Abstract: A cover fitted about the periphery of an opening in a chamber wall is provided with a channel having therein both high frequency sealing elements which prevent escape of electromagnetic radiation, and a vacuum sealing element. The HF sealing elements are spaced apart by less than 1/4 of the minimum wavelength .lambda. to be shielded against, and are preferably held in the channel by a profiled element which also holds the vacuum sealing element. Alternatively, the channel may be provided in the chamber wall, or the sealing elements may he held in place without any channel.