Abstract Possible diffraction of u.h.f. and microwave beams in the atmosphere by pressure variations produced by sonic beams, similar to the optical diffraction in ultrasonic experiments, is investigated theoretically. Calculations indicate that appropriate sound fields can give rise to diffracted microwave signals reaching distances beyond the horizon. The types of sound fields required to produce a substantial signal in the first order diffraction maximum, are described. These signals can be stronger than those produced by scattering from irregular variations of refractive index caused by turbulence.