  We explore the radio properties of powerful (rest-frame luminosity
$\gtrsim10^{28}$ W Hz$^{-1}$ at 500 MHz) high-redshift (z > 3.5) quasars. The
aim of this study is to gain a better understanding of radio-loud sources at
the epoch when they reach the highest space density. We selected 29 radio-loud
quasars at low radio frequencies (76 MHz). Their radio spectra, covering the
range from 76 MHz to 5 GHz, are generally well reproduced by a single power
law. We created samples that were matched in radio luminosity at lower redshift
(from z~1.3 to z~2.8) to investigate any spectral evolution. We find that the
fraction of flat-spectrum radio quasars (FSRQs) strongly increases with
redshift (from ~8% at z=1.2 to ~45% at z>3.5). This effect is also observed in
quasars with lower luminosities (down to $\sim 10^{27}$ W Hz$^{-1}$). The
increase in the fraction of FSRQs with redshift corresponds to a decrease in
the steep-spectrum radio quasars. This result can be explained, assuming that
the beaming factor and the slope of the luminosity function do not change with
redshift, if high-redshift radio-loud sources can be recognized as quasars only
when they are seen at a small viewing angle ($\lesssim 25^\circ$), while most
of them, about 90%, are obscured in the UV and optical bands. We also found a
trend for the size of radio sources to decrease with increasing redshift.
Because projection effects are insufficient to cause this trend, we suggest
that the large amount of gas causing the nuclear obscuration also hampers the
growth of the more distant sources.
