2001MNRAS.325.1167S

We report on the search for distant radio-loud quasars in the Cosmic Lens All Sky Survey (CLASS) of flat spectrum radio sources with S_5GHz>30mJy. Unresolved optical counterparts were selected from APM scans of POSS-I plates, with e<19.0 and red o-e>2.0 colours, in an effective area of ∼6400deg². Four sources were found to be quasars with z>4, of which one was previously known. This sample bridges the gap between the strong radio surveys with S_5GHz>200mJy and the samples of radio-weak quasars that can be generated via radio observations of optically selected quasars. In addition, four new quasars at z>3 have been found. The selection criteria result in a success-rate of ∼1:7 for radio-loud quasars at z>4, which is a significant improvement over previous studies. This search yields a surface density of 1 per 1600deg², which is about a factor of ∼15 lower than that found in a similar search for radio-quiet quasars at z>4. The study presented here is strongly biased against quasars beyond z>4.5, since the e-passband of the POSS-I only samples the spectra shortward of 1200Å at these redshifts.