Lightning Performance of Vertical Antenna Ground Systems

Most vertical antennas used in the AM standard broadcast band in the United States have extensive radial ground systems beneath them which serve as image planes. These same image planes can function as excellent ground systems for dissipating lightning discharges. When subjected to a lightning stroke, the "surge impedance" of a ground system is almost always lower than its measured resistance since the soil adjacent to the eldctrode may break down under the stress caused by intense electric fields. An equation is given for calculating the resistance of a counterpoise, and an HP-29C calculator program is included. Methods and instrumentation for measuring soil resistivity and ground-system resistance are also described.