Geometric Dispersion in Corrugated Coaxial Cables

Geometric dispersion of a periodic coaxial transmission line (TL) with outer conductor sinusoidal corrugations and uniform inner conductor is examined. This is of practical importance when the TL is long and/or used for wideband pulse propagation. Eigenfrequency simulations with a finite element code, and an analytic approach based on a modal decomposition of the electromagnetic field and Floquet theory, are used to determine the dispersion relation. To gain insight into the behavior of the coaxial TL dispersion relation for the TEM-like mode in the long-wavelength limit, approximations are developed for the dispersion relation of a simpler planar TL with rectangular corrugations. Finally, comparisons between coaxial TLs with other corrugation profiles, including rectangular and triangular, are made.