Propagation Effects on the Electromagnetic Field Radiated by Lightnining to Tall Towers

We investigate the effect of the finite conductivity of the ground on the amplitude and waveshape of electromagnetic fields radiated by lightning return strokes to tall towers. The study is based on the engineering return stroke models extended to take into account the presence of a vertically-extended strike object. The propagation along a finitely-conducting ground is taken into account using Cooray’s approach. Simulations are presented for a homogeneous ground and considering three cases: (1) a return stroke initiated at ground level, (2) a return stroke to a 168-m tall tower corresponding to the Peissenberg tower, and (3) a return stroke to a 553-m tall tower corresponding to the CN Tower. It is shown that the propagation along an imperfectly conducting ground causes the amplitude of the field to decrease and its risetime to increase with decreasing ground conductivity. In addition, it was found that some of the fine structure of the electromagnetic field associated with transient processes along the struck tower vanishes due to propagation effects.