  The dependence of the turnover frequency on the linear size is presented for
a sample of GPS and CSS radio sources derived from complete samples. The
dependence of the luminosity of the emission at the peak frequency with the
linear size and the peak frequency is also presented for the galaxies in the
sample. The luminosity of the smaller sources evolve strongly with the linear
size. Optical depth effects have been included to the 3D model for the radio
source of Kaiser (2000) to study the spectral turnover. Using this model, the
observed trend can be explained by synchrotron self absorption. The observed
trend in the peak-frequency -- linear-size plane is not affected by the
luminosity evolution of the sources.
