On the influence of elevated strike objects on directly measured and indirectly estimated lightning currents

The knowledge of lightning current parameters comes essentially from direct measurements obtained using instrumented towers or triggered lightning, and from measurements of lightning electromagnetic fields assuming empirical or theoretical relations between fields and currents (indirect estimation). In this paper, the effects of an elevated strike object on both the direct and indirect estimation of the current parameters is examined. The authors first give the expressions which allow for what they call the `decontamination´ of measured lightning currents from the tower reflections, assuming the tower as a perfectly conducting strike object. They show that even though the reflections might not be discernible in experimental recordings for tower heights as those used in Berger et al.´s and Garbagnati and Lo Piparo´s investigations, the peak current might be significantly affected. They then extend the transmission line (TL) return-stroke model of Uman and McLain, by taking into account the presence of an elevated strike object and the multiple reflections at the bottom and at the top of it. In general, radiated fields calculated considering and neglecting the presence of the strike object differ. It is shown that relatively short strike objects, such as triggered lightning launching structures, may not appreciably affect the calculated field peak values and overall wave shapes, but could affect the field derivatives