A theoretical and experimental investigation of antenna near-field effects as applied to incoherent backscatter measurements at Arecibo

The 'near-field' (or Fresnel zone) properties of the Arecibo spherical reflector and 430 MHz line feed have been investigated along with the influence of these properties on incoherent backscatter determinations of lower ionosphere electron concentrations. Previous attempts to measure or estimate these effects have been plagued by either inaccurate techniques or lack of clear documentation regarding the procedures employed. We thus review the form of the classical radar equation used for incoherent backscatter measurements and relate this form of the equation with range dependent gain to experimentally accessible far-field beam pattern measurements. Near-field characteristics are investigated from a theoretical viewpoint which is justified via far-field agreement between a theoretical antenna model and experiment. We conclude that in the Arecibo case the 'near-field correction' to the appropriate far-field radar equation is no more than 20% at a range of 50 km. We also comment on beam patterns which are optimum for incoherent backscatter applications.