Effective frequency of the expected synchrotron emission in the observers frame as a function of redshift for different star formation rates and cosmic ray energies. For comparison, we indicate the highest (10 GHz) and lowest frequencies (70 MHz) accessible via phase 1 and 2 of the SKA.The cosmic ray energies E are in units of GeV, while Σ_SFR is given in units of M_⊙ yr^-1 kpc^-2. We assume here no evolution of the B − Σ_SFR-relation with redshift, i.e. ξ = α / 6 = 0.