Dispersion engineering and mode control using multiple pairs of non-identical coupled transmission lines

We present a novel propagation model that describes how a pair of non-identical coupled transmission lines (CTLs) can be used to generate a higher order dispersion propagation medium. In practice, this can achieved using a `butterfly' structure composed of four coupled transmission lines. Higher order dispersion for this structure is achieved by coupling the modes supported on each of the transmission lines. Placing the `butterfly' structure in a regular cylindrical waveguide generates a TM₀₁ mode with degenerate band edge (DBE) dispersion. This attractive DBE property can be used to significantly enhance coupling of the electron beams to RF waves for high power microwave sources (HPM) such as travelling wave tubes (TWTs). In this paper, we demonstrate a design that realizes a degenerate band edge and its associated slow wave characteristics using a pair of non-identical CTLs.