# A Multi-Coupling 3D Frequency-Selective Surface with High Selectivity and Wide Dual Passband
## 3 Fabrication and Measurement  
To validate the effectiveness of the proposed design, a prototype of the dBFSS, consisting of 10 \(\times\) 8 elements, was fabricated with a total size of 96 \(\times\) 88 \(\times\) 21 mm\({}^{2}\). During the manufacturing process, due to the limitations of the 3D printing process and cost constraints, we encountered significant difficulties in realizing a structure with smaller dimensions. To solve this problem, we removed the substrate and enlarged the structure. The complete structure and measurement setup are illustrated in Figure 5. The parameters of the fabricated model used for the measurement are provided in Table 2. Due to the fabrication limitation, the dielectric material was removed from the 3D-DBFSS for the experimental setup. The same parameters and conditions were used in the simulation for comparative purposes. The 3D-DBFSS was enclosed with absorbers, and a vector network analyzer (VNA) was employed to minimize unwanted transmissions from the edges of the dBFSS structure. For the measurement process, two horn antennas were used in a microwave chamber--one serving as the transmitter and the other as the receiver. The measurement procedure was conducted in two steps: in the first step, transmission coefficients were measured without the dBFSS structure; in the second step, the measurements were taken with the dBFSS in place. The results were then normalized with respect to the first step's measurements. To evaluate the transmission coefficients at oblique incidence for TE mode, the holder containing the dBFSS was rotated horizontally. By rotating the holder 90\({}^{\circ}\) along the vertical axis, the TE mode was switched to the TM mode, and the transmittance was measured again at various incidence angles. The measurements, taken with and without the dBFSS, were \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Parameter** & **h** & **d** & **L1** & **L2** & **L3** & **w1** & **w2** & **w3** \\ \hline
**Value (mm)** & 21.6 & 8 & 2.8 & 7.2 & 2.4 & 0.4 & 4 & 3 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Parameters of the model used in the measurement. Figure 5: Protype of the 3D-DBFSS and measurement setup. (**a**) 3D-DBFSS side view and local details. (**b**) Test setup side view. used to normalize the response, and the transmission coefficients for obliquely incident dual-polarized waves are presented in Figures 6 and 7. Some discrepancies between the simulated and measured results were observed. These deviations can be primarily attributed to two factors. First, the presence of metal particles and surface defects introduced by the 3D printing process resulted in significant disturbances to the surface current. As shown in Figure 5a, the surface of the fabricated structure is rough and contains numerous defects, which contribute to performance variations. Second, the diffraction effects caused by the finite size of the dBPSS structure further contributed to the observed differences. Due to the constraints of the available 3D printing equipment, the overall size of the front face of the structure was limited to 96 \(\times\) 88 mm\({}^{2}\), which exacerbated the edge diffraction effects on the transmission coefficient curve. Despite these challenges, a strong correlation between the simulated and measured results was still observed, demonstrating good agreement overall. To assess the effectiveness of the proposed 3D-DBFSS, the frequency selection characteristics and bandwidth are evaluated and compared with those of other dual-band bandpass FSS designs reported in the literature, as shown in Table 3.  
Figure 6: Simulated and measured transmission coefficients at oblique incidence of TE polarized waves. Figure 7: Simulated and measured transmission coefficients at oblique incidence of TM polarized waves.