This paper presents a new grid–based method for investigating the evolution of the steep–spectrum radio luminosity function , with the aim of quantifying the high–redshift cut–off suggested by previous work .
To achieve this , the Combined EIS–NVSS Survey of Radio Sources ( CENSORS ) has been developed ; this is a 1.4 GHz radio survey , containing 135 sources complete to a flux density of 7.2 mJy , selected from the NRAO VLA Sky Survey ( NVSS ) over 6 deg ^ { 2 } of the ESO Imaging Survey ( EIS ) Patch D. The sample is currently 73 % spectroscopically complete , with the remaining redshifts estimated via the K – z or I – z magnitude–redshift relation .
CENSORS is combined with additional radio data from the Parkes All–Sky , Parkes Selected Regions , Hercules and VLA COSMOS samples to provide comprehensive coverage of the radio power vs. redshift plane .
The redshift distributions of these samples , together with radio source count determinations , and measurements of the local luminosity function , provide the input to the fitting process .

The modelling reveals clear declines , at > 3 \sigma significance , in comoving density at z > 0.7 for lower luminosity sources ( \log P = 25 - 26 ) ; these turnovers are still present at \log P > 27 , but move to z \mathrel { \lower 2.58 pt \hbox { $ \buildrel { \textstyle > } \over { \scriptstyle \sim } $ } } 3 , suggesting a luminosity–dependent evolution of the redshift turnover , similar to the ‘ cosmic downsizing ’ seen for other AGN populations .
These results are shown to be robust to the estimated redshift errors and to increases in the spectral index for the highest redshift sources .

Analytic fits to the best–fitting steep spectrum grid are provided so that the results presented here can be easily accessed by the reader , as well as allowing plausible extrapolations outside of the regions covered by the input datasets .