A compact, high speed, miniaturized adaptive controller for a microwave power generator. A pair of directional couplers are arranged between a microwave source and a circulator to measure forward and reverse power. An electromagnet in the circulator may be controlled in response to a VSWR calculation. In some arrangements, the directional couplers may be loop-type directional couplers that are integrated into a waveguide magnetron launching assembly. The directional couplers may use a tapered coaxial line that presents an impedance that reduces along the length thereof. This enhances the directivity of the loop coupler and improves performance of the controller.