**2. SIW filters**

Traditional planar transmission structures, such as microstrip and coplanar waveguide (CPW), have been widely used in filter design. Compared with these two structures, the substrate-integrated waveguide technology has the advantages of high Q coefficient, low cost, and high integration, and is widely used in the design of narrowband and broadband band-pass filters. The substrate-integrated waveguide technology can realize a variety of filters with different characteristics, including multi-band filters, wide stop-band filters, and reconfigurable filters. In the early days, SIW filters focused on the realization of circuit functions, and reports on the miniaturization of SIW filters appeared around 2005 [5]. There are various miniaturization technologies for SIW filters, and miniaturization has become the main trend in the development of SIW filters. This section introduces the application of SIW technology in filters from five aspects.

#### **2.1 Conventional substrate-integrated waveguide filters**

The design of the SIW bandpass filter can follow a similar approach to the design of gas-filled waveguide filters, or it can be based on the coupled matrix approach. In [6], a three-pole Chebyshev filter is designed through the inductive column synthesis technology, and its insertion loss and return loss are more than 1 dB and 17 dB, respectively. This SIW-based filter reduces size, weight, and cost significantly.

**Figure 2.** *Configuration of proposed SIW filter [7].*

A novel wide-stopband SIW filter using an angular cavity is performed in Ref. [7]. For each corner cavity, two transmission zero points can be introduced, and the position of each transmission zero point can be easily controlled by adjusting the appropriate geometric dimensions. **Figure 2** shows the configuration of the proposed SIW filter.
