The effects of channel tortuosity on electromagnetic radiations from lightning return strokes.

The analysis of radiation from a lightning return stroke requires both channel geometry and the current waveform that propagates along the channel. Analysis and interpretation in the past have been performed with simplified channel geometries or simplified current models. Our objective has been to gain insight into the effects of channel tortuosity on radiated electric field waveforms. Both theoretical investigations and experimental observations were conducted. In the theoretical part, a numerical solution of Maxwell's equations was developed and used to examine the impact of channel tortuosity on calculated electric field waveforms using artificial channel geometries and two different return stroke current models (compound exponential transmission line and the modified model of Lin et al.). In the experimental part, simultaneous channel geometries and electric field waveforms were recorded in Oklahoma and Florida. Two flashes from each location were used to compare measured electric field waveforms with calculated waveforms for reconstructed tortuous and equivalent linear channels. From both theoretical investigations and experimental observations, it is concluded that the magnitude of the effect due to tortuosity is highly dependent upon the current model and the range to the lightning flash. When the compound exponential transmission line model is used, the calculated electric field reflects clearly the tortuosity of the channel. When the more realistic and complex modified model of Lin et al. is used, the effect of channel tortuosity is smaller. Even in this case, however, the impact is still quite significant, especially at large ranges and if the tortuosity occurs in the lower portion of the channel