# Waveguide Manufacturing Technologies for Next-Generation Millimeter-Wave Antennas
## 3 Direct Metal Laser Sintering (DMLS)  
Direct metal laser sintering (DMLS) belongs to a family of additive manufacturing techniques known as selective laser sintering (SLS). SLS techniques are based on the successive deposition of layers of powdered material. For each layer, a high-power laser heats the regions that correspond to the object that must be built, melting the material to join the powder particles [3; 34; 35]. This process is illustrated in Figure 5. The unused material of a certain layer acts as support for successive layers, enabling the building of overhanging parts or even hollow structures, as long as they have some aperture that allows the evacuation of the powder after fabrication. This process opens up the possibility of monolithic (i.e., in a single part) manufacturing of waveguide devices, avoiding the problems associated with manufacturing in several parts, such as the alignment issues or power losses mentioned in the previous section. In DMLS, a powdered metal alloy is used as the building material, and therefore the manufactured objects are completely metallic [21]. This technique is of special interest for the manufacturing of waveguide devices, as it avoids the need to perform a metallization of the built waveguide. The main disadvantage of DMLS is that the obtained parts exhibit a rough finish on their surfaces. This roughness, when present at the internal faces of a waveguide device, affects the device's insertion losses. Nevertheless, DMLS is still a very  
Figure 4: Measured (solid) and simulated (dotted) performances of the triple-radiation pattern monopulse horn antenna: (**a**) sum radiation pattern, \(D_{\mathit{irectivity,Mass}}=24.85\) dBi ; (**b**) difference in azimuth radiation pattern, \(D_{\mathit{irectivity,Mass}}=22.66\) dBi ; (**c**) difference in elevation radiation pattern, \(D_{\mathit{irectivity,Mass}}=22.44\) dB ; and (**d**) reflection coefficient for each radiation pattern. young technology, and it is expected that it will evolve to achieve finer finishing in the near future. A recent interesting work that illustrates the virtues of DMLS for the fabrication of waveguide devices can be found in [36], with a novel waveguide reconfigurable phase shifter (WRPS) adjusted by means of two tuning screws (TSs). Then, four of these phase shifters are integrated in a waveguide-fed phased array to provide reconfiguration capabilities. The phase shifters are used to adjust the relative phase differences between the radiating elements of the array, which allows control of the direction of the radiation pattern main lobe. It must be noted that, due to the spacing constraints between the radiating elements of the array, it was necessary to build the four phase shifters in a single WRPS module. In this situation, it would have been extremely complicated to use a subtractive machining approach. Due to the reduced spacing between the shifters, there would not have been enough room to insert the required assembling screws and alignment pins. Here, additive manufacturing became the solution for overcoming the limitations of traditional manufacturing approaches. As Figure 6 shows, the block of phase shifters has been built using DMLS. There are two reasons for choosing this technique over other additive manufacturing techniques based on printing the device in plastic and subsequently applying a metallic coating. First, the threaded holes needed to introduce the TSs would have been much more difficult to manufacture if the device were built in plastic. Although it is possible to incorporate threaded inserts into these kinds of designs, for the correct operation of this specific device a good electrical contact between the TSs and the waveguide walls is necessary, and would not have been be attained by the insert. In addition, the relatively elongated shape of the phase shifters might have posed certain issues during the metallization stage, causing some internal regions to be inadequately covered by the conductive layer.