**7.1 Cascaded 3-dB directional coupler structure**

The planar structure of cascading two 3-dB SIW couplers is the earliest SIW crossover. As shown in **Figure 43**, the proposed crossover has a measured return loss of lower than 13-dB, isolation of better than 20 dB, and an insertion loss of less than

**Figure 43.** *The crossover at 60 GHz [60].*

0.5 dB within 5% bandwidth [60]. However, cascading inevitably brings the problem of excessive circuit size. To reduce the size, a two-layer cascaded 3-dB coupler is proposed based on the two-layer folded SIW directional coupler [61]. The electric field and magnetic field are coupled through the middle slot.

## **7.2 SIW resonators**

For the SIW resonator, on the one hand, using signal resonance to cascade multiple cavities on one channel can realize signal filtering; on the other hand, using the orthogonality of the resonant modes in the cavity, the signals in two channels can be isolated. The combination of the two can realize the dual-function integration of filtering and crossover in one device, which will reduce the complexity of the circuit and help reduce signal transmission loss.

In a general filter crossover circuit structure, according to the duality principle, each channel needs three cavities, and the design and implementation of the crossover require at least five SIW cavities, as shown in **Figure 44**.

The commonly used dual-mode SIW resonators are mostly TE201 and TE102 modes. As shown in **Figure 45**, the electric field distributions of the two modes are orthogonal to each other. The strongest electric field of one mode is the weakest of the other mode. The four ports are arranged according to the strength of the electric field, and the two signals are transmitted in the TE201 and TE102 modes respectively, which can realize the cross isolation of the signals. On this basis, a filter crossover circuit with cascading five dual-mode resonators is designed [62].

From the perspective of reducing circuit area and heat dissipation loss, the industrial field has higher and higher requirements for device performance. SIW crossovers are developing in the direction of better filtering performance, smaller size, and higher integration. Three ways to improve development are summarized below.

**Figure 44.** *Traditional filter crossover.*

**Figure 45.** *Distributed electric fields of TE102 and TE201 in rectangular SIW cavity.*
