Diffraction analysis of a double-shielded antenna in the Fraunhofer and Fresnel regimes: Model predictions

We analytically investigate the use of a wire mesh ground screen (fence) an d a halo of extension panels around a helically fed parabolic reflector in order to estimate the ground contribution to the antenna noise temperature in an experiment aimed at surveying the sky at decimeter wavelengths. We us e geometric diffraction theory to model the effect of these screening and b locking shields when scanning in azimuth at tilt angles from zenith in the range 0 degrees greater than or equal to Z greater than or equal to 45 degr ees. We report estimates based on existing formulas for monofilar axial-mod e helical antennas with expected low-level sidelobes in the direction of th e halo region. As long as there is no significant coupling between the near -field patterns of both the feed and the diffracting halo, estimates using the Fraunhofer approximation agree with those calculated with the Fresnel a pproach at a tilt angle Z(eq), which increases with the proximity of the di ffracting edge from the near-/far-field boundary of the feed pattern. Our e stimates show that for a fence of some IO-dB attenuation and high enough to level out the horizon profile at the prime focus of the antenna, the diffr acted components dominate the contribution for tilt angles Z less than or s imilar to 35 degrees The fence is the main diffractor when Z greater than o r similar to 20 degrees, but for Z greater than or similar to 25 degrees it s contribution becomes insensitive to the presence of the halo. On the othe r hand, if the attenuation is low (<1dB), the increase in ground solid angl e with tilt angle makes the contribution due to transmission and ground exp osure the dominant one.