An apparatus is disclosed for wireless power transmission. The apparatus may include a resonator circuit configured to generate a magnetic field to couple to a power receiving unit. The resonator circuit may include a transmit coil and an electromechanical reactive device having one or more moveable components, and having a reactance that is determined by a physical arrangement of those components. A power circuit connected to the resonator circuit can provide power to drive the transmit coil to generate the magnetic field. The electromechanical reactive device of the resonator circuit can be actuated to change the physical arrangement of the one or more moveable components, and hence change the reactance.