  Radio spectra of radio galaxies are often ascribed a simple power law form
($S_{\nu} \propto \nu^{\alpha}$) with the spectral index, $\alpha$, being of
order -0.7. However, all radio galaxies deviate from this simple power law
behaviour. In this paper we derive simple expressions for the average
rest-frame spectra of FRI and FRII radio galaxies. These will be used to
describe the spectral curvature of the parent (FRI and FRII) populations in
models of radio source evolution.
