The authors present an analysis in the frequency domain of lightning electromagnetic fields making use of a computer code developed by the authors. The analysis is made for different distances from the lightning channel, for two different current waveshapes corresponding to the first and the subsequent negative return stroke, and evidencing the contribution of the various terms (static, induction and radiation) to the total field. The calculated frequency spectra match satisfactorily with the average spectra obtained by others by means of measured oscillogram. Features of the calculated spectra, such as the peak near 4 kHz, the decrease with 1/f from about 104 to 106 Hz and with 1/f2 above 106 Hz are in reasonably good agreement with the experimental spectra. Dips in the electromagnetic field spectrum at large distances from the lightning channel and at high frequencies have been justified