Estimation of the Equivalent Impedance of Lightning Return-Stroke Channel From Current Waveforms Measured at Tall Towers

Two waveforms of current measured at a height of 497 m of the 634 m high Tokyo Skytree and three waveforms of current measured at a height of 533 m of the 540-m high Ostankino tower have been reproduced in simulations using the finite-difference time-domain method in the two-dimensional cylindrical coordinate system. The lightning return-stroke channel is represented by a dielectric-coated vertical conductor. The dielectric coating has high relative permeability, such as 20, 40, or 60, and serves to reduce the propagation speed of current wave along the channel-representing vertical conductor and to increase its equivalent impedance relative to that of the vertical conductor without dielectric coating. The channel-representing vertical conductor and the tall tower are excited by a voltage source inserted between them. A procedure to determine the output waveform of the voltage source from measured current waveforms has been developed. The magnitudes of measured initial current peaks ranged from 5 to 12 kA. The equivalent impedance of the lightning return-stroke channel has been inferred to be roughly 800 to 900 Ω.