an electromagnetic radio - frequency or microwave device having a treatment chamber with a transmission wall , which is permeable for the electromagnetic waves , to an adjacent chamber and with a design of the transmission wall for influencing the intensity distribution of the transmission of the electromagnetic waves into the treatment chamber allows low - loss control of the intensity of the transmitted waves in that the transmission wall has a large number of antenna elements which are directed at an angle to the wall plane of the adjacent chamber , are in the form of rods , and project centrally through through - openings which are bounded by electrically conductive material .