We present an Extremum Seeking approach for resonance control of radio frequency (RF) resonant cavities without phase measurements. The controller minimizes reflected power from an RF cavity by utilizing model-independent ES. Unlike phase measurement-based resonance controllers, this approach does not require cable length-based calibration. Furthermore, this adaptive approach automatically tracks temperature-induced cable length changes which could otherwise require re-calibration or uncompensated would introduce time-varying offsets. We give a theoretical overview of the problem, a general overview of the controller, and present experimental results.