## 3 Experimental Results and Discussions  
### Design of the Waffle Grid and Waveguide Transitions  
A proprietary industrial-grade SLA 3D printer developed by the National Institute of Additive Manufacturing of China was used to fabricate the proposed antenna array. The resolution of the prototyping process is 50 \(\upmu\)m and the printing tolerance is \(\pm\)0.1 mm, which provides sufficient accuracy for the intended application. To metalize the 3D-printed prototype, a 10 \(\upmu\)m thick layer of copper was then electroless plated onto the resin, forming a conductive layer. The thickness of the copper layer is more than 20 times the skin depths at Ka/K-bands (about 0.47 \(\upmu\)m for copper at 20 GHz). Hence, the proposed antenna has the same electromagnetic properties as an all-metal antenna. It should be noted that the effective electrical conductivity of the plated copper layer is smaller than the typical value of 5.8 \(\times\) 10\({}^{7}\) S/m for copper, according to [30]. Therefore, we used an effective conductivity of 1.5 \(\times\) 10\({}^{7}\) S/m reported in this reference to model the plated copper in EM simulations of the antenna.  
Photographs showing the fabricated prototype at different stages throughout the fabrication process are shown in Figure 10. Figure 10a shows the initial 3D-printed versions of the antenna array and waveguide transitions, which were then metalized, as shown in Figure 10b, and polished, as shown in Figure 10c. The unassembled antenna components are shown in Figure 10d. The complete antenna structure that was assembled using four M3 screws and had two standard waveguide ports mounted to it is shown in Figure 10e. Owing  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \(\mathbf{RGW_{1}}\) & \(\mathbf{RGW_{2}}\) & \(\mathbf{RGW_{3}}\) & \(\mathbf{RGW_{4}}\) \\ \hline width & 0.8 & 0.8 & 1.2 & 1.17 \\ height & 1.6 & 1.53 & 2.9 & 2.82 \\ length & 0.45 & 0.45 & 0.45 & 0.45 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Dimensions of the RGW in the corporate-feed network (unit: mm).  
Figure 9: Grid layer and input-port transitions of the proposed antenna. (**a**) Waffle-grid layer. (**b**) Ka-band transition. (**c**) K-band transition. The parameters of the waffle grid and waveguide transitions follow: \(\tau\)5 = 5.5 mm, \(sl\) = 11.5 mm, \(sl\) = 2.5 mm, \(sw\) = 1.5 mm, \(ad1\) = 1.8 mm, \(gw1\) = 3.36 mm, \(gh1\) = 8.12 mm, \(ad2\) = 2.8 mm, \(gw2\) = 4.3 mm, \(gh2\) = 10.6 mm.  
to the contactless feature of GWs, assembling different layers of the antenna prototype only required bolting. With the SLA 3D printing technology, the average cost of each prototype is less than USD 50, which is about ten times cheaper than using the traditional CNC process. Each complete antenna structure weighs 35 g.  
Figure 11 illustrates a sample of some crucial parts of the prototype array. Surface roughness due to the SLA printing and metallization are visible as ripples that can be seen in several places (see blue circles in Figure 11). However, since the surface of the prototype was polished, such ripples are merely 1 \(\upmu\)m thick, and their effect is negligible. In addition, the right-angle corners of metallic pins and RGWs became rounded corners when fabricated (see purple circles in Figure 11), which have also been taken into account in the simulated model. Capacitive slits placed in a GGW for improving the impedance matching can be seen in Figure 11c. Because of fabrication error in the vertical dimension, the metallic pins may not have exactly the same height (see red circles in Figure 11). As mentioned in Section 2, the air gap between the pins and the top metallic plate is intended to be 0.1 mm. Since this distance is much less than a quarter of a wavelength, slight variations in the air gap do not affect the EBG characteristics. Even if the pins are in contact with the top metallic plate (e.g., air gap = 0 mm), the gap waveguide would degenerate into a laminated waveguide with quasi-closed geometry [31], which can also act as an EBG structure.