  In this work a new generalization of the theory of coupled modes for
non-periodic structured waveguide is presented. Based on a set of eigen waves
of a homogeneous periodic waveguide, a new basis of vector functions is
introduced that takes into account the non-periodicity of the waveguide.
Representing the total field as the sum of these functions with unknown scalar
coefficients, a system of coupled equations that determines the dependence of
these coefficients on the longitudinal coordinate has been obtained. It was
shown that the single-wave equation has an additional phase term. In the frame
of proposed approach, the z-dependent series impedance and local wave vector
were introduced for structured waveguides.
