Abstract: A device for coupling electromagnetic radiation from a source into a microwave chamber (20) is configured to be attached to a window (23) of the microwave chamber (20). The device comprises a waveguide (26) and a shield operable to cover an outer part of the window (23) in such a manner as to prevent electromagnetic radiation passing through the outer part. The shield defines an opening (25) configured to expose an inner part of the window (23) in such a manner as to allow electromagnetic radiation to pass through the inner part from the waveguide (26). The opening (25) has a cross-sectional area substantially equal to the cross-sectional area of the waveguide (26).