Localization of the onset of the radio emission depending on (a) the expansion velocity in units of c and (b) the initial magnetic field strength, both in logarithmic scale. The other parameters have the values: cm, erg s⁻¹, , γ_min = 1, γ_max = 10⁶, p = 2, δ = 10 and z₀ = 0.001 pc. In sub-figure (a) G and in sub-figure (b) u_exp = 0.01 c. The profiles of the magnetic field strength and electrons luminosity are decreasing as R⁻¹.