Noncylindrical Helix Waveguide

Helix waveguide composed of closely wound insulated copper wire covered with a jacket of dielectric material and surrounded by a coaxial metallic shield is a good transmission medium for circular electric waves.1 In long distance communication with these waves helix waveguide is useful as a mode filter, for negotiating bends and particularly as a transmission line proper. The different applications of helix waveguide require different properties of jacket and shield. Corresponding design rules have been worked out..2 The loss of circular electric waves in a metallic waveguide decreases steadily with increasing frequency only if the guide is perfectly round. The same is true for the helix waveguide. To maintain the low-loss properties of the circular electric wave, the helix waveguide must be manufactured to a high degree of roundness and uniformity. 233 234 THE B E L L SYSTEM TECHNICAL J O U R N A L , J A N U A R Y 1901 As long as the guide is cylindrical, i.e., any deviation from roundness is independent of distance along the guide, increased circular electric wave loss is the only effect of such deviation from roundness. But if at the same time this deviation changes with length, the transmission characteristics of the guide will be further degraded by mode conversionreconversion effects. At any change of cross-sectional shape of the guide, power of the circular electric wave will be scattered into unwanted modes, and vice versa. The amount of power scattered depends not only on the magnitude of change but also on the rate of change with length of these deviations from roundness.