**2.5 Tunable SIW filters**

A new method to design tunable filters is shown in **Figure 18** [23]. The proposed filter consists of a conventional SIW resonator and an additional via. The vial contains an open-loop slot in the top metal wall, where the size, location, and orientation of the

**Figure 16.** *Geometry of the proposed filters. (a) Loaded with CSRR and SICSRR, (b) loaded with SICSRRs [21].*

**Figure 17.** *Geometry of the triple-order EMSIW BPF with QMSIW cavity [22].*

**Figure 18.** *Geometry of the proposed filter [23].*

open-loop slot determine the current and field distribution paths, which in turn controls the tuning range, resulting in an 8% tuning range.

A tunable filter based on microstrip patch resonators is proposed [24]. The overall layout of the filter is shown in **Figure 19**. The designed filter is similar to the SIW structure, that is, an electrical wall composed of through-holes is placed around the microstrip patch to reduce radiation, thereby reducing the insertion loss of the filter. In addition, the varactor diode is selected as the tuning element because of its small size, fast tuning speed, and large tuning range. The final designed filter covers six different states from 2.0 GHz to 2.53 GHz, and its insertion loss is 1.68 dB–3.90 dB.


**Table 1** shows the performance parameter of the filters following different technological approaches.

#### **Table 1.** *Comparison of different filters.*
