**2.2 Multilayer substrate-integrated waveguide filters**

The emergence of multi-layer technology can extend the traditional twodimensional planar structure to a three-dimensional direction, greatly increasing the flexibility and freedom of filter design, and can increase the coupling mode. At present, the realization process of multi-layer structure mainly includes low temperature cofired ceramic (LTCC) process and multi-layer PCB process. The second part mainly introduces the multi-layer structure design of the filter applied to the substrate-integrated waveguide.

The first multilayer technology is the substrate-integrated-folded waveguide (SIFW), which was proposed by Grigoropoulos et al. [8]. The use of substrateintegrated folded waveguides in filter design can greatly reduce the overall circuit size of the filter.

**Figure 3** shows a filter based on a SIFW resonator design [9]. The filter was developed using LTCC technology to achieve miniaturization. Resonators of the same layer, such as resonator 1 and resonator 4, resonator 2 and resonator 3, are horizontally coupled through the corresponding sensing window of the through hole. The different layers of resonators, such as Resonator 1 and Resonator 2, Resonator 3 and Resonator 4, are vertically coupled through a slot in the common wall between the two layers. Compared with the traditional planar direct coupled waveguide filter, the area of the filter can be reduced to 26.3%.

The second multilayer technology is the ridged substrate-integrated waveguide (RSIW). As shown in **Figure 4** [10], RSIW is the introduction of a longitudinal metal ridge into a classical waveguide without any degradation in RF performance. At the same time, the use of ridge waveguides can increase the bandwidth by 37%.

#### **Figure 3.** *Configuration of proposed SIFW filter [9].*

**Figure 4.** *The dimensional structure of the RSIW [10].*
