A microwave heating apparatus and a method of heating a load using microwaves is disclosed. The microwave heating apparatus comprises a cavity arranged to receive a load, a microwave generator, a transmission line arranged to transmit the generated microwaves to the cavity and a sensing device arranged to measure electromagnetic field strengths at different positions along the transmission line. The positions are selected such that the measured field strengths provide information about the phase of a reflection coefficient being representative of the ratio between the amount of microwaves reflected back towards the microwave generator and the amount of microwaves transmitted in the transmission line from the microwave generator. The microwave heating apparatus further comprises a control unit configured to control feeding of the microwaves to the cavity based on the measured field strengths.