  We present a novel propagation medium using a pair of non-identical coupled
transmission lines. The medium is referred to as `butterfly' structure and is
composed of four coupled transmission lines. These four coupled transmission
lines generate higher-order dispersion modes by coupling the modes supported on
each of the transmission lines. When these transmission line pairs are placed
inside a circular waveguide, they support a strong TM01-like Degenerate Band
Edge mode. This property can be used to enhance coupling of the electron beams
to RF waves for high-power Traveling Wave Tubes. In this paper, we present the
slow wave characteristics and Degenerate Band Edge mode formation using the
coupled pair of transmission lines.
