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Silicone rubber insulators are widely used as outdoor insulation due to their excellent surface performance. However, the decrease even the loss of hydrophobicity will take place under various environments, which causes the essential concern on the hydrophobicity evaluation. Dynamic dropping test (DDT) method is proposed as one of the evaluation methods. In this paper, based on DDT method the PD-induced electromagnetic wave is investigated for the hydrophobicity evaluation. An antenna is utilized to receive the high-frequency electromagnetic wave produced by the surface discharges, which are associated with the hydrophobicity variation. The electromagnetic signals are analyzed by a signal-processing system based on the wavelet-transform theory. The obtained 3-D images of surface discharges contain the relationship among the frequency, the time and the amplitude, which can reveal different ageing condition of the insulators. Based on the 3-D images, the fractal dimension (FD) and the lacunarity of 2-D contour line spectra are discussed to quantitatively give the complexity of surface discharges. It is found that the distribution range of discharge becomes wider and the frequency increases with the ageing time in the 3-D images, and the FD value shows the increasing tendency with the increase of ageing level. Therefore, the PD-induced electromagnetic wave can recognize the characteristics of the surface discharges, which can be utilized to evaluate the hydrophobicity of silicone rubber insulators.