A Study of the Effects of Helix Misalignment on the Cold Parameters of a Sheath Helix Slow-Wave Structure

The cold parameters including the dispersion properties and the average interaction impedance of a sheath eccentric helix slow-wave structure in a circular waveguide are investigated. With the applications of orthogonal properties of the helix modes and Graf´s addition theorem for modified Bessel functions, the closed-form characteristic equations concerning the helix modes are derived rigorously. The obtained characteristic equations show that the misaligned structure admits only the field solutions existing in the form of the superposition of hybrid modes. Numerical calculations show that the normalized phase velocity and the average interaction impedance of the helix dominant mode decrease as the misalignment of the inner helix increases and that more hybrid modes must be considered for satisfactory convergence with the increase in the misalignment. The effects of different misalignments on the cold parameters of the eccentric structure are investigated and the results are discussed.