an apparatus for heating containers , and in particular plastic performs . at least one microwave generating unit generates an electromagnetic alternating field in the form of microwaves a microwave transfer unit transfers the microwaves generated by the microwave generating unit to a resonator unit a transport means transports the plastic preforms relative to the resonator unit . the resonator unit includes a resonator housing forming a receiving chamber for heating the plastic preforms . the resonator unit is arranged such that the electric field lines of an electric field that heats the container are arranged obliquely in relation to a longitudinal direction of the container .