Transmission Characteristics of Helix Waveguide
A simple frequency transformation method to obtain propagation constant and mode coupling coefficients of a helix waveguide is introduced. Theoretical TE/sub 01/ mode losses agree well with measured results. Also the transmission characteristics of 5,700 of helix waveguide recently manufactured are described. The inner diameters of them are 51 mm and 60 mm and their unit length is 5 meters.
Normal modes and mode conversion in 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 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.
Helix waveguide theory and application
Generalized telegraphist's equations have been derived for curved helix waveguide and coefficients obtained for conversion from normal modes of the helix waveguide to normal modes of the metallic waveguide. A radial wave impedance at the helix interface is used to calculate the effect of composite jacket structures. Three different applications of the helix waveguide for circular electric wave transmission are discussed: As a mode filter, the helix waveguide should have a lossy jacket which causes a high transmission loss for all unwanted modes. For sharp intentional bends with tapered curvature, the helix waveguide should have a jacket of low-loss dielectric material surrounded by a highly conducting coaxial shield. For an all-helix waveguide, in order to reduce both mode conversion-reconversion effects at imperfections and loss in curvature the jacket should be medium lossy and also surrounded by a metallic shield. The distance between helix ami shield should in all applications be about a quarter of the radial wavelength in the material. Measurements on unwanted mode transmission and TE 01 curvature loss confirm the analysis.