A resonator unit (16) comprising a resonator chamber (40) with a first opening (42) for receiving a container (10) in a predefined position and for heating the container (10) by means of microwaves coupled into the resonator chamber (40), and with a second opening (44) via which the microwaves are coupled into the resonator chamber (40), wherein the geometry of the resonator chamber (40) relative to the predefined position of the container (10) in the first opening (42) is adapted by a device for adapting the geometry so that an electric field produced in the resonator chamber (40) in a working mode is symmetrical in relation to the container or the impedance of the resonator unit (16) equipped with a container (10) is approximately constant for containers (10) of different configurations.