Abstract: A method and control system for regulating the output power of a magnetron in a microwave oven having a food heating cavity, a magnetron, and a waveguide coupling the magnetron to the heating cavity. The reflection coefficient within the waveguide is measured and a control signal is generated to switch the magnetron between first and second levels of power output when the reflection coefficient reaches a pre-determined value indicating that the food item within the oven has begun to defrost. In the preferred embodiment, the present invention incorporates digital signal processing utilizing a microprocessor to calculate the reflection coefficient and generate the control signal based upon digital signal inputs that are a function of the detected microwave energy within the waveguide.