**1. Introduction**

3D-printing has made a profound impact in the development of radio-frequency (RF) devices in the past decade. Initially, this manufacturing technique represented an opportunity to implement new ideas as well as to enjoy fast, cost-effective, and monolithic prototyping [1–8]. However, non-negligible constraints were found in the first attempts to 3D-print RF components. Such constraints were associated, for example, to surfaces rough finish, the appearance of undesired supports, and the need for electroless plating. Neither the resulting manufacturing tolerances were optimal, nor the quality of the printed pieces. With time, as a result of collective efforts, it has been understood that the full benefit of this technique (i.e., short lead time, low cost, single piece prototyping, and high RF quality) implies a change of mindset in RF design [9–13]. As a result, today we are witnessing the most disruptive impact of 3Dprinting: the challenge of the bounds of our creativity and the need for new co-design guidelines.

3D-printing consists in the layer-by-layer additive manufacturing (AM) of objects. Different materials can be employed to build the piece. For example, plastic polymers can be used to build objects through stereolithography (SLA) [6, 7]. The case of the present chapter considers selective laser melting (SLM), which allows to use metal alloys as the building material [10, 14–16]. Examples found in the literature include aluminum (which is the most extended one, AlSi10Mg), titanium, stainless steel, or Invar. This option is the preferred one when dealing with aerospace applications. The main associated advantage is that it directly leads to a body that can easily meet the stringent mechanical and thermal conditions associated to space or other harsh environments. Additionally, metallic 3D-printed parts are compatible with high-power handling, specially if they are monolithic, as passive inter-modulation issues are minimized.

The main disadvantage of metal 3D-printing is the surface finish of the 3D-printed part. The resulting surface roughness is larger than what is normally obtained with conventional manufacturing techniques, which results in higher insertion losses. Consequently, 3D-printed parts might exhibit higher loss than the counterpart produced with CNC milling. For that reason, postprocessing and additional surface metallization techniques are currently being developed in order to improve the conductivity of a metal part [17]. The effective conductivity of a 3D-printed waveguide depends on many aspects such as the frequency of operation, the chosen printing direction, or the shape; however, an average value for a raw (not metallized) finish is 0*:*<sup>5</sup> <sup>10</sup><sup>7</sup> <sup>S</sup>*=*m. With treatments, values of up to 3 x 107 <sup>S</sup>*=*m can be achieved. This is still less than the ideal conductivity of aluminum; however, it has been noticed that the devices do give competitive loss thanks to the avoidance of assembly, which might create leakage or reflections.

Current metal 3D-printers offer printing volumes that range from 250 mm 250 mm 250 mm [18] to 400 mm 400 mm 400 mm [19], which are well suited for devices operating from X-band. At the time this chapter is being written, one can find stories in the media where the main space primes have announced the use of 3D printing in their future high-throughput satellites [20–22]. On the other hand, in 2021, a European Cooperation for Space Standardization (ECSS) standard with requirements for processing and quality assurance of powder bed fusion technologies for space applications has been issued [23]. All this together proves that the space industry recognizes the maturity of metal 3D-printing and accepts its use for present and future systems.

The authors of this chapter have been involved in several R&D projects related to 3D-printing of RF components and antennas. Such projects have been mainly conducted in France, at the laboratory IETR (Institut d'Electronique et des Technologies du numéRique), and they have been funded by the European Union, the European Space Agency (ESA), the Centre National d'Etudes Spatiales (CNES), and the Region of Brittany. Such projects have allowed the conception of general design guidelines for successful SLM prototyping, which are next provided in Section 2. In the following sections, advanced 3D-printed parts are described. Section 3 includes the implementation of waveguide components, such as filters and ortho-mode transducers. Section 4 considers the development of horns and slotted antennas. The case of periodic structures implementing frequency selective filtering and polarization conversion is considered in Section 5. Finally, conclusions are drawn in Section 6.
