Normal Modes and Mode Conversion in Helix Waveguide
01 January 1961
255 25(3 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 19G1 ture of the guide axis, deformation of the cross section or deviation of the winding from a low and uniform pitch adds to the loss and degrades the transmission characteristics. In a perfect helix waveguide circular electric waves propagate undisturbed. Imperfections cause coupling between circular electric waves and other modes. Power is lost by conversion to unwanted modes and reconversion distorts any smooth transmission characteristics. In order to control mode conversion and reconversion in helix waveguide with practical imperfections, and also to design helix waveguides for mode filters and intentional bends, the unwanted mode characteristics and unwanted mode coupling must be investigated. Earlier calculations have resulted in a characteristic equation which implicitly determines the properties of helix waveguide-modes, and also in explicit expressions for various coupling coefficients. ' Numerical evaluations of these equations have been very informative. They were, however, not complete enough to reveal all the unwanted mode properties and could not serve as a basis for helix waveguide design in every application. The results of a more exhaustive numerical evaluation of helix waveguide equations will be presented here. In a few typical examples these results will be applied to helix waveguide design problems. First the equations which describe wave propagation in perfect and imperfect helix waveguide will be listed.