Mode Conversion in Metallic and Helix Waveguide*

Helix waveguide, composed of closely wound insulated copper wire covered with an absorptive or reactive jacket, transmits circular electric waves with low loss. Mechanical imperfections, such as curvature and deformation, cause mode conversion and degrade the transmission. Generalized telegraphist's equations describe propagation in an imperfect helix waveguide with coupling between the many propagating waves. The coefficients of coupling depend strongly on the outside jacket. However, the sum of the squares of the coupling coefficients is independent of the jacket for circular electric waves. As a consequence, the average circular electric wave loss in a helix waveguide with random imperfections is also nearly independent of the jacket and the same as in a metallic pipe.