# Ultrawideband Low-Profile and Miniaturized Spoof Plasmonic Vivaldi Antenna for Base Station
## 3 Design of Plasmonic Antipodal Vivaldi Antenna  
The typical AVA is illustrated in Figure 3a, which is denoted by Structure A, where the metal layers are on both sides of the 0.8-mm-thickness F4B substrate (\(\varepsilon_{r}=2.65,\leavevmode\
obreak\ tan\delta=0.001\)) with a dimension of \(94\leavevmode\
obreak\ \mathrm{mm}\times 70\leavevmode\
obreak\ \mathrm{mm}\). A microstrip feeding line is adopted for broadband impendence matching. The port width of the microstrip feeding line is fixed to 1.35 mm. The exponential profile curves employed in this design can be calculated by the following equations:  
\[y=c_{1}e^{Rx}+c_{2} \tag{1}\]  
\[c_{1}=\frac{y_{2}-y_{1}}{e^{Rx_{2}}-e^{Rx_{1}}} \tag{2}\]  
\[c_{2}=\frac{y_{1}e^{Rx_{2}}-y_{2}e^{Rx_{1}}}{e^{Rx_{2}}-e^{Rx_{1}}} \tag{3}\]  
The tentative plasmonic AVA is shown in Figure 3b, which is named as Structure B. The final plasmonic AVA is shown in Figure 3c, which is denoted by Structure C, where periodic elliptic grooves are cut from the feeding section to the terminal of the antenna for both structures. Furthermore, the rectangular gradual grooves with a width of 1 mm around the feeding section are further etched to realize the miniaturization of the AVA for Structure C. The detailed structures of the gradual grooves are shown in the insets of Figure 3b,c. The numerical simulations were performed by using CST Microwave Studio. The simulated return losses changing with the grooves are shown in Figure 4. First, from Figure 4a, it can be observed that the impedance matching at lower frequencies is better for Structure C, compared with Structure  
Figure 3: **(a)** The typical antipodal Vivaldi antenna (AVA). **(b)** The tentative plasmonic AVA. **(c)** The final plasmonic AVA. The dimensions of the antenna: \(R=0.16\), \(h=94\leavevmode\
obreak\ \mathrm{mm}\), \(w=70\leavevmode\
obreak\ \mathrm{mm}\), \(l_{1}=26\leavevmode\
obreak\ \mathrm{mm}\), \(d_{1}=2.5\leavevmode\
obreak\ \mathrm{mm}\), \(d_{2}=1.25\leavevmode\
obreak\ \mathrm{mm}\), \(l_{2}=8.02\leavevmode\
obreak\ \mathrm{mm}\), and \(d_{3}=2\leavevmode\
obreak\ \mathrm{mm}\). Figure 2: **(a)** The efficiencies of the typical monopole antenna with different lengths (\(h\)); **(b)** the efficiencies of the plasmonic monopole antenna with different groove depths (\(h=25\leavevmode\
obreak\ \mathrm{mm}\)); and **(c)** the efficiencies of the plasmonic monopole antenna when the groove depth \(b\) is \(0\leavevmode\
obreak\ \mathrm{mm}\), \(1.4\leavevmode\
obreak\ \mathrm{mm}\), and \(3.6\leavevmode\
obreak\ \mathrm{mm}\). B, and the operating frequency red-shifts. Thus, loading the rectangular gradual grooves can make the AVA further miniaturized. Then, the typical parameters of structures B and C are the maximum groove depth \(l_{1}\) and the groove gap \(d_{2}\). It can be seen that the operating frequency red-shifts when the maximum groove depth \(l_{1}\) is bigger, while the groove gap \(d_{2}\) mainly affects the input impedance at higher frequency. The optimized maximum groove depth \(l_{1}\) and the groove gap \(d_{2}\) are set to be 26 mm and 1.25 mm, respectively, to make sure that the lowest operating frequency is 1.8 GHz when the \(S_{11}\) is not larger than \(-\)10 dB. It is expected that the etched grooves would change the surface currents and then the effective impedance. In order to understand the operating principle of the AVA and plasmonic AVA, surface current distributions at 1.8 GHz, 4 GHz, and 6 GHz are illustrated in Figure 5. First, it can be seen that the surface currents distribution at the region A of the AVA is larger than that of the plasmonic AVA at 1.8 GHz. After loading the periodic slots, electromagnetic waves are concentrated at the end of the coupling section and the inner end of the tapered slots (region B), as shown in Figure 5a. Hence, radiated electromagnetic energy can be concentrated in the axial direction of the tapered slot (\(y\)-direction). The same thing is true for the cases of the plasmonic AVA at 4 GHz and 6 GHz, as shown in Figure 5b,c.