# Waveguide Manufacturing Technologies for Next-Generation Millimeter-Wave Antennas
## 4 Stereolithography (SLA) and Post-Metallization Process
A reflection coefficient below \(-\)10 dB, good agreement between simulated and measured diagrams, and a difference between measured and simulated gain of less than 3 dB were obtained. Corrugated horn antennas at W-band were also presented in [46], showing 1 mm deep metallized corrugations with high-precision and satisfactory experimental results. Finally, experimental tests were carried out on waveguide designs and horn antennas on standard WR3.4 waveguide that offered good performance up to 300 GHz [45]. In some of these studies, the importance of the preparation stage to achieve good mechanical performance is highlighted (e.g., in the fixing of the screws at  
Figure 10: Initial prototypes manufactured in a commercial 3D printer and metallized with a copper coating. the waveguide flange). A disadvantage of this chemical process is the activation of the preliminary step, which makes it a relatively costly proceeding. The Swissto12 patented process contains the whole 3D printing plus copper coating operations, and is especially recommended for space or satellite applications, leading to medium-to-high costs if cellular deployment is intended. On the other hand, another kind of plastic metallization process for millimeter band applications can be analyzed. The direct-plating technology of JetMetal [47] avoids some disadvantages of other plating processes, especially by providing significant cost reduction due to the short time required to apply it (i.e., only a couple of minutes). As can be observed in Figure 11, a compressed-air and double-nozzle painting gun is used to simultaneously spray a double solution of reducer (palladium free) and oxidant (composed of metallic silver ions Ag+) agents onto the 3D printed plastic device. The ratio of the reducing and oxidating agents forming the sprayed liquid film and their reaction conditions are properly controlled on the plastic surface of the material compounding the 3D printed device. In this manner, the silver metal coating is absorbed by the plastic, which is the key point of this process, avoiding the risk of crumbling and peeling observed in chemical platting. The main disadvantage of this metallization method, compared to the other methods mentioned, is that the spray must have direct access to the areas to be metallized. Therefore, this method cannot be applied to structures that have intricate topologies or that contain internal channels (unless it is possible to implement openings that allow access to them). Several examples of antenna devices operating in the millimeter band, manufactured by using 3D SLA printing + JetMetal metalization coating, can be found in [48; 49; 50; 51]. The metallization process, material composition, deposition time, and metallic layer thicknesses are extensively detailed in [50]. This work shows the comparison of a 77 GHz reference horn antenna fabricated by electroforming, SLA + copper electroplating, and SLA + silver spray metallization, demonstrating that the spray metallization method provides good results with a difference between simulated and measured gain of 0.6 dB. A tunable microwave device at Ku-band (a rotary same attenuator) is also manufactured by the spray metallization technique in [51]. The authors successfully checked these 3D printing and post-metallization processes in a single band conformal slotted array [52], as well as in a dual-band and dual-polarization conformal slotted array for 5G applications [53], as shown in Figure 12a. In both cases, conical-beam antennas based on a slotted cylindrical waveguide path were designed. The latter was manufactured in resin material by SLA technology, then plated by metallic pulverization to experimentally validate the proposed antenna topology and the fabrication processes. The combination of these manufacturing processes led to optimized costs, precision, and reliability. Initially, the idea was to directly print the structure on a metal 3D printing process, such as DMLS.