Spillover and diffraction sidelobe contamination in a double-shielded experiment for mapping Galactic synchrotron emission

We have analyzed observations from a radioastronomical experiment to survey the sky at decimetric wavelengths along with feed pattern measurements in order to account for the level of ground contamination entering the sidelob es. A major asset of the experiment is the use of a wire mesh fence around the rim-halo shielded antenna with the purpose of levelling out and reducin g this source of stray radiation for zenith-centered 1-rpm circular scans. We investigate the shielding performance of the experiment by means of a ge ometric diffraction model in order to predict the level of the spillover an d diffraction sidelobes in the direction of the ground. Using 408 MHz and 1 465 feed measurements, the model shows how a weakly-diffracting and unshiel ded antenna configuration becomes strongly-diffracting and double-shielded as farfield diffraction effects give way to near-field ones. Due to the asy mmetric response of the feeds, the orientation of their radiation fields wi th respect to the secondary must be known a priori before comparing model p redictions with observational data. By adjusting the attenuation coefficien t, of the wire mesh the model is able to reproduce the amount of differenti al ground pick-up observed during test measurements at 1465 MHz.