  There is mounting evidence that mechanical radio source feedback is important
in galaxy evolution and in order to quantify this feedback, detailed models of
radio source evolution are required. We present an extension to current
analytic models that encompasses young radio sources with physical sizes on
sub-kiloparsec scales. This work builds on an existing young source dynamical
model to include radiative losses in a flat environment, and as such, is the
best physically-motivated Compact Symmetric Object model to date. Results
predict that young radio sources experience significant radiative loss on
length scales and spectral scales consistent with observed Compact
Steep-Spectrum sources. We include full expressions for the transition to
self-similar expansion and present this complete model of radio source
evolution from first cocoon formation to end of source lifetime around 10^8
years within the context of a simplified King profile external atmosphere.
