Presents an analysis in the time and frequency domains of the electromagnetic field originated by negative lightning return strokes, calculated at ground level. The analysis is performed on the basis of the transmission line model modified by the authors, which gives results in good agreement with experimental data. The influence of several parameters which deeply affect the time waveshape and the frequency spectra of the electromagnetic field has been investigated. These parameters are: the rise time and the peak value of the lightning current pulse, its velocity of propagation and the rate of decreasing of the intensity of the current pulse propagating along the channel. The influence of other parameters such as the lightning channel height and the distance of the observation point is also analyzed. It is shown that the height of the channel does not affect the electromagnetic field. Two simplified expressions of the electromagnetic field used for the calculation of overvoltages induced on transmission lines are discussed in order to assess their validity limits