The Australia Telescope 20 GHz ( AT20G ) Survey is a blind survey of the whole Southern sky at 20 GHz ( with follow-up observations at 4.8 and 8.6 GHz ) carried out with the Australia Telescope Compact Array ( ATCA ) from 2004 to 2007 .

The Bright Source Sample ( BSS ) is a complete flux-limited sub-sample of the AT20G Survey catalogue comprising 320 extragalactic ( |b| > 1.5 ^ { \circ } ) radio sources south of \delta = -15 ^ { \circ } with S _ { 20 GHz } > 0.50 Jy .
Of these , 218 have near simultaneous observations at 8 and 5 GHz .

In this paper we present an analysis of radio spectral properties in total intensity and polarisation , size , optical identifications and redshift distribution of the BSS sources .
The analysis of the spectral behaviour shows spectral curvature in most sources with spectral steepening that increases at higher frequencies ( the median spectral index \alpha , assuming S \propto \nu ^ { \alpha } , decreases from \alpha _ { 4.8 } ^ { 8.6 } = 0.11 between 4.8 and 8.6 GHz to \alpha _ { 8.6 } ^ { 20 } = -0.16 between 8.6 and 20 GHz ) , even if the sample is dominated by flat spectra sources ( 85 per cent of the sample has \alpha _ { 8.6 } ^ { 20 } > -0.5 ) .
The almost simultaneous spectra in total intensity and polarisation allowed us a comparison of the polarised and total intensity spectra : polarised fraction slightly increases with frequency , but the shapes of the spectra have little correlation .
Optical identifications provided an estimation of redshift for 186 sources with a median value of 1.20 and 0.13 respectively for QSO and galaxies .