# An Ultra-Wideband Vivaldi Antenna System for Long-Distance Electromagnetic Detection
## 2 Antenna Configuration Design and Performance  
### Ultra-Wideband Wide Beam Antenna  
In general, the bandwidth of a traditional Vivaldi antenna is determined by the transition from the feeding microstrip line to the slot line and the dimensions of the antenna. For the planar slit gradient antenna, the spacing of the narrow end of the slot line opening determines the highest working frequency, while the spacing of the wide end determines the lowest working frequency [22]. Considering that the working frequency range is 2.5-4 GHz, the end width of the slot line can be set as 50 mm and the starting end width can be set as 2 mm. Since the radiation of the antenna is generated by the current extending along the gradient slots on both sides [23], widening the width of the antenna will make the current flow through a longer path and generate a wider main beam. However, it also affects the impedance of the antenna. By further optimized simulation, the width of the antenna's metal sheet can be obtained.  
Figure 1 shows the configuration of the proposed transmitting antenna. The antenna is fabricated on a 150 \(\times\) 150 \(\times\) 1.6 mm FR4 substrate with a dielectric constant of 4.3. The structure of the Vivaldi antenna is composed by dielectric substrate, metal ground plane, and feeding microstrip transmission line. The exponential tapered slot, which is on the ground plane, can be expressed as:  
\[y=C_{1}e^{ax}+C_{2} \tag{1}\]  
\[C_{1}=\frac{y_{2}-y_{1}}{e^{ax_{2}}-e^{ax_{1}}} \tag{2}\]  
\[C_{1}=\frac{y_{2}-y_{1}}{e^{ax_{2}}-e^{ax_{1}}} \tag{3}\]  
where (\(x_{1}\), \(y_{1}\)), (\(x_{2}\), \(y_{2}\)) are the peak and bottom point, respectively, of the exponential tapered shape and \(a\) is the exponential factor of the antenna. The optimized dimensions of the proposed antenna are tabulated in Table 1.  
Xd and Yd are the short axis and long axis of the elliptical resonator cavity, respectively. The values l3 and wh are the length and width of the rectangular transitional slot, respectively. The value l4 is the length of the gradient slot, while w3 is the widest width of the gradient slot.  
After optimizing the parameters of feed line structure, the working frequency range was 1.4-8 GHz. As shown in Figure 2a, the return loss of the simulated and measured antenna was adjusted to \(-\)10 dB over the frequency band from 1.4 GHz to 8 GHz. Furthermore, the simulated and measured normalized E-plane radiation patterns of the antennae operating at different frequencies are shown in Figure 3. Higher-order modes are generated at higher frequencies, resulting in some ripples in the radiation patterns, which is observed in Figure 3. The measured and simulated beam width of the transmitting antenna with structure at different frequencies are shown in Table 2. The antenna is a wide beam antenna and the half-power beamwidth (HPBW) can be maintained above 90\({}^{\circ}\) in the frequency range 2.5-5 GHz, and the maximum beam width is 122\({}^{\circ}\). What is worth mentioning is that the gain is shown in Figure 2b, which is greater than 3.26 dBi and that the directivity consistency is good.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Xd & 32.0 \\ Yd & 12.0 \\ l3 & 22.8 \\ wh & 1.0 \\ l4 & 112.5 \\ w3 & 48.0 \\ w & 150.0 \\ \(1\) & 150.0 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Transmitting Antenna parameters (mm).  
Figure 1: Structure of the transmitting antenna.  
Figure 3: _Cont_.  
Figure 2: (**a**) Return loss of the measured and simulated transmitting antenna. (**b**) The gain of the simulated transmitting antenna.