**1. Introduction**

Substrate-integrated waveguides (SIWs) are planar structures that emulate a dielectric-filled rectangular waveguide (RWG) in a single circuit board, in which the lateral metallic walls are replaced with a periodic array of metallic vias (see **Figure 1**) [1, 2]. Thus, SIWs are good candidates to be used as building blocks for the implementation of microwave waveguide filters with different topologies, benefiting from the advantages of such technology (mainly low cost and easy integration), combined with the well-known advantages of conventional rectangular waveguides (complete shielding and high-power-handling capability).

In the following sections, we begin analyzing the main properties of ordinary SIWs with the homogeneous substrate, and those whose substrate is periodically loaded with either cylindrical air holes or with metallic cylinders, thus achieving a reduced/ higher effective permittivity, respectively. After that, different topologies of bandpass filters in SIW technology are briefly described, starting from classical iris-type SIW filters and moving to more novel topologies, consisting of step impedance filters based on high and low dielectric constant sections, extending this concept to halfmode SIW structures, with the aim to reduce the size of the filters, showing in all cases good performances in terms of insertion and return losses in their passbands, along with deep and wide rejection bands.

**Figure 1.**

*The geometry of the classical SIW structure.*
