Curved oversized EM waveguides: adiabatic modes and parabolic equation

Two alternative representations of the wave field in a curved oversized electromagnetic waveguide are discussed: adiabatic modes, matched to the local value of the axis curvature, and full-wave solution of the vectorial parabolic equation. The latter, although being asymptotically equivalent to the adiabatic mode theory, displays additional diffraction effects. A simplified complex WKB analysis is performed in order to estimate residual mode conversion in smooth waveguide bends. Application of the vectorial PE to radio propagation in tunnels is illustrated with numerical examples