A method for heating a medium , in particular for generating a plasma, using an HF signal, with the steps: - generating an HF supply signal with a defined first operating frequency and a defined first signal power, - coupling the HF supply signal into the medium via a transmission path so that the medium is heated by the HF supply signal , - determining a first HF signal reflection along the transmission path , and - changing the first operating frequency depending on the first HF signal reflection in order to reduce subsequent HF signal reflections, characterized in that the defined first operating frequency is changed, in particular increased , by a defined first frequency value during a first test interval in order to couple the first HF supply signal into the medium with a changed, in particular increased, first operating frequency for a limited time, and that the defined first operating frequency is changed in the reverse manner, in particular reduced , by a defined second frequency value during a second test interval in order to couple the first HF supply signal into the medium for a limited time with a first operating frequency that has been changed in the reverse manner, in particular reduced, wherein a second HF signal reflection is determined during the first test interval, which correlates in time with the changed, in particular increased, first operating frequency, and wherein a third HF signal reflection is determined during the second test interval, which correlates in time with the first operating frequency, which has been changed in the reverse manner, in particular reduced, and wherein the HF supply signal is generated at a defined second operating frequency after the second test interval has elapsed and is coupled into the medium , wherein the defined second operating frequency is selected as a function of the first, second and third HF signal reflections. - generating an HF supply signal with a defined first operating frequency and a defined first signal power, - coupling the HF supply signal into the medium via a transmission path so that the medium is heated by the HF supply signal , - determining a first HF signal reflection along the transmission path , and - changing the first operating frequency depending on the first HF signal reflection in order to reduce subsequent HF signal reflections, characterized in that the defined first operating frequency is changed, in particular increased , by a defined first frequency value during a first test interval in order to couple the first HF supply signal into the medium with a changed, in particular increased, first operating frequency for a limited time, and that the defined first operating frequency is changed in the reverse manner, in particular reduced , by a defined second frequency value during a second test interval in order to couple the first HF supply signal into the medium for a limited time with a first operating frequency that has been changed in the reverse manner, in particular reduced, wherein a second HF signal reflection is determined during the first test interval, which correlates in time with the changed, in particular increased, first operating frequency, and wherein a third HF signal reflection is determined during the second test interval, which correlates in time with the first operating frequency, which has been changed in the reverse manner, in particular reduced, and wherein the HF supply signal is generated at a defined second operating frequency after the second test interval has elapsed and is coupled into the medium , wherein the defined second operating frequency is selected as a function of the first, second and third HF signal reflections. <EOS>