**6. SIW diplexer**

The diplexer is a three-terminal device used for transmitting and receiving signals. One port is connected to the antenna and the other two are used for transmitting


#### **Table 4.**

*Comparisons with different phase shifters.*

and receiving signals, respectively. The two processes are completed independently without any effect on each other. Thus, isolation, insertion loss, cost, and size should be paid more attention to. Diplexers can be divided into the following classes according to the shunting method of signals. Traditional diplexers mainly consist of two filters in different frequency bands and a T-junction [51–54], which increase the design complexities and the insertion loss. In recent years, there are two main ways to replace the T-junction. Firstly, multiple dual-mode resonators (MDMR) are used to replace the T-junction [55]. At the same time, good isolation is achieved through the orthogonality of the dual modes. However, the design flexibility of these diplexers is limited by the dual-mode coupling mechanisms. The second way is employing the scheme of a common dual-mode resonator (CDMR) and multiple single-mode resonators [56–59]. This method solves the problem of flexibility and achieves circuit miniaturization. Three structures are shown in **Figure 36**. In addition, there are also some other hot spots and trends, such as the miniaturization design of diplexers based on sub-mode SIW, balanced SIW diplexers, and the tunable design of SIW diplexers.

#### **Figure 36.**

*(a) T-junction scheme (b) multiple dual-mode resonators scheme (c) a common dual-mode and multiple singlemode resonators scheme.*
