**5.2 Dynamic tunable phase shifter**

By incorporating switches into the circuits, a phase shifter can achieve multiple phase changes easily. According to the different types of switches, controllable phase shifters are divided into electrically controlled tunable phase shifter,

**Figure 31.** *Structure of the proposed SIW self-compensating phase shifter.*

**Figure 32.** *Photograph of five prototypes [44].*

magnetically controlled tunable phase shifter, and mechanical tunable phase shifter. Compared with the fixed phase shifter, the variable phase shifter is more flexible to meet the requirements of the system, which facilitates the evolution of phased array techniques strongly. Early SIW multiple phase shifters were designed with a multi-phase channel side by side, which increased the size [47]. At present, most of the adjustable phase shifters are single-channel structures, and the phase shift is adjusted by switching the equivalent dielectric constant of the transmission line in the channel.

A cylindrical metal post inserted in SIW is equivalent to a T-network high-pass filter [48]. When a PIN diode is buried within a SIW, the diode shows the same behavior as the metal post [49]. The difference is that diode on or off can change the value of the phase shift on the SIW. Accordingly, a new approach to designing a dynamic phase shifter is proposed. A bias circuit is placed on top of the SIW to control the diode states, as shown in **Figure 33**.

Different from diode tuning, a SIW phase shifter which can be reconfigured using liquid metal is presented [50]. A series of holes that can be filled or emptied of liquid metal is placed on the SIW. The holes with filled liquid metal can be seen as a wall to block the passage of energy. **Figure 34** illustrates the operating paths in different states. By using the different electrical lengths among the three paths, the device achieves coarse steps of phase change, from 0° up to 180°, in steps of 60°. Utilizing reactive loading (placing a single hole in each path), the phase shifter can achieve a phase shift in steps of 10°, from 0° up to 180°.
