In this communication, a substrate integrated wave (SIW) cavity is applied in feeding dielectric resonator antennas (DRAs) and designing reconfigurable circuits for multifunction applications. The TE120 mode of the SIW cavity is utilized to differentially feed a dielectric resonator (DR) for high-performance antennas with low cross-polarization and symmetrical patterns. The TE110 mode is used to design a filter or a power divider with continuously tunable operating frequency. Considering the characteristics of the field distributions, the filter or power divider part shows no effect on the antenna part, meaning that different functions are independent. The proposed multifunction applications of the SIW cavity are listed, simulated, and verified by experiments.