The measurement of the electric field created by cosmic-ray induced air showers is nowadays a well-established technique. Due to technical limitations, the low-end part of the frequency spec- trum of the field has not been thoroughly exploited or understood, even though some experiments have indicated a large electric field at low frequencies. In this work, we present a new equation for the electric field of a particle track in time-domain valid for all frequencies, and therefore suited for the treatment of the near-field regime. After that, we calculate the low frequency radio signal of extensive air showers in the far-field and near-field regimes using a new version of the SELFAS Monte Carlo code that includes our equation.