A theory is presented that describes a wave-mixing experiment involving two moderately powerful transmitters at different locations and a sensitive radar at a third location. The two transmitters produce a plasma wave at their difference frequency, and the sensitive radar is used as a diagnostic. The scattering cross section of this coherent process is calculated and compared with the ever present incoherent cross section. In the presence of ionospheric irregularities that backscattered power may be substantially reduced. The experiment is shown to be a promising diagnostic tool.