The present invention relates a microwave applicator for heating a sample by microwave radiation which is transmitted via an at least partially tapering transmission duct from a microwave source to a cavity adapted to receive the sample to be heated. The transmission duct has at least one external wall, said wall defining an internal space for the propagation of said microwave radiation and comprising an interface which is at least partially permeable to said microwave radiation. The interface is at least partially arranged within said tapering portion of the duct. The duct is adapted to form a jacket surrounding the cavity with said interface forming an inner wall of said jacket.