Abstract: A semi-flexible double-ridge waveguide comprises a corrugated tube formed into a special dumbbell-shaped cross-section defined by parameters which are conveniently optimized to realize improved power-handling capability as well as improved attenuation and VSWR factors across extended dominant-mode operational bandwidths. The dumbbell-shaped cross-section efficiently removes the problems typically associated with the use of conventional rigid waveguide, including difficulty of installation as well as the need for precise alignment of components, by combining flexibility and ease of manufacture, even for long lengths of waveguide, through use of a continuous, uncomplicated and relatively inexpensive process.The dumbbell-shaped cross-section is totally devoid of corners and other abrupt protrusions and is defined by a geometric equation in which specific parameters can be correlatively optiminzed to improve desired electrical properties of the waveguide.