## 3 Design of the Wide Passband SIW-SSPP Filters with Multi-Sub-Bands
### The Wide Bandpass Filter with Multi-Sub-Bandpass
#### 3.2.2 Tri- Sub-Bandpass Filter Loaded with Dual CSRR Structures  
After realizing the dual-passband filter, this section introduces an additional CSRR with different parameters loaded on the back of the filter, following the same procedure. As illustrated in Figure 7a to obtain the third sub-passband and verify whether this method can achieve further band segmentation. The final structure of the filter is presented in Figure 7b. \(move_{x3}\) = 14 mm and \(move_{y3}\) =1.38 mm represent the distances from the CSRRs to the center point, while \(r_{3}\) = 0.6 mm and \(r_{4}\) = 0.4 mm denote the outer and inner radii of the newly added CSRR, respectively. \(g_{3}\) = 0.1 mm specifies the opening width, and \(d_{2}\) = 0.08 mm indicates the ring spacing. The detailed parameters of the new structure are summarized in Table 2, and along with the data in this table, all the parameters of the filters are provided. Considering the inherent discrepancies between theoretical simulations and actual measurements, the data in the tables have been optimized through parameter tuning with CST simulations to more accurately represent the device performance.  
The S-parameter results for the tri-sub-bandpass filter are presented in Figure 7d. Within the 24 GHz-25.3 GHz frequency range, S\({}_{11}\) is below \(-13\) dB with an insertion loss of 0.5 dB; in the 26 GHz-27.9 GHz range, S\({}_{11}\) is below \(-13.5\) dB with an insertion loss of 0.7 dB; and in the 28.6 GHz-30 GHz range, S\({}_{11}\) is below \(-12\) dB with an insertion loss of 0.8 dB. The loading of the two CSRRs splits the original single passband into three passbands, offering a feasible method to suppress unwanted signals in wireless communication systems and improve system stability. Figure 7c shows the surface current of the tri-bandpass filter  
\begin{table}
\begin{tabular}{c c c c c c} \hline \multicolumn{2}{c}{**Locations**} & \multicolumn{2}{c}{**Archimedean Spiral Parameters**} & \multicolumn{2}{c}{**CSRRs Parameters**} \\ \hline \(move_{x1}\), \(move_{y1}\) & 8.5, 1.4 & \(r_{Acc1}\) & 0.63 & \(r_{1}\), \(r_{2}\) & 0.65, 0.45 \\ \(move_{x2}\), \(move_{y2}\) & 16, 1.45 & \(r_{Acc2}\) & 0.21 & \(r_{3}\), \(r_{4}\) & 0.6, 0.4  
at different frequency points. It can be observed that the proposed structure effectively suppresses current outside the upper and lower cutoff frequencies of the passbands, as well as in the frequency ranges segmented by the loaded CSRRs, while allowing current to pass freely within the three passbands. For example, at around 25.5 GHz and 28.5 GHz, the corresponding CSRRs exhibit significant resonance. Based on the S-parameter plots and surface current analysis, it is demonstrated that the design, built upon the SIW and comb-shaped SSPP structure, optimizes the passband and high-frequency cutoff characteristics by loading Archimedean spirals, while the loading of two types CSRRs enables triad bandpass operation.  
Figure 7: Tri-band filter loaded with two CSRRs. (**a**) schematic structure; (**b**) overall structure; (**c**) surface current; (**d**) S-parameters.