The coherent transition-radiation emission from an underdense relativistic beam of electrons, travelling through a dense plasma ripple, was studied. The evolution of this radiation field is described by a set of self-consistent pendulum-wave equations. Analytic calculations of the small-signal gain and numerical computations of the nonlinear saturation of this emission are presented. It is shown that such a device may provide a source of tunable coherent radiation ranging from the microwave to the infrared region.