## 3 Fabrication of the WG ZOR BPF and Test of the Prototype  
The designed waveguide filter is fabricated and measured to validate the proposed method and geometry. Since the CNC milling technique is conducted for fabrication, taking into account that the end-mill tip cannot realize the aforementioned shapes of the cross-sections of the thin resonators 100%. The round corners appear instead of the sharp right-angle ones. Though the design has been done with the unit of 10 \(\upmu\)m as a coarse approach to ease the mechanical tolerance, the round corners are inevitable. The secondary procedure of design is done to keep the function of the WG BPF satisfactory, as in Figure 6. This leads to the modified values for the geometrical parameters and the values are seen in Table 7.  
Despite the geometrical change in the front view of the flat metamaterial resonators, it is necessary to keep the frequency response compliant with the specifications. The corners become round \(R\) and weaken the slot capacitance and the inductance on the edges of the metal patches and their intermediate strip. The rounded corner leads to lower capacitance due to a wider gap and lower inductance due to a shorter current path. This means at a microwave frequency band it degrades the initial performance of the BPF. In the realistic case, Figures 3c and 6b are rendered as Figure 7a-c. Some of the physical dimensions have to be fine to be in the unit of 100 \(\upmu\)m; there is no more being coarse in order to achieve the required frequency response. Setting up the structure in the electromagnetic analysis software with the values for the geometrical parameters, the transmission and reflection coefficients are obtained as in Figure 7d,e, which meets the design requirement. This is very different from cavity filters presented by [15; 16; 17] in terms of shape and length, and is physically realized as follows. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{3}{c}{**Resonator\#1**} \\ \multicolumn{1}{c}{**Variable**} & **Value [mm]** & **Variable** & **Value [mm]** \\ \hline _L1_ & 12.2 & _W1_ & 1 \\ _L2_ & 4.7 & _W2_ & 1 \\ _L3_ & 1.5 & _W3_ & 1 \\ \hline \multicolumn{3}{c}{**Resonator\#2**} \\ \hline _Variable_ & **Value [mm]** & **Variable** & **Value [mm]** \\ \hline _L1_ & 8.8 & _W1_ & 1 \\ _L2_ & 4.8 & _W2_ & 1 \\ _L3_ & 3.3 & _W3_ & 1 \\ \hline \multicolumn{3}{c}{**Resonator\#4**} \\ \hline _Variable_ & **Value [mm]** & **Variable** & **Value [mm]** \\ \hline _L1_ & 8.8 & _W1_ & 1 \\ _L2_ & 4.7 & _W2_ & 1 \\ _L3_ & 3.4 & _W3_ & 1 \\ \hline \multicolumn{3}{c}{**Gap**} \\ \hline _Variable_ & **Value [mm]** & **Variable** & **Value [mm]** \\ \hline _gap1_ & 15.8 & _gap3_ & 15.2 \\ _gap2_ & 15.5 & _gap4_ & 15.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: The values of the physical dimensions of the eighth-order filter. The aluminum ingot is carved into WG metamaterial resonators and coupling sections in the milling process as in Figure 7a and they are pieced together to the eight-pole BPF as in Figure 7b,c. The original structure becomes a little longer and wider because it has parts for mechanical assembly with bolts and a flange body for WG port connecting. However, the size of the core has the length of \(3.5\,\lambda_{g}\) for the eighth-order filtering. With this harness, the WG BPF is tested to see the frequency response. This experiment is conducted as in Figure 8d, which produces S\({}_{11}\), and S\({}_{21}\) is as in Figure 8e. The insertion loss and reflection coefficients are about \(-0.9\) dB and \(-19\) dB in the passband. The roll-off this manufactured metamaterial filter makes is satisfactory with attenuation of almost \(-40\) dB. There occurs a discrepancy between the simulated and measured results that the frequency is shifted downward a bit.