Novel classes of bandpass filters in substrate integrated waveguide technology

The evolution of the Internet of Things (IoT), Wireless Sensor Network (WSN) and the emerging market of the fifth generation mobile (5G) is leading the microwave community towards new challenges, created by growing demands for interconnected components. IoT and WSN are groups of autonomous spatially distributed elements (sensors or tags) that communicate between each other for a specific purpose: to track an object or being, measuring their vitals by responding to requests relating to the physical parameters of their surrounding environment. The use of these components spans across many occupational fields and, for both the academic and the industrial fields, they present new and exciting possibilities. In IoT, for example, the wide distribution of “things” requires easily manufactured and disposable elements to be mass produced at a low cost. Furthermore, the deployment of complete wireless systems and sensor nodes requires the implementation of a technology with an efficient scale of integration. In this scenario, the Substrate Integrated Waveguide (SIW) technology could become an important player by being an optimal trade-off between highly efficient yet bulky metallic waveguide components and low cost yet easily embeddable planar components. In addition, from the System-in-Package (SiP), adopted in the design of RF circuits, the System-on-Substrate (SoS) paradigm can be achieved with this technology. For these reasons, this Doctoral Thesis examines the potentialities of Substrate Integrated Waveguide (SIW) technology applied, in particular, to microwave bandpass filter design. The application of these components scopes across many fields - from base stations to satellite communications. The decision to realize these components based on the SIW technology is expounded with the previous motivations. The Thesis is organized in 5 Chapters, describing the novel classes of bandpass filters designed and realized during my Ph.D. In particular, Chapter 1 is devoted to a critical Introduction of the SIW technology with a selection of bandpass filters that have inspired this work. Chapter 2 describes the study of a new class of SIW filters based on the periodical perforation of the dielectric substrate: the local effective permittivity, reduced by the perforation of the substrate, creates waveguide sections below cut-off, defining an iris-type like filter design. In order to reduce the size of this structure the Half-mode and Folded Half-mode SIW filters have been designed, realized and measured. Chapter 3 introduces a new class of dual-mode air-filled SIW cavity filters. This structure combines the advantages of an air-filled and a dual-mode structure, namely low loss and size reduction. A theoretical model, based on an equivalent transmission line, relates the filter characteristics to the variation of a few geometrical parameters. From this analysis, doublets, that are building blocks for the design of higher order filters, are deeply investigated. In Chapter 4, a further study, based on the dual-mode air-filled SIW cavity (Chapter 3) is presented. In particular, this topology, in contrast to the previous structure, is realized considering a complementary geometry, obtained by removing the lateral dielectric portions of the substrate – instead of the central portion of the SIW cavity. This new structure gives a degree of freedom related to the possibility to locate the transmission zeros both below or above the pass-band of the filter. A novel class of mushroom shaped resonator SIW filter is described in Chapter 5. With this topology is possible to obtain the maximum number of transmission zeros, equal to the number of resonators, in an in-line configuration, for an improved selectivity. In addition, the overall length, compared to the classical in-line filters with inductive obstacles, is smaller. In the Conclusions, the results of the work are briefly summarized with an overview of the improvements achieved.