Measurements of microwave scattering by ultrasound in air at oblique incidence

Scattering of electromagnetic waves against acoustic waves has been studied in acousto-optics and radio acoustic sounding of the atmosphere, among others. Strong interaction requires phase matching, which occurs at two angles between waves. These depend on the wavelength ratio. The scattered electromagnetic wave is frequency shifted by the acoustic frequency, either up or down depending on the angle between waves. This work presents experimental verification of this scattering for 27.3 GHz microwaves and 40 kHz ultrasound in air. Both up- and down-shifting of the scattered wave is shown. Previous work has mostly focused on either nearly perpendicular or parallel incidence. In contrast, this work considers oblique incidence with frequencies differing from those used in acousto-optics and radio acoustic sounding.