A feedthrough for feeding a microwave signal through a wall of a casing comprises a signal waveguide having at least two portions having different cross-sections. The portion having the smaller cross-section is filled with plastically-deformable dielectric material which, under the action of pressure and heat, is bonded to the walls of the portion in which it is located. The dimensions of the portion are such as to provide an impedance match with the adjacent portions. An antenna couples electromagnetic energy between the waveguide and a strip transmission line. A metal cap seals the end of the waveguide and screens the antenna.