A theory has been developed to explain the observation at a ground station of a signal at the modulation frequency of a high-power HF signal transmitted 180 km to the north of the station (Getmantsev et al., 1974). The theory is based on the following model. A modulated wave traveling in the ordinary mode traverses upward into the ionosphere and is reflected. At a short distance below the reflection layer, the carrier and one of the sidebands of which the modulated wave is composed interact to produce a whistler wave. The whistler generated by this mixing process then propagates to the ground where it is detected at the original modulation frequency.