Raúl García

In this work, we describe a new topology for interdigital bandpass filters implemented in waveguide technology. Compared to classical coaxial resonators, which are typically based on cylindrical metallic posts, the resonators that we propose are based on rectangular boxed cavities loaded with flat metallic strips of a finite thickness. The main advantages of the proposed topology are low‐cost practical realization and manufacture simplicity. The design process, which is based on the classical circuit‐based approach combined with an efficient segmentation of the component, uses the well‐known Aggressive Space Mapping technique, with the aim of reducing the overall computation effort. In order to validate the new topology and the design procedure, interdigital bandpass filters of orders 3 and 5 operating at the S‐band have been designed. For validation purposes, the results obtained are successfully compared to simulations provided by two different full‐wave electromagnetic tools (i.e., Ansys HFSS and CST Studio Suite). Furthermore, a multipactor study has also been performed, with the aim of exploring the power handling capability of the proposed filters in view of possible space applications. Finally, the design of classical interdigital filters based on cylindrical posts is also discussed, with the aim of comparing both topologies regarding compactness, as well as in terms of their electrical and high‐power performance.

The electrical response of low-frequency band-pass filters based on periodic substrate integrated waveguide (SIW) technology typically shows permitted and forbidden frequency bands. Therefore, this type of filters can be designed using a conceptually very simple and efficient procedure based exclusively on the study of the dispersion properties of the periodic structure. In this paper, we go a step further with the design of a periodically air-filled SIW band-pass filter in which part of the dielectric substrate is removed to reduce insertion losses, and whose unit cell parameters, which are directly related to the center frequency ( $f_{c}$ ) and bandwidth (BW) of the first passband, and also to the first stopband or bandgap (BG) of the structure, have been appropriately selected for filtering purposes, thus providing some useful design rules. Furthermore, we apply the concept of glide symmetry for achieving a much larger fractional bandwidth (FBW) than that obtained in conventional air-filled SIW filters found in the technical literature. Finite implementations of both periodic structures with and without glide symmetry have been analyzed, showing their filtering response for validation purposes. Additionally, to overcome the matching level restrictions in the resulting air-filled periodic SIWs, a microstrip-to-SIW transition including a novel coupling iris is proposed. A prototype of the proposed air-filled glide-symmetric periodic SIW filter has been manufactured and experimentally validated, illustrating the potential of this technique to obtain large FBWs that can not be achieved in conventional air-filled SIW filters. The proposed filter proves to be a good candidate for millimeter wave applications.