Normal modes and mode conversion in helix waveguide

TLDR: In this paper, a program has been set up for automatically finding the complex roots by iterative approximation starting from the known roots at infinite jacket conductivity, the characteristic equation is solved for all practical values of wall impedance of the jacket and all modes of interest.

Abstract: 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 coupling between the circular electric waves and unwanted modes and degrade the transmission In designing a helix waveguide for a particular application, a jacket must be found that minimizes the transmission degradation Unwanted mode characteristics and their coupling coefficients must be known; these quantities are given by the roots of a transcendental equation involving complex Bessel functions A program has been set up for automatically finding the complex roots by iterative approximation Starting from the known roots at infinite jacket conductivity, the characteristic equation is solved for all practical values of wall impedance of the jacket and all modes of interest The representation of the mode characteristics as a function of wall impedance leads to a definite designation of modes in heterogeneous waveguide The TE pn modes of helix waveguide with n ≠ 1 can have only a limited attenuation These limits determine the design of mode filters Manufacturing imperfections increase the average TE 01 loss independently of the wall impedance Random curvature with large correlation distance is produced by laying tolerances, but its contribution to the average loss is minimized in a helix waveguide with very large wall impedance