Influence of the height of an elevated strike object on the enhancement of lightning radiated fields

We discuss in this paper the influence of the height of an elevated strike object on the magnitude of lightning radiated electric and magnetic fields. For the analysis, the transmission line (TL) return stroke model for the lightning channel is adopted and the strike object is represented by a uniform lossless transmission line. Analytical expressions derived recently by the authors for the radiated electric and magnetic fields are used to investigate the influence of the height of the elevated strike object Two specific cases of electrically-long and electrically-short strike objects are considered in the analysis. It is shown that, for both cases, the presence of a strike object results in an enhancement of the radiated electromagnetic field, compared to that of return strokes initiated at ground level. However, the enhancement is considerably larger for tall structures compared to short ones. The paper investigates additionally the effect of other parameters such as return stroke speed and reflection coefficients at the top and at the bottom of the strike object on the field enhancement

Far-field-current relationship based on the TL model for lightning return strokes to elevated strike objects

José-Luis Bermudez, Mario Paolone, Davide Pavanello, Marcos Rubinstein

New general expressions relating lightning return stroke currents and far radiated electric and magnetic fields are proposed, taking into account the effect of an elevated strike object, whose presence is included as an extension to the transmission line (TL) model. Specific equations are derived for the case of tall and electrically short objects. The derived expressions show that, for tall structures (when the round-trip propagation time from top to bottom within the tower is greater than the current zero-to-peak risetime), the far field is enhanced through a factor with respect to an ideal return stroke initiated at ground level. The enhancement factor can be expressed in terms of the return stroke wavefront speed v, the speed of light in vacuum c, and the current reflection coefficient at the top of the elevated strike object. For typically negative values of this top reflection coefficient, lightning strikes to tall towers result in a significant enhancement of the far electromagnetic field. Expressions relating the far electromagnetic field and the return stroke current are also presented for electrically short towers and for very long return stroke current wavefronts. For the case of return strokes initiated at ground level (h=0), these expressions represent a generalization of the classical TL model, in which the reflections at the ground are now taken into account. We describe also simultaneous measurements of return stroke current and its associated electric and magnetic fields at two distances related with lightning strikes to the 553-m-high Toronto Canadian National (CN) Tower performed during 2000 and 2001. The derived expressions for tall strike objects are tested versus obtained sets of simultaneously measured currents and fields associated with lightning strikes to the CN Tower, and a reasonable agreement is found. Additionally, it is shown that the peak of the electromagnetic field radiated by a lightning strike to a 553-m-high structure is relatively insensitive to the value of the return stroke velocity, in contrast to the lightning strikes to ground

2005

On the enhancement of radiated electric and magnetic fields associated with lightning return strokes to tall structures

José-Luis Bermudez, Mario Paolone, Marcos Rubinstein

We propose in this paper new expressions relating lightning return stroke currents and far radiated electric and magnetic fields, taking into account the presence of an elevated strike object. The derived expressions show that the presence of an elevated strike object enhances the radiated electromagnetic field. The enhancement with respect to a return stroke initiated at ground level is expressed simply through a factor equal to (1+c/v), where v and c are the return stroke speed and the speed of light, respectively. The computed results using the proposed expression are compared with sets of simultaneously-measured currents and fields associated with lightning strikes to the Toronto CN Tower, and a reasonable agreement is found.

2001

Lightning Electromagnetic Fields at Very Close Distances Associated with Lightning Strikes to the Gaisberg Tower

Seyed Abbas Mosaddeghi, Davide Pavanello, Marcos Rubinstein, Abdolhamid Shoory, Pierre Zweiacker

In this paper we present and discuss measurements of electric (vertical and radial) and magnetic fields from leaders and return strokes associated with lightning strikes to the 100 m tall Gaisberg tower in Austria obtained in 2007 and 2008. The fields were measured at a distance of about 20 m from the tower. Simultaneously, return stroke currents were also measured at the top of the tower. The data include, for the first time at such close distances, simultaneous records of vertical and horizontal electric fields. The vertical electric field waveforms appeared as asymmetrical V-shaped pulses. The initial, relatively slow, negative electric field change is due to the downward leader, and the following, fast, positive electric field change is due to the upward return stroke phase of the lightning discharge. The horizontal (radial) electric field due to the leader phase has a waveshape similar to that of the vertical electric field. However, the horizontal field due to the return stroke is characterized by a short negative pulse of the order of 1 mu s or so, starting with a fast negative excursion followed by a positive one. The return stroke vertical electric field changes appear to be significantly smaller than similar measurements obtained using triggered lightning. This finding confirms the shadowing effect of the tower, which results in a significant decrease of the electric field at distances of about the height of the tower or less. The vertical and horizontal E field changes due to the return stroke were also found to be larger on average than the leader electric field changes. In a significant number of cases (33%), the vertical electric field waveforms due to the return stroke were characterized by a first peak exceeding the typical late-time flattening due to the electrostatic term. This is in contrast with similar measurements related to triggered lightning which do not exhibit such a first peak. About one quarter of the measured vertical electric field waveforms (18 pulses out of 76) featured an unusual waveform characterized by a positive leader field change followed by a bipolar return stroke field change with a zero crossing time of about 60 mu s.