# Wideband Ring-Monopole Flexible Antenna with Stub for WLAN/C-Band/X-Band Applications
## 2 Antenna Design and Parametric Study  
### Antenna Design  
The proposed antenna is shown in Figure 1, and Table 1 shows the length of each element of the antenna. The feeding method uses a CPW feeding line with a transmission line width of 1.5 mm (Wf) and a distance between the transmission line and the ground of 0.2 mm (g) on both sides. The step structure of the ground plane helps impedance matching with an optimized length by parametric study and the stub is bar-shaped with a length of 10.5 mm (Wsh1) and a height of 1.9 mm (Lsh1). The t-strip line is designed by 10.5 mm (Lt1) bars and 8.9 mm (Wt2) bars. The antenna uses polyimide with a thickness of 0.2 mm and a dielectric constant of 3.5 as a substrate and has a size of 32 \(\times\) 40 mm\({}^{2}\). It is designed to operate in WLAN/X-band/C-band using ANSYS HFSS.  
The antenna was designed according to the procedure shown in Figure 2, and the S-parameter of the antenna according to each procedure is shown in Figure 3. As shown in Figure 2a, antenna 1 was designed with a ring-type monopole using a CPW feeding method. The ring-type monopole can have broadband characteristics and is more resistant to cracks than the solid type. As shown in Figure 3, antenna 1 operates in the 2.46-2.55 GHz, 3.96-5.11 GHz. Thus, it was necessary to improve the bandwidth of antenna 1. To improve the impedance matching, a slot was added to the ring of antenna 2 in Figure 2b. As shown in Figure 3, the S-parameter of the antenna 1 was only \(-\)15 dB at 4.6 GHz, but the S-parameter of antenna 2 was \(-\)25 dB at 4.6 GHz. In antenna 3 of Figure 2c, a step structure was added to the ground for impedance matching [30]. The S-parameter of antenna 3 shows that it operates at 2.33-3.02 GHz, 3.92-5.22 GHz, 7.33-9.40 GHz due to the step structure of the ground plane that achieves impedance matching. After that, the stub was placed between the ring and the feed part as shown in antenna 4 of Figure 2d. The proposed antenna usesa stub to make a resonance point between the existing C-band and X-band to achieve a wide bandwidth without a notch [31; 32]. When the stub operates as a \(\lambda\)/4 monopole at 5-7.5 GHz, the antenna can cover the entire C-band and X-band. The length of the stub is designed to include both C-band and X-band. As a result, the S-parameter of antenna 4 shows that it operates at 2.38-3.27 GHz and 4.95-11.65 GHz. In antenna 5 of Figure 2e, a t-strip line inside a ring-shaped monopole was added to improve the bandwidth. The t-strip line is designed to have a resonance point at 3.5 GHz [33]. Note that antenna 5 finally operated at WLAN/C-band/X-band and 5G sub-bands (3.4-3.7 GHz).  
Figure 4 is the current distribution of the final designed antenna, which explains the operating principle of the proposed antenna. Figure 4a shows the current distribution of 2.4 GHz. It can be seen that the ring part is a radiating element in the 2.4 GHz band, the lowest frequency band of the proposed antenna because of a relatively strong current in the ring part unlike the other frequencies in Figure 4b,c. The proposed antenna did not operate at 3.5 GHz before the t-strip line was added. Figure 4b shows the current distribution of 3.5 GHz and a strong current flow in the t-strip line, which means that the proposed antenna operates at 3.5 GHz due to the t-strip line. The stub was designed to be 10.5 mm long to operate as a \(\lambda\)/4 monopole in 5-7.5 GHz. Figure 4c shows a strong current flowing through the stub at 7.5 GHz. This means that the stub works as designed.  
\begin{table}
\begin{tabular}{c c c c c c} \hline W & L & Wt1 & Lt1 & Wt2 & Lt2 \\ \hline
32 & 40 & 0.5 & 10.5 & 8.9 & 0.6 \\ \hline Wg & Lg & r1 & r2 & Wsh1 & Lsh1 \\ \hline
2.9 & 2.4 & 28.4 & 16.8 & 10.5 & 1.9 \\ \hline Lc1 & Lc2 & Lc3 & Lc4 & Wc1 & Wc2 \\ \hline
1.9 & 5 & 2.4 & 1.5 & 7.8 & 1.5 \\ \hline Wc3 & Ls1 & Wf & Lf & g & \\ \hline
5.75 & 0.7 & 1.5 & 11.2 & 0.2 & \\ \hline \end{tabular}
\end{table}
Table 1: Antenna parameters (unit: mm).  
Figure 1: Proposed antenna structure.