We present the results of numerical simulations of artificial ducts during high-power HF heating performed by a novel model accounting for the effect of self-action. This effect interferes with the HF-plasma matching in the heated region and hence with electron heating. The model satisfactorily explains recent experimental observations. It helps for choosing the heating parameters optimal for duct formation, such as proper duration of the heating pulse and its frequency. It also suggests that distortion of the ducts caused by the self-action effect can be avoided by down-chirping the heating frequency. The down-chirping rates needed to suppress such distortions are evaluated.