In this paper, the lightning current in the time and frequency domains is described using the Heidler function, where the function is approximated by exponential functions. The introductory chapter of the thesis describes its task. In the second chapter, the Heidler function is introduced as an approximation of the lightning current. During t<0, the lightning current is equal to zero. At t = 0, the lightning current begins to change. Since there is no analytical solution to the Heidler approximation from the transition from the time to the frequency domain, in the third chapter the function is approximated based on the natural logarithm e, where the collocation method at a point combined with the least squares method is used to obtain the unknown values of the weight functions for the slope factor n <3, 4, …, 15> . The approximated function must satisfy the real function in m sampled points uniformly along the observation interval. It is determined that the approximation function consists of three parts: the approximation function for u < us , the approximation function for the range us <= u <= 1 and the approximation function for the range u > 1. In the fourth chapter, the values of the approximation of the Heidler function for the frequency domain are obtained, where the transition from the time domain to the frequency domain is achieved using the inverse Fourier transform. By analyzing the real and approximated functions in the fifth chapter, a small percentage of error is obtained, which can be concluded from the presented Figures 5.1-5.13. It is seen that the approximation of the Heidler function follows the original function in its entire range. In the frequency domain for n=10, a high degree of accuracy is also achieved, which is confirmed by Figure 5.14. It should be emphasized that if any slope factor n <3, 4, …, 15> was taken, a high degree of accuracy would again be achieved.