Frequency-dependent impedance of vertical conductors and a multiconductor tower model

The paper proposes a frequency-dependent characteristic impedance of a vertical multiconductor system above an imperfectly conducting earth and a multiconductor multistair model with a number of legs between tower arms to represent an actual tower for a lightning surge analysis. Surge impedances calculated by the proposed formula and model show a satisfactory accuracy compared with a number of experimental and field test results of vertical conductors and actual towers. It is observed that an imperfectly conducting earth causes no significant effect on the characteristic impedance of a tower, and the wave propagation velocity is nearly the light velocity in free space in the entire frequency range