An electromagnetic cooking device includes a heating cavity and at least one electromagnetic energy source configured to generate radiation in communication with the heating cavity via a supply cavity. The cooking apparatus further includes a first port configured to emit the radiation into a first region of the heating cavity from the supply cavity and a second port configured to emit the radiation into a second region of the heating cavity from the supply cavity. A septum is disposed along the supply cavity. The septum is configured to adjust a proportion of the radiation emitted from the first port and the second port into the heating cavity.