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We analytically investigate the use of a wire mesh ground screen (fence) and 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 use geometric diffraction theory to model the effect of these screening and blocking shields when scanning in azimu that tilt angles from zenith in the range 0 degrees greater than or equal to Z greater than or equal to 45 degrees. We report estimates based on existing formulas for monofilar axial-mode helical antennas with expected low-level sidelobes in the direction of the 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 approach at a tilt angle Z(eq), which increases with the proximity of the diffracting edge from the near-/far-field boundary of the feed pattern. Our estimates 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 diffracted components dominate the contributionmore » for tilt angles Z less than or similar to 35 degrees. The fence is the main diffractor when Z greater than or similar to 20 degrees, but for Z greater than or similar to 25 degrees its contribution becomes insensitive to the presence of the halo. On the other hand, if the attenuation is low (