Abstract: A long flexible waveguide is fabricated by corrugating a thin-walled tube circular in section and then molding the corrugated tube so as to make the shape of its section substantially rectangular. The long flexible waveguide has a substantially rectangular section wherein a corrugating pitch P falls within a range of .lambda..sub.g /10<P<2 K.sqroot.D.sup.. t where .lambda..sub.g denotes a wavelength in the waveguide, K a constant, D an outer diameter of the tube before the corrugation, and t a thickness of the wall of said tube, and wherein a radius of curvature R of each inside corner of the molded tube falls within a range of h .ltoreq.R .ltoreq.0.46 b where h denotes a depth of the corrugate form of said tube, and b a dimension of said tube on the side of a minor axis of said section. Rectangular waveguide thus fabricated has excellent electric characteristics and is produced under good working conditions.

Abstract: A method and apparatus for forming a corrugated waveguide which has pairs of sub-blocks for forming the corrugated metal tube into the desired shape repeatedly by mating the sub-blocks, separating and advancing them intermittently so that the desired elliptic section or predetermined shape of section of the corrugated tube may be gradually approximated.