included adding a director and slotting the metal patch. It should be noted that both methods will affect the standing wave ratio of the antenna. The antenna adopting a gradual slotting method yields better results in the direction of the main beam when compared with the method of ordinary slotting, and such a method can help increase the gain of the antenna. At the same time, the triangular director of the antenna can also help increase the gain of the antenna and reduce the beam width of the antenna. To improve radiation characteristics of the antenna, a series of symmetric slots were installed in the extremities of the antenna. The modifications are shown in Figure 4. The modified antenna with optimized slots can secure a compact structure with improved radiation patterns and better impedance bandwidth. Each slot operates as an RLC resonator where the resonant wavelength can be estimated by the following expression:  
\[l=\frac{\lambda_{0}}{4}\sqrt{\frac{2}{1+\varepsilon_{r}}} \tag{4}\]  
where \(l\) is the length of slot and \(\varepsilon_{r}\) is dielectric constant of the substrate. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Frequency/GHz** & **HPBW (Measured)/\({}^{o}\)** & **HPBW (Simulated)/\({}^{o}\)** \\ \hline
2.5 GHz & 108\({}^{\circ}\) & 122.3\({}^{\circ}\) \\
3 GHz & 122\({}^{\circ}\) & 128.3\({}^{\circ}\) \\
3.5 GHz & 109\({}^{\circ}\) & 120\({}^{\circ}\) \\
4 GHz & 90\({}^{\circ}\) & 91.9\({}^{\circ}\) \\
5 GHz & 95\({}^{\circ}\) & 104.7\({}^{\circ}\) \\
6 GHz & 86\({}^{\circ}\) & 91.8\({}^{\circ}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The beam width of the transmitting antenna at different frequency. Figure 3: Simulated E-plane radiation patterns of the transmitting antenna with structure at (**a**) 2.5 GHz; (**b**) 3 GHz; (**c**) 3.5 GHz; and (**d**) 4 GHz; (**e**) 5 GHz; (**f**) 6 GHz. After CST modelling and multiple simulation analysis of the parameters, the final optimized triangular slot height registered 8 mm and the width was recorded as 4 mm. The length of the triangle slot adopted an arithmetic sequence, and the minimum slot height is 5 m. The height difference between two adjacent rectangles is 3 mm. At the same time, three additional triangular slots are opened near the wide side of the antenna, which can contribute a lot without affecting other electrical properties of the antenna. The final optimization results of the various parameters of narrow-beam antenna are shown in Table 3.  
The final model of the designed ultra-wideband narrow-beam Vivaldi antenna was simulated. As shown in Figure 5a, the return loss of the simulated narrow beam Vivaldi antenna was adjusted to \(-\)10 dB over the frequency band from 2.5 to 8.5 GHz. The simulated E-plane radiation pattern of the antenna at different frequencies are shown in Figure 6. In addition, the HPBW of the antenna with structures at different frequencies are shown in Table 4. It also lists out the comparison between the proposed antenna and other published related antennas. From Table 4, the proposed Vivaldi narrow-beam antenna structure was smaller in dimension than the antennae as reported in [19; 24; 25; 26; 27; 28]. In addition, the comparative results show the improvement in maintaining an HPBW less than 70\({}^{\circ}\) in the frequency range between 2.5 and 8 GHz. The HPBW of proposed antenna is smaller than the antennas in [19; 24; 27], and the bandwidth is larger than the antennas in the others. Figure 4: Vivaldi narrow-beam antenna structure (front and back).