**3.2 Filtering power divider**

Loading a resonator or multiple resonators on the power divider can realize the dual function of filtering and power division. **Figure 22** shows a triple-band FPD and a quad-band FPD [27]. Two unequal CSRRs are engraved face to face on the top of the SIW rectangular cavity to achieve two operating frequencies. An outer pair of U-shaped slots (USS) or two pairs of USS is engraved on the bottom of the SIW to achieve the third or the fourth operating frequency. The fabricated FPD presents good isolation and return loss.

**Figure 22.** *Prototypes of Triple-band and quad-band PDs [27].*

**Figure 23.** *Fabricated dual-band balanced SIW FPD.*

Compared to the above FPD, the balanced SIW FPD is relatively less explored, especially for dual-band applications. **Figure 23** shows a single-layer dual-band SIW FPD which has three bisected substrate-integrated cavities [28]. By adjusting the position of metal vias, each cavity is designed to not only form three-pole dual passbands but also attain high in-band common-mode (CM) rejection. An isolation resistor is loaded across each output pair to attain high isolation easily.
