On the interpretation of ground reflections observed in small-scale experiments simulating lightning strikes to towers

Using the finite-difference time-domain (FDTD) method for solving Maxwell's equations, we have simulated small-scale experiments intended to study the interaction of lightning with towers. In these experiments, employing the time-domain reflectometry (TDR), the tower was represented by a conical conductor placed between two horizontal conducting planes, and a relatively high grounding impedance (about 60 Q, constant or decreasing with time) of the bottom plane was inferred, based on the assumption that a conical conductor could support propagation of unattenuated waves in either direction. We have shown, using the FDTD simulations, that a current pulse suffers no attenuation when it propagates downward from the apex of the conical conductor to its base, but it attenuates significantly when it propagates upward from the base of the conical conductor to its apex. We show that the current reflection coefficient at the base of the conical conductor is close to 1, so that the equivalent grounding impedance of the conducting plane is close to zero. Our analysis suggests that the relatively high grounding impedance of conducting plane inferred from the small-scale experiments is an engineering approximation to the neglected attenuation of upward propagating waves. When the dependence of cone's waveguiding properties on the direction of propagation is taken into account, the results of small-scale experiments simulating lightning strikes to towers can be interpreted without invoking the fictitious grounding impedance of conducting plane. Representation of a vertical strike object by a uniform transmission line terminated in a fictitious grounding impedance appears to be justified in computing lightning-generated magnetic fields and relatively distant electric fields, but may be inadequate for calculating electric fields in the immediate vicinity of the object. This study was motivated by the growing interest in extending lightning return stroke models to include a tall strike object and calculating associated electric and magnetic fields.