Reciprocity and the scattering matrix of waveguide modes

The implications of the Lorentz reciprocity theorem for a scatterer connected to waveguides with arbitrary modes, including degenerate, evanescent, and complex modes, are discussed. In general it turns out that a matrix $\mathit{CS}$ is symmetric, where $C$ is the matrix of generalized orthogonality coefficients and $S$ is the scattering matrix. Examples are given, including a scatterer surrounded by waveguides or free space, and discontinuities of waveguides.