2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/2391Radio spectra of radio galaxies are often ascribed a simple power law form ($S_ν \propto ν^α$) with the spectral index, $α$, 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.4 pages, 3 figures, submitted to 'Particles & Fields in Radio Galaxies' eds Laing & Blundell, ASP conference seriesAstrophysicstext