Lightning: earthing electrodes harmonic response

The grounding behavior (resistive, capacitive, inductive) to a lightning discharge is related to the electrodes´ geometrical dimensions, shapes and the soil characteristics (resistivity ρ, permittivity ε, permeability μ). The border between the capacitive and inductive behaviors corresponds to a frequency value (critical frequency f_c), analyzed with the T line model. A practical relation (f_c, ρ/1) is obtained from variable frequency tests made over different electrodes