## 2 Design and Simulation
The substrate used for the hairpin filter design is Rogers' RO4350b, which has negligible dielectric losses at gigahertz frequencies, and its specifications are listed in Table 2. Solid-state switches are employed for switching purposes. An SP4T switch from mini circuits with model number HSWA4-63 DR+ is integrated. It is a low-loss, high-speed, and high-isolation switch. Figures 8 and 9 show the layouts and S-parameter responses of hairpin bandpass filters, which are simulated in an advanced design system (ADS). The layout dimensions for all four proposed filters are mentioned in Table 3. In Figure 8a, the simulation results of BPF1 (2.0-2.2 GHz) along with its layout are shown. BPF1 features a peak insertion loss (IL) of approximately 0.9 dB, with the minimum return loss (RL) below 25 dB in the band. Similarly, Figure 8b illustrates that BPF2 (2.3-2.5 GHz) has a maximum IL of 0.85 dB, accompanied by an acceptable RL. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**SP4T Switch Specifications** & \multicolumn{2}{c}{**Substrate Specifications**} \\ \hline
**Switch Model** & **HSWA4-63 DR+** & **Substrate** & **Rogers’ RO4350b** \\ \hline Isolation (0.9 GHz) & 61 dB & Dielectric Constant & 3.48 \\ Insertion Loss (0.9 GHz) & 0.9 dB & Substrate Thickness & 0.508 mm (20 mils) \\ Switching Speed & 255 ns & Loss Tangent & 0.0037 \\ Rise Time & 100 ns & Metal Thickness & 0.035 mm \\ Fall Time & 100 ns & - & - \\ Operating Temperature & \(-40\)\({}^{\circ}\)C to 105 \({}^{\circ}\)C & - & - \\ RF Power Input & 34 dBm & - & - \\ \hline \hline \end{tabular}
\end{table} 
Table 2: SP4T switch and substrate specifications. Figure 7: Three-dimensional configuration of proposed hairpin filter. The insertion loss (IL) and return loss (RL) characteristics of the third hairpin-based bandpass filter, denoted as BPF3 (3.05-3.3 GHz), are depicted in Figure 9a. It shares the same maximum IL as BPF2, with the minimum RL falling below 20 dB within the designated frequency band. Moving on to the s-parameter outcomes for BPF4, illustrated in Figure 9b, the maximum IL is recorded at 1.6 dB, while the minimum RL remains below 20 dB. Since all of the filter frequencies are apart, there is already a significant level of isolation between the four filters. To be on the safe side, the attenuation is set to be greater than 40 dB after determining the relevant fractional bandwidth of each filter. Each filter's insertion loss is kept in simulation close to zero so that, after integrating switches, their loss can be taken into account to prevent the signal's power loss. Figure 8: Design layout of 2.1 GHz and 2.4 GHz hairpin filter and its simulated results. Figure 9: Design layout of 3.2 GHz and 4.0 GHz hairpin filter and its simulated results. Additionally, each filter's dimensions are displayed alongside the results. Every filter has an inverse relationship between frequency and filter size. Each hairpin is assumed to bend at a 135\({}^{\circ}\) angle. The majority of the filter bank's dimensions remain constant; however, changing certain of them has a significant impact on the frequency and IL of the corresponding filter. After calculations, not all dimensions are accurate. The calculated parameters were entered into ADS and subsequently tuned for the optimum outcomes based on the results. Therefore, optimisation was crucially important in obtaining the results that are displayed. For radar applications, the frequencies for this filter bank are assumed to be in the S-band. This demonstrates that high power is required on the transmitting side of radar for signal transmission, whereas on the receiving side, the radar receives the high-power signal and passes it through the LNA, which lowers the signal's power before it passes through the filter bank, making it the ideal device for use at the receiving side. On the other hand, high-power filters are needed at the transmitting backend, where waveguides can be employed as high-power filters. The strength of the signal is the first factor to be taken into account while discussing radar.