# Waveguide Manufacturing Technologies for Next-Generation Millimeter-Wave Antennas
## 4 Stereolithography (SLA) and Post-Metallization Process
In addition, the authors have tested the tightening of the screws several times in the same hole, without any significant degradation of the metallic coat. Nevertheless, if an extra-protection of these punctual areas is desired, metallic washers can be embedded inside the plastic to permit screw contact over them instead of over the plastic surface, as was implemented in a filter device for space applications manufactured by Swissto12 [57]. To demonstrate the strength of the manufacturing method described in this section, the experimental results of the conformal array shown in Figure 12 are presented. This antenna operates in a dual 5G band with dual polarization. In particular, the dual-mode feeder shown in Figure 11(c) has two input ports: port 1, implemented in a coaxial transition, and port 2, implemented in a rectangular waveguide transition. Port 1 generates the TM\({}_{01}\) mode into the cylindrical waveguide, exciting the transversal slots. They are designed to operate in the n257 band (26-30 GHz). The slot arrangement around the cylindrical waveguide perimeter provides a uniform excitation of the slots, generating a linearly polarized (\(\theta\)-polarization) conical-beam radiation pattern. A comparison between the simulations (extracted from CST Microwave Studio) and the measurements is shown in Figure 13. In particular, the \(\phi\) = 0\({}^{\circ}\) normalized radiation pattern at 28 GHz is shown in Figure 12(a). A great concordance between the simulated pattern and the measured copolar (CP) radiation pattern was obtained, as well as a low cross-polar (XP) radiation. The difference between CP and XP maximum levels, denoted as cross-polar discrimination (XPD), was higher than 15 dB. In the case of the \(\phi\) = 90\({}^{\circ}\) cutting plane (Figure 12(b)), a great similarity between simulation and measurements, as well as a XPD > 20 dB, was achieved. The similitude between CP radiation patterns, for both \(\phi\) = 0\({}^{\circ}\) and \(\phi\) = 90\({}^{\circ}\) cutting planes, demonstrated the omnidirectional performance of the conical-beam pattern. Finally, the experimental radiation patterns at 26 and 30 GHz are shown in Figure 12(c). Considering the typical main beam scanning of the progressive-wave array antennas, the obtained results may be considered as stable in band. Figure 13: Conformal array antenna normalized radiation patterns: (**a**) simulated and measured \(\phi\) = 0\({}^{\circ}\) patterns at 28 GHz; (**b**) simulated and measured \(\phi\) = 90\({}^{\circ}\) patterns at 28 GHz; and (**c**) measured \(\phi\) = 90\({}^{\circ}\) patterns at 26 and 30 GHz. Second, port 2 of the dual-mode feeder shown in Figure 12c was implemented in waveguide technology. A TE\({}_{10}\)-TE\({}_{01}\) converter was used to transform the fundamental TE\({}_{10}\) mode of the input rectangular waveguide into the non-fundamental TE\({}_{01}\) mode of the cylindrical waveguide. This propagation mode produced an excitation of the longitudinal slots, which were designed to operate in the n260 band (37-40 GHz) [1]. Therefore, a linearly polarized (\(\phi\) -polarization) conical-beam radiation pattern was generated. Figures 14a and 14b present the CP and XP radiation patterns at 38.5 GHz for \(\phi=0^{\circ}\) and \(\phi=90^{\circ}\) cutting planes, respectively. The great concordance between simulations and measurements demonstrated the high quality of the manufacturing process. Moreover, the \(\phi=90^{\circ}\) radiation patterns at 37 and 40 GHz are presented in Figure 14c, showing a notable stability in band. Finally, the realized gain and the total radiation efficiency were evaluated experimentally. The realized gain was higher than 13.5 dBi for both frequency bands, while the total efficiency oscillated between 83% and 90%. Very slight differences between simulated and measured gains were obtained (around 0.3 dB), demonstrating excellent antenna surface finishing. Therefore, it can be concluded that the final prototype provides a high-performance in terms of gain and efficiency.