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

We have analyzed observations from a radioas- tronomical experiment to survey the sky at decimetric wavelengths along with feed pattern measurements in or- der to account for the level of ground contamination enter- ing the sidelobes. A major asset of the experiment is the use of a wire mesh fence around the rim-halo shielded an- tenna with the purpose of levelling out and reducing this source of stray radiation for zenith-centered 1-rpm circu- lar scans. We investigate the shielding performance of the experiment by means of a geometric diraction model in order to predict the level of the spillover and diraction sidelobes in the direction of the ground. Using 408 MHz and 1465 MHz feed measurements, the model shows how a weakly-diracting and unshielded antenna conguration becomes strongly-diracting and double-shielded as far- eld diraction eects give way to near-eld ones. Due to the asymmetric response of the feeds, the orientation of their radiation elds with respect to the secondary must be known a priori before comparing model predictions with observational data. By adjusting the attenuation coe- cient of the wire mesh the model is able to reproduce the amount of dierential ground pick-up observed during test measurements at 1465 MHz.