This paper presents a new class of quasi-elliptic pass-band filters in substrate-integrated waveguide technology, which exhibits compact size and modular geometry. These filters are based on mushroom-shaped metallic resonators, and they can be easily implemented using a standard dual-layer printed circuit board manufacturing process. The presented filters exploit nonresonating modes to obtain coupling between non-adjacent nodes in the case of in-line geometry. The resulting structure is very compact and capable of transmission zeros. In this work, the singlet configuration is preliminarily investigated, and a parametric study is performed. The design of three-pole, four-pole, and higher-order filters is illustrated with examples and thoroughly discussed. A four-pole filter operating at the frequency of 4 GHz has been manufactured and experimentally verified, to validate the proposed technique.Keywords: New and emerging technologies and materials, Filters
I . I N T R O D U C T I O NThe deployment of a variety of novel applications in the framework of the wireless sensor networks (WSN) [1][2][3] and Internet of Things (IoT) [4][5][6] demands the development of novel classes of radio frequency (RF) and microwave components and antennas, which combine easy manufacturing, simple integration, low cost, compact size, limited loss, and low weight. Both WSN and IoT are expected to lead to the deployment of an extremely large number of wireless systems, which integrate passive and active components as well as antennas in a single, compact device. From a market point of view, the possibility to easily integrate an entire wireless system by adopting a cost-effective manufacturing process represents the key factor for the success and widespread development of these new applications.Among the available manufacturing and integration technologies for RF and microwave circuits, a good candidate able to satisfy all the aforementioned requirements is represented by the substrate-integrated waveguide (SIW) technology [7,8]. SIW technology permits to integrate in planar form waveguide-like components, by adopting a dielectric substrate with top and bottom ground planes and rows of metal cylinders to emulate the side walls of the waveguide. The fabrication of SIW structures can be based on wellestablished and low-cost manufacturing techniques, such as the standard printed circuit board (PCB) technology. SIW structures guarantee large design flexibility and easy manufacturing, combined with relatively low losses and selfpackaging. Moreover, SIW allows the simple integration with active and non-linear devices, as well as the implementation of complete circuits on a single substrate, according to the system-on-substrate (SoS) paradigm [9].Microwave filters represent a class of components very suitable for implementation in SIW technology. The main reason is related to the low losses of SIW structures, which are usually smaller than in other planar technologies (as the microstrip line or the coplanar waveguide): due to this feature, SIW...
