Effect of the anistropy of density fluctuations on the directivity of the radio emission. Top: normalised directivity as histograms calculated from the results of the simulations (described in Sect. 2.2), with anisotropy factors α = 0.25, 0.30, 0.40, 0.50, 0.60, 0.75, and 1.0. The result of the exponential fit is shown as a line for each curve and the values of the parameter Δμ resulting from this fit are displayed in the low right corner. Bottom: evolution of the parameter Δμ with the anisotropy factor α. The distribution is fitted with an exponential model: Δμ ∝ e^(a₀α), which gives a₀ = 2.2 ± 0.3. The result of this fit is shown as a dashed line.