**4.1 Electronic control switch**

An electronic control single-pole single-throw (SPST) switch is shown in **Figure 25**. Firstly, make a narrow slot in the SIW top wall, then, the diodes are capacitively coupled to the slot by metal pads on a very thin insulating layer. With the pin diodes being switched between on-state and off-state, the propagation could be transformed between two different types of modes. When the diodes are turned on, the top slot is closed and the waveguide becomes an ordinary SIW. When the diodes turn off, the electric field and propagation characteristics of the structure can be seemed as HMSIW, as long as the slot has an appropriate size [32]. The difference in cut-off frequency between SIW and HMSIW gives a switching bandwidth from 3 to 4.7 GHz. The fabricated switch is presented with isolation of 50 dB and 3 dB IL in 3–3.5 GHz.

In addition, PIN diodes are also used in a single pole double throw (SPDT) switch. Inductive posts with rectangular slots are embedded in the SIW to increase the impedance of the SIW, which can disturb the incident signal flow. The PIN diodes and bias networks are placed to control the operating mode of the inductive posts, as shown in **Figure 26**. The proposed SPDT switch has measured isolation(S31) of greater than 10 dB, isolation(S32) of greater than 15 dB, and an IL of less than 2.55 dB from 8.24 to 10.36 GHz [33]. However, SIWs are narrow at higher frequency ranges and cannot use switched vias. Thus, this method is no longer suitable at higher millimeter-wave frequencies.

Different from the above internal through-hole design, a SIW switch with a resonant slot is made innovatively [34]. The proposed switch in **Figure 27** has an IL of less than 1.3 dB and isolation of more than 10 dB in the switchable frequency band. The

**Figure 25.** *Structure of switched waveguide [32].*

**Figure 26.** *Structure of the SIW SPDT switch [33].*

**Figure 27.** *A switch with a resonant slot [34].*

resonant slot vanishes when the PIN diode is in the forward-biased state, while the resonant slot presents a high impedance load on the SIW when the PIN diode is in the reverse-biased. This novel device can be particularly suitable for higher millimeterwave frequency due to its simple structure.
