Radio spectra with the peak intensity drifts from the simulation results and those calculated from theory. Exemplary, the drift of the peak intensity from higher to lower frequencies with time is shown for the case ζ = 30, δ = 2 (left panel), and ζ = 70, δ = 2 (right panel), both seen from the halo view. The black dotted curve shows the peak intensities from the simulation results, while the dark-red dashed curve shows the theoretical peak intensities calculated with Eq. (3).