# Wideband-Narrowband Switchable Tapered Slot Antenna for Breast Cancer Diagnosis and Treatment
## 2 Design of the Proposed Frequency Switchable Tapered Slot Antenna
### Design of the Narrowband Resonator  
The narrowband characteristic in wideband TSA can be achieved by inducing current to the narrowband resonator in the back layer of the TSA. Figure 4a-d shows the typical shapes for frequency switching; bent dipole, square ring, round ring, and meander line. In terms of losses and size, the narrowband resonator must have low losses and be small in size. To verify the resonator characteristics, a radio frequency (RF) signal was induced between the vertical slot on the ground plane, and the coupling energy was induced to the resonator on the top layer. Table 1 shows the results for the simulated insertion loss (S21) and the size of resonators, respectively. The S21 results were similar but slightly lower for the round ring and meander line. Moreover, the size of the meander line resonator was smaller than the other shapes. Therefore, we selected the meander line as a narrowband resonator, which was suitable for electrically small antennas with closely coupled element shapes [22].  
Figure 4: Resonance shapes (**a**) bent dipole, (**b**) square ring, (**c**) round ring, and (**d**) meander line.  
Figure 3: Reflection coefficients of conventional TSA with and without flared slots.  
The frequency response of the meander line structure is defined by the width, the number of turns, and the gap size of the meander line, which can be represented by an equivalent lumped circuit model [23; 24]. In Figure 5a, the meander line resonator, which was designed on the back of the slotline, induces a coupling current to achieve band-pass characteristics at 2.45 GHz. The meander line resonator is represented by the LC equivalent sub-circuit and will be discussed later.  
The meander line structure was based on the cascade connection of several individual line segments. The equivalent lumped circuit of the transmission line segments could be calculated using the T-equivalent network from the equations provided by [23] to reasonably approximate relative changes in the resonant frequency. Changes in the narrowband resonance in terms of the number of turns of the meander line and line length are represented in Figure 6a and b, respectively. For the proposed antenna, both a compact size of 6.8 mm \(\times\) 8.1 mm (0.11\(\lambda_{\text{r}}\)\(\times\) 0.13\(\lambda_{\text{r}}\)) and band-pass filter operation could be achieved by adopting the meander line structure, the S-parameter results of which are shown in Figure 5b, which verified the band-pass operation at 2.45 GHz.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Shapes** & **Insertion Loss (S21), dB** & **Size (\(\lambda_{\text{r}}\): Effective Wavelength at 2.45 GHz)** \\ \hline Bent dipole & \(-1.71\) & \(15\) mm \(\times\) 16.25 mm (0.25\(\lambda_{\text{r}}\)\(\times\) 0.27\(\lambda_{\text{r}}\)) \\ Square ring & \(-1.67\) & \(10.5\) mm \(\times\) 10.5 mm (0.178\(\lambda_{\text{r}}\)\(\times\) 0.178\(\lambda_{\text{r}}\)) \\ Round ring & \(-1.65\) & \(7.8\) mm \(\times\) 7.8 mm (0.132\(\lambda_{\text{r}}\)\(\times\) 0.132\(\lambda_{\text{r}}\)) \\ Meander line & \(-1.65\) & \(7.6\) mm \(\times\) 7.3 mm (0.129\(\lambda_{\text{r}}\)\(\times\) 0.124\(\lambda_{\text{r}}\)) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Characteristics of narrowband resonators.  
Figure 5: Meander line resonator with band pass characteristics: (**a**) Geometry of the meander line structure and (**b**) S-parameter characteristics.  
Figure 6: The reflection coefficient charts in terms of (**a**) the number of turns and (**b**) the vertical length of the meander line.