We present an iterative approach to correct the results obtained using the transmission line (TL) approximation for lines of finite length above a perfectly conducting X ground. The method can be applied both in the frequency- and in the time-domain. The proposed iterative procedure converges rapidly to the exact analytical solution for the case of an infinite line, and to the NEC solution for a line of finite length. Moreover, with only one iteration, an excellent approximation to the exact solution can be obtained