The early time high altitude electromagnetic pulse conducted environment calculation is revisited by using the transmission line (TL) theory and including high-frequency corrections that were not present in the earlier studies. Waveform salient parameters, such as the current peak, rise time, and pulsewidth, are studied as a function of a number of parameters involved in the calculation. The obtained cumulative distribution functions of the parameter probabilities are discussed and compared with the existing standards for the HEMP protection. The high-frequency correction to the TL model results in a decrease of the current amplitudes of 15–20%. For the highest currents, rise times in the range of 25 ns are obtained. A less significant impact was found in the calculation of the pulsewidth, which was increased to about 150 ns.