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) 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 azimuth at tilt angles from zenith in the range 0° ≥ Z ≥ 45°. 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 Zeq, 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 10-dB attenuation and high enough to level out the horizon profile at the prime focus of the antenna, the diffracted components dominate the contribution for tilt angles Z $\lesssim$ 35°. The fence is the main diffractor when Z $\gtrsim$ 20°, but for Z $\gtrsim$ 25° its contribution becomes insensitive to the presence of the halo. On the other hand, if the attenuation is low (<1dB), the increase in ground solid angle with tilt angle makes the contribution due to transmission and ground exposure the dominant one.