A transmitter includes an alternating current power source with tunable parameters. A tunable parameter may be capacitance, inductance, or frequency.A metric related to power transfer from the transmitter through a coil is used to determine an amount to modify a parameter. One metric is equal to |Vs|·|Is|·cos(φ), where |Vs| is magnitude of the power source voltage, |Is| is magnitude of the power source current, and φ is phase difference between the power source voltage and power source current. Another metric is equal to |Vs|·|Is|·cos(φ)·(Vcoil_ref/Vcoil), where |Vs| is magnitude of the power source voltage, |Is| is magnitude of the power source current, φ is phase difference between the power source voltage and power source current, Vcoil_ref is a normalization value, and Vcoil is voltage across the coil.The transmitter may further include a phase tracking and adjustment loop; for example, a phase-locked loop (PLL).