Advanced manufacturing of passive waveguide components

According to the American Society for Technologies and Materials (ASTM), Additive Manufacturing (AM) is defined as “a process of joining materials to make objects from three dimensional (3D) model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies”. AM commercial processes are available from the end of 1980s. The first application of these technologies has been rapid prototyping (RP), used to check the design. Recently, thanks to improvements in terms of accuracy and material properties, AM technologies are used to build end-use components. Typical fields of application are automotive and aerospace industries, customized bio-medical implants and prosthesis, visual merchandising and jewelry industry. In the last five years, the exponential growth of publications regarding the application of AM processes to the microwave fields proves the increasing interests of research institutions and industries in these technologies. The development of antenna-feed chains for satellite communications (SATCOM) is an application domain in which AM technologies are envisaged to boost advancement in RF devices. In the next generation of High Throughput Satellite (HTS), hundreds of these payloads will be boarded on satellites. If standard manufacturing processes are considered, antenna-feed chains are manufactured in several parts that are assembled together. As a consequence, mass and envelope cannot be optimized because of the use of screws. Moreover, oxidation of contacting flanges can generate high levels of passive intermodulation products. AM is a promising candidate for the development of HTS, since it offers a good design flexibility and a more efficient development of new components. Moreover, different microwave components could be integrated in a single part, with a reduction of mass and envelope. However, the manufacturing accuracy and the surface finishing are not as high as those obtainable with conventional manufacturing processes. As a consequence, RF devices manufactured with AM technologies often exhibit a significant deviation between the simulated and the measured scattering parameters. An optimized choice of processes and materials, an accurate tuning of the process parameters, and an AM-oriented electromagnetic design are key steps in order to overcome these problems. In the present doctoral research activity, microwave filters have been considered as a relevant benchmarks to test AM technologies. The design of filters is a challenging task because the high amplitude standing waves developing inside the components lead to high sensitivity to mechanical tolerances and high losses. Moreover, strict mechanical constraints must be applied in the design of filters aimed at high-power applications. For these reasons, an electromagnetic robust design of Ku/K-band filters, with typical electrical requirements of SATCOM applications, have been considered to test SLM and stereolithography processes. The layout of the thesis is as follows. In chapter 1, an overview of the seven categories of AM technologies is presented. Moreover, the state of the art about applications of AM to RF components is reported. This analysis has been carried out considering the number of papers published in IEEE journals and conferences until 2016. In chapter 2, SLM process is described in more detail. Then, in order to test the application of SLM to the manufacturing of passive waveguide components, the design and measurements of test lines in Ku/K and Ka bands and of a resonant cavity in K-band are reported. Chapter 3 reports the characterization of dielectric materials used in stereolithography and PolyJet processes. A description of these two technologies is reported at the beginning of this chapter. Chapter 4 reports the design and manufacturing of microwave waveguide filters through SLM and stereolithography processes. First, an overview on the satellite communications RF payloads is reported. Then, an AM oriented design of Ku/K-band filter is presented. Finally, an integration of a filter, a H-plane bend and a 90° twist in a single component is presented. The whole research activity has been carried out in the framework of a collaboration between the Applied Electromagnetics & Electronic Devices (AE&ED) group of the Istituto di Elettronica e di Ingegneria dell’Informazione e delle Telecomunicazioni (IEIIT), that is a research structure of Consiglio Nazionale delle Ricerche (CNR) of Italy, and Center for Sustainable Futures-CSF@Polito of the Italian Institute of Technology (IIT).