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

¹ Divisão de Astrofísica, Instituto Nacional de Pesquisas Espaciais (INPE), CP. 515, São José dos Campos, 12201-970, Brazil
² Lawrence Berkeley National Laboratory, University of California at Berkeley, 1 Cyclotron Road, Bldg. 50, MS 205, Berkeley, CA 94720, U.S.A.
³ Dipartimento di Fisica, Universitá di Milano, Via Bassini 15, 20133 Milano, Italy
⁴ Escola Federal de Engenharia de Itajubá, Av. BPS 1303, Itajubá, 37500-000, Brazil
⁵ TRW R1/2128, 1 Space Park Drive, Redondo Beach, CA 90278, U.S.A.
⁶ Newfield Wireless, Inc., 2907 Claremont Av., Suite 111, Berkeley, CA 94705, U.S.A.
⁷ Centro Internacional de Física, Bogotá, Colombia

Send offprint request to: C. Tello; e-mail: tello@das2.inpe.br

Received: 24 January 2000
Accepted: 7 June 2000

Abstract

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 sidelobes. 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 reducing this source of stray radiation for zenith-centered 1-rpm circular scans. We investigate the shielding performance of the experiment by means of a geometric diffraction model in order to predict the level of the spillover and diffraction sidelobes in the direction of the ground. Using 408 MHzand 1465 MHzfeed measurements, the model shows how a weakly-diffracting and unshielded antenna configuration becomes strongly-diffracting and double-shielded as far-field diffraction effects give way to near-field ones. Due to the asymmetric response of the feeds, the orientation of their radiation fields with respect to the secondary must be known a priori before comparing model predictions with observational data. By adjusting the attenuation coefficient of the wire mesh the model is able to reproduce the amount of differential ground pick-up observed during test measurements at 1465 MHz.