# A Multi-Coupling 3D Frequency-Selective Surface with High Selectivity and Wide Dual Passband
## 4 Conclusions  
In this work, a 3D-DBFSS is proposed, along with an innovative design concept. The design incorporates three distinct gaps within a single-layer structure, which interact through different coupling mechanisms to form a multi-mode resonant cavity. This resonant configuration results in eight transmission poles and three transmission zeros, providing excellent performance while significantly reducing the overall thickness of the FSS.  
The proposed 3D-DBFSS exhibits several desirable characteristics, including steep edge responses, as well as insensitivity to variations in incidence angle and polarization. Compared to other dual-band FSS designs, the proposed structure has several advantages. Firstly, it provides sufficient bandwidths for both passbands, which are 5.3 GHz and 4 GHz, respectively--adequate for practical applications. Secondly, the selection index K, which quantifies the frequency selectivity, is significantly higher than those of previously surveyed designs. By employing multiple gaps in a single unit cell, the design achieves multiple transmission poles and zeros, providing a novel approach to reducing the thickness of the FSS.  
Furthermore, compared to previous designs, the proposed 3D-DBFSS effectively meets the requirements for dual-frequency communication, featuring dual-wideband operation, sharp roll-off characteristics, and a reduced radar cross-section (RCS). These attributes make the proposed FSS highly suitable for practical dual-band communication  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Reference**} & \multicolumn{2}{c}{**\(\mathbf{BWL_{3 dB}}\) Central**} & \multicolumn{2}{c}{**\(\mathbf{BWL_{3 dB}}\)**} & \multicolumn{2}{c}{**\(\mathbf{BWL_{3 dB}}\) Central**} & \multicolumn{2}{c}{**\(\mathbf{BWL_{3 dB}}\)**} & \multirow{2}{*}{**KL**} & \multirow{2}{*}{**KU**} \\  & \multicolumn{1}{c}{**Frequency (GHz)**} & & & & & \\ \hline
[23] & 2.5 & 0.64 & 5.5 & 1.75 & 35\% & \textless{}35\% \\
[1] & 2.5 & 0.91 & 4.8 & 3.19 & 11.4\% & 33.6\% \\
[24] & 3.3 & 0.5 & 7 & 2.52 & 25\% & 68\% \\
[25] & 3.6 & 0.25 & 4.2 & 0.4 & 24\% & 67\% \\
[26] & 1.1 & 0.7 & 15.1 & 2.4 & 56\% & 32\% \\
[27] & 3 & 0.5 & 13.5 & 1.3 & 12\% & 65\% \\
[28] & 2.5 & 1 & 5 & 1.1 & 27\% & \textless{}27\% \\
[29] & 10 & 1.9 & 24 & 2.9 & 38\% & 34\% \\
[30] & 26 & 5.5 & 58 & 0.2 & 25\% & 65\% \\ This work & 12.55 & 5.3 & 24 & 4 & 82.8\% & 80.2\% \\ \hline \hline \end{tabular}
\end{table}
Table 3: Comparison of the existing dual-band bandpass designs in the literature.  
applications, offering enhanced performance in terms of bandwidth, selectivity, and overall structural efficiency.  
Conceptualization, X.F., R.F., Z.W., N.L., P.O. and M.B.; Methodology, X.F., R.F., Z.W., N.L., P.O. and M.B.; Software, X.F., R.F., Z.W., N.L., P.O. and M.B.; Validation, X.F., R.F., Z.W., N.L., P.O. and M.B.; Formal analysis, X.F., R.F., Z.W., N.L., P.O. and M.B.; Investigation, X.F., R.F., Z.W., N.L., P.O. and M.B.; Resources, X.F., R.F., Z.W., N.L., P.O. and M.B.; Data curation, X.F., R.F., Z.W., N.L., P.O. and M.B.; Writing--original draft, X.F., R.F., Z.W., N.L., P.O. and M.B.; Writing--review & editing, X.F., R.F., Z.W., N.L., P.O. and M.B.; Visualization, X.F., R.F., Z.W., N.L., P.O. and M.B.; Supervision, X.F., R.F., Z.W., N.L., P.O. and M.B.; Project administration, X.F., R.F., Z.W., N.L., P.O. and M.B.; Funding acquisition, X.F., R.F., Z.W., N.L., P.O. and M.B. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Beijing Naturalscience Foundation grant number L243029.  
Not applicable.  
The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.  
The authors declare no conflict of interest.