Small angular scale ( high \ell ) studies of cosmic microwave background anisotropies require accurate knowledge of the statistical properties of extragalactic sources at cm-mm wavelengths .
We have used a 30Â GHz dual-beam receiver ( OCRA-p ) on the Toruń 32-m telescope to measure the flux densities of 121 sources in VSA fields selected at 15Â GHz with the Ryle Telescope .
We have detected 57 sources above a limiting flux density of 5Â mJy , of which 31 sources have a flux density greater than 10Â mJy , which is our effective completeness limit .
From these measurements we derive a surface density of sources above 10Â mJy at 30Â GHz of 2.0 ~ { } \pm~ { } 0.4 per square degree .
This is consistent with the surface density obtained by Mason et al .
( 20 ) who observed a large sample of sources selected at a much lower frequency ( 1.4Â GHz ) .
We have also investigated the dependence of the spectral index distribution on flux density by comparing our results with those for sources above 1Â Jy selected from the WMAP 22Â GHz catalogue .
We conclude that the proportion of steep spectrum sources increases with decreasing flux density , qualitatively consistent with the predictions of de Zotti et al .
( 39 ) .
We find no evidence for an unexpected population of sources whose spectra rise towards high frequencies , which would affect our ability to interpret current high resolution CMB observations at 30Â GHz and above .