# A Multi-Coupling 3D Frequency-Selective Surface with High Selectivity and Wide Dual Passband  
[PERSON]  
1 School of Integrated Circuit Science and Engineering, Beihang University, Beijing 100191, China; [EMAIL_ADDRESS] (X.F. ); 173256197308163.com (Z.  
edges. To address these limitations, an additional connection structure can be introduced between the layers, resulting in the formation of a three-dimensional FSS (3D-FSS). This 3D configuration has gained significant attention due to the additional degrees of freedom it provides in the design process, enabling enhanced performance characteristics. Specifically, 3D-FSS designs are capable of achieving ultra-wide passbands, insensitivity to both angle and polarization, flat in-band filter responses, steep edges, and wide out-of-band rejection. Existing 3D-FSS designs are primarily based on several methodologies, including the waveguide principle, equivalent circuit modeling, and effective medium concepts. In these designs, inter-layer structures serve a variety of roles, such as acting as waveguide walls for electromagnetic confinement, providing resonant paths as transmission lines, or functioning as materials with left-handed properties to achieve a desired dielectric constant. These inter-layer structures are typically implemented using periodic arrays of microstrip lines, metallic plates, via holes, or resonant metallic lines, contributing to enhanced electromagnetic performance. With the rapid advancement of wireless communication technologies, single-frequency communication equipment has become increasingly insufficient due to the limited spectrum resources available. Consequently, researchers have developed wide passband FSSs with superior performance to address these challenges [13; 14; 15]. Furthermore, enabling devices to operate across multiple frequency bands has become a major research focus, resulting in significant interest in dual-frequency and multi-frequency FSS designs [16; 17; 18; 19; 20]. Currently, multiple passbands are largely achieved through the use of multilayer FSS designs; however, these approaches often suffer from narrow bandwidths and insufficiently steep edges, limiting their practical utility. The miniaturized conical FSS designed in reference [21] exhibits excellent angular stability (80\({}^{\circ}\) in planar state and 75\({}^{\circ}\) in conical state) and dual-polarization compatibility in both planar and conical configurations. This work fills the research gap in conical bandpass FSS and provides a new scheme for the conformal design of radomes. The consistency between the measured and simulated results verifies its engineering feasibility and highlights its significant practical application value. Ref. [22] proposes a design of broadband dual-polarization switchable active frequency-selective surface (AFSS) based on PIN diodes. By controlling the bias states of PIN diodes, dynamic switching between the reflection mode and transmission mode is achieved. The design features a wide bandwidth and dual-polarization characteristics, showing application potential in stealth technology and other fields. In this work, a dual-passband 3D-FSS unit cell with multiple coupling structures is proposed. In the second part of this paper, the design and simulation of a dual-passband 3D frequency-selective surface (3D-FSS) unit cell with multiple coupling structures are carried out. In the third part, a 3D-DBFSS prototype composed of 10 \(\times\) 8 units is fabricated. Due to the limitations of 3D printing technology, there are differences between the actual fabrication parameters and the simulation parameters. The equipment, environment, and steps for measurement are introduced. The simulation and measurement results are compared, and the reasons for the differences are analyzed. Finally, the design innovation points of the 3D-DBFSS are summarized, and its excellent characteristics, such as steep edge response, insensitivity to the incident angle, and polarization, are generalized.