Abstract-A model of the ring waveguide of a fixed cross-section and variable distribution of the surface impedance of waveguide's wall has been considered. For a class of circular hodographs of surface impedance the analytical solution of the corresponding boundary-value problem has been obtained. This solution has been used for simulating a 'cycle slipping' phenomenon, known from the observations of VLF signals propagating over long paths in the earth-ionosphere waveguide, with the goal of clarifying the cause for its initiation. Numerical experiments have shown that this phenomenon, in the context of the model in question, is a consequence of the interconversion of two dominant waveguide modes in circumstances where their propagation constants are close.