A microwave heating apparatus comprising: a cavity arranged to receive a load; a microwave generator arranged to generate microwaves; a transmission line arranged to transmit the generated microwaves to the cavity; a sensing device arranged to measure electromagnetic field strengths at different positions along the transmission line, said positions being 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 apparatus also comprises a control unit configured to control feeding of the microwaves to the cavity based on the measured field strengths, wherein the microwave generator includes a frequency-controllable microwave generator, preferably a solid state microwave generator, and wherein the sensing device is arranged to measure electromagnetic field strengths for different frequencies of the generated microwaves, the microwave heating apparatus further comprising a processor configured to extract at least one of the phase and amplitude of the reflection coefficient for different frequencies of the generated microwaves using the measured field strengths, wherein the control unit is configured to control the feeding of the microwaves to the cavity based on at least one of the extracted phase and the extracted amplitude of the reflection coefficient for different frequencies of the generated microwaves, characterized in that the sensing device is arranged to measure the electromagnetic field strengths at four different positions spaced from each other along the transmission line, and wherein the spacing between two adjacent positions is approximately equal to λ/8+n×λ/2, wherein λ is a wavelength corresponding to an operating frequency of the microwave generator or a mid-value of wavelengths available in an operating frequency band of said microwave generator, and n is an integer. a cavity arranged to receive a load; a microwave generator arranged to generate microwaves; a transmission line arranged to transmit the generated microwaves to the cavity; a sensing device arranged to measure electromagnetic field strengths at different positions along the transmission line, said positions being 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 apparatus also comprises a control unit configured to control feeding of the microwaves to the cavity based on the measured field strengths, wherein the microwave generator includes a frequency-controllable microwave generator, preferably a solid state microwave generator, and wherein the sensing device is arranged to measure electromagnetic field strengths for different frequencies of the generated microwaves, the microwave heating apparatus further comprising a processor configured to extract at least one of the phase and amplitude of the reflection coefficient for different frequencies of the generated microwaves using the measured field strengths, wherein the control unit is configured to control the feeding of the microwaves to the cavity based on at least one of the extracted phase and the extracted amplitude of the reflection coefficient for different frequencies of the generated microwaves, characterized in that the sensing device is arranged to measure the electromagnetic field strengths at four different positions spaced from each other along the transmission line, and wherein the spacing between two adjacent positions is approximately equal to λ/8+n×λ/2, wherein λ is a wavelength corresponding to an operating frequency of the microwave generator or a mid-value of wavelengths available in an operating frequency band of said microwave generator, and n is an integer.