  We discuss our ATCA 18.5 and 22 GHz flux density measurements of the Southern
extragalactic sources in the complete 5-GHz sample of Kuehr et al. (1981). The
high-frequency (5-18.5 GHz) spectral indices of steep-spectrum sources for
which we have 18.5-GHz data (66% of the complete sample) are systematically
steeper than the low-frequency (2.7-5 GHz) ones, and there is evidence of an
anti-correlation of high-frequency spectral index with luminosity. The
completeness of the 18.5-GHz data is much higher (89%) for flat-spectrum
sources (mostly quasars), which also exhibit a spectral steepening. Taking
advantage of the almost complete redshift information on flat-spectrum quasars,
we have estimated their 5-GHz luminosity function in several redshift bins. The
results confirm that their radio luminosity density peaks at z_peak \simeq 2.5
but do not provide evidence for deviations from pure luminosity evolution as
hinted at by other data sets. A comparison of our 22-GHz flux density with WMAP
K-band data for flat-spectrum sources suggests that WMAP flux densities may be
low by a median factor of \simeq 1.2. The extrapolations of 5-GHz counts and
luminosity functions of flat-spectrum radio quasars using the observed
distribution of the 5-18.5 GHz spectral indices match those derived directly
from WMAP data, indicating that the high-frequency WMAP survey does not detect
any large population of FSRQs with anomalous spectra.
