Diffraction fields of echosonde antennas

Abstract

The scalar diffraction integral is used in the present paper to obtain transforms of aperture field-distributions of an echosonde (acoustic echo-sounding) antenna. A highly convergent Lommel series of the form Ω0m(ν, η), is used to obtain the antenna Fresnel-zone field near the boresight of the antenna, The zero-order Hankel transforms of the real aperture-field distributions, which can also be used to synthesize the antenna-pattern, are obtained for circularly symmetric aperture-fields. The zero-order Lommel transforms are applied in the case of complex aperture-fields using the type of radial phase-changes (across the aperture) previously discussed [Adekola et al., Radio Sci. 12, 11–22 (1977)], The zero-order Lommel transforms arising from uniform excitations are expressed in closed forms using the Anger—Weber functions. Echosonde antennas are useful for investigating the thermal-structures and dynamics of the lower atmosphere [S. A. Adekola, J. Acoust. Soc. Am. 60, 230–239 (1976)]. Halfpower beamwidth-variations between 13.45° and 3.16° are obtained within 1–5 kHz. Sidelobe attenuations are between 50 and 60 dB. The theory is in good accordance with available experimental data. Atmospheric turbulence-components producing the strongest scatter range from 0.165 m at 1 kHz through 0.035 m at 5 kHz. The antenna-aperture is of several wavelengths from 3.7λ at 1 kHz through 25.7λ at 5 kHz. [Work supported by NRC.]