A resonator for use in a wireless power transfer system is provided. The resonator includes a core including a front surface, a back surface, and an annular sidewall extending between the front surface and the back surface, wherein an annular groove is defined in the front surface and surrounds a post, and wherein a cavity is defined in the back surface, the post and the cavity aligned with a longitudinal axis of the core. The resonator further includes a coil element disposed within the annular groove.