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6 Summary and conclusions

Levelling and reducing the contamination of the antenna temperature by ground emission is an important requirement in survey experiments for mapping the non-thermal component of the Galactic emission background. In the zenith-centered 1-rpm circular scans of the GEM experiment this is achieved by using a wire mesh fence around a rim-halo shielded antenna. Without the fence, a prohibitive variable component of ground contamination compromises the data taken with this portable 5.5-m dish in the Southern Hemisphere at 1465MHz with a mean amplitude of $0.52\pm0.29$þK above the level of a uniform azimuth-independent component. With the fence, the level of a uniform component was obtained by comparing differential measurements of the antenna temperature toward selected regions of the sky with model predictions of the spillover and diffraction sidelobes.

First of all, the model allowed us to investigate the shielding performance of the experiment using the fully measured beam patterns of the GEM backfire helical feeds at 408MHz and 1465MHz. We concluded that far-field diffraction effects dominate a weakly-diffracting and unshielded antenna scenario whereas near-field effects dominate a stronger-diffracting and double-shielded scenario. Furthermore, the shielding efficiency of the experiment could be quantified in terms of the normalized cumulative ratio $Q_{\rm n}$ of the spillover-induced transmission to the overall sidelobe contamination in the zenith angle range $0\ifmmode^\circ\else\hbox{$^\circ$ }\fi\le Z\le 45^\circ$. If the shielding is low enough, spillover sidelobe suppression will ensue, since the ground temperature angular distribution can introduce an upper cut-off in the relative power response of the feed. A critical element in the analysis is introduced, however, by the need to account for the asymmetric response of the feed and which seems, most likely, to result from imperfect alignment of the feed axis on the measuring stand and along the optical axis of the secondary. We used the near sidelobe pattern (out to some $30\ifmmode^\circ\else\hbox{$^\circ$ }\fi$ from axis) of the radiotelescope to ressolve the issue.

Finally, we applied atmospheric and Galactic corrections to the differential measurements before comparing the residual signal with the model predictions for the level of ground contamination. The choice of sky directions away from the Galactic Plane led to contributions from the sky between $Z=0\ifmmode^\circ\else\hbox{$^\circ$ }\fi$ and $Z=30\ifmmode^\circ\else\hbox{$^\circ$ }\fi$ which were as high, but not larger, than the ones expected from the emission of the atmosphere. The former were derived from a template sky with a sensitivity of 20þmK based on GEM data taken at 1465MHz in the Southern sky with a $HPBW \approx 5.4^\circ$.

The corrected test measurements match the model predictions if we introduce a screening efficiency factor $\beta$ which shows strict and separate linear correlations with the differential ground contamination and its diffraction components generated at the shields. Consequently, it suffices that the (total) differential ground contamination be known, for its spillover and diffracted components to be identified uniquely. With the refined model ( $\beta=0.675\pm0.052$) a uniform level of ground contamination is estimated at $1.146\pm0.075$þK with a spillover-to-diffraction component ratio of $5.7\pm0.5$. This is a spillover dominated scenario with $Q_{\rm n}=0.67\pm0.01$ and decreasing diffraction sidelobes with increasing Z.

Acknowledgements
The authors are particularly in debt to A.M. Alves, L. Arantes, E.R. Rodrigues, A.P. da Silva and Rogério R. de Souza for technical and observational support. We are also grateful to the LIT-INPE Antennas Group for its collaboration during the feed pattern measurements. The GEM project in Brazil is presently being supported by FAPESP through grants 97/03861-2 and 97/06794-4. M. Bersanelli acknowledges the support of the NATO Collaborative Grant CRG960175.


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