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We observed with the VLBA at 2.3 and 8.6 GHz a complete flux-density limitedsample of 482 radio sources with declination >+75 degrees brighter than 200 mJyat 1.4 GHz drawn from the NVSS catalog. 34% of the sources show parsec-scaleemission above the flux density detection limit of 30 mJy; their accuratepositions and parsec-scale structure parameters are determined. Among all thesources detected at least at the shortest VLBA baselines, the majority, or 72%,has a steep single-dish spectrum. The fraction of the sources with a detectableparsec-scale structure is above 95% among the flat-spectrum and close to 25%among the steep-spectrum objects. We identified 82 compact steep-spectrumsource candidates, which make up 17% of the sample; most of them are reportedfor the first time. The compactness and the brightness temperature of thesources in our sample show a positive correlation with single-dish and VLBAspectral indices. All the sources with a significant 8 GHz variability weredetected by the VLBA snapshot observations, which independently confirmed theircompactness. We demonstrated that 54% of the sources detected by the VLBA at2.3 GHz in our sample have a steep VLBA spectrum. The compact radio emission ofthese sources is likely dominated by optically thin jets or mini-lobes, not byan opaque jet core. These results show that future VLBI surveys aimed to searchfor new sources with parsec-scale structure should include not onlyflat-spectrum sources, but also steep-spectrum ones in order to reach anacceptable level of completeness.

Parsec-scale properties of steep and flat spectrum extragalactic radio sources from a VLBA survey of a complete north polar cap sample