In this paper, we have studied the accuracy of field-to-current conversion factors (FCCFs) presented by Baba and Rakov (2007) for currents inferred from electromagnetic field produced by lightning strike to tall objects, considering the perfectly and finitely conducting ground, respectively. For the perfectly conducting ground, the different FCCFs for the initial peak current at the object top, the short-circuit current peak, the largest peak current at the object top, and the peak current at the object bottom have different accuracy ranging from underestimation of 18% to overestimation of 10% for the reflection coefficients at the two ends of object ρ t = À 0.5 and ρ b = 1.0, and from underestimation of 25% to overestimation of 10% for ρ t = À 0.5 and ρ b = 0.7; and their accuracy decreases with the increase of current risetime RT. For the finite conductivity with 0.01 S/m and 0.001 S/m, FCCFs will cause many errors if we do not take into account the propagation effect along the finitely conducting ground, and their errors obviously increase with the decrease of the conductivity. For example, for ρ t = À 0.5 and ρ b = 1.0, the errors are about 20% when the conductivity is 0.01 S/m while the errors are about 55% when the conductivity is 0.001 S/m for lightning strike to the 168 m high object. Therefore, we revised FCCFs by considering the propagation effect of finite conductivity on the electromagnetic field radiated by lightning strike to tall objects and found that our revised FCCFs have much better accuracy for the lossy ground than that presented by Baba and Rakov (2007).

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In this paper we have studied the propagation effect of frequency-dependent soil (FDS) on the far vertical electric fields radiated by subsequent lightning strike to tall objects with heights from 50 to 300 m. It is found that the field propagation attenuation along FDS is obviously less than that case where the parameters are assumed to be constant (low-frequency conductivity at 100 Hz, LFC), and with the decrease of LFC, the corresponding field attenuation increases. When LFC is equal to or larger than 0.01 S/m, the effect of FDS can be ignored. However, when LFC is 0.0001 S/m, the field peak for FDS may reach as much as 2.5 times of that earth with LFC for strike to a 300-m-tall object.Index Terms-Attenuation, frequency-dependent conductivity, lightning subsequent return stroke, risetime of current wave (RT), strike to tall object.