IEEE Std 1260-2018 pdf download – IEEE Guide on the Prediction, Measurement, and Analysis of AM Broadcast Reradiation by Power Lines.
As the electromagnetic waves emanating from the radio station travel outward from the antenna array, they may meet various manmade structures containing metal. The passing waves induce electric currents to flow in the metal. These induced currents radiate their own electromagnetic waves at the same frequency as the radio station. The waves produced by the induced currents are called reradiation. The reradiating waves may alter the effective far-field pattern of the AM station. A decrease in received signal can mean a loss of listeners for the station, while an increase can cause the pattern to exceed its allowable limit in certain directions. Vertical structures are most efective at reradiation when they are close to a quarter wavelength ()4) tall. The AM broadcast band of 535- -1705 kHz results in corresponding N4 heights of 140 m to 44 m. This range of heights includes many buildings, transmission line structures, antenna towers, wood poles with vertical ground wires, and even down guys without series insulators. Horizontal structures, such as power lines, also can reradiate signals by picking up current on their tower-skywire-tower loops. In the case of a power line, reradiation is directly proportional to the AM radio frequency currents in its towers and overhead ground wires. These currents are dependent on the wavelength, tower design, and tower spans. If a loop consisting of two towers, the span between them (provided at least one overhead ground wire is present), and the ground image is a multiple of the wavelength of the AM station, then a resonance may be set up that causes a high current to flow. As an example, 50 m tall towers with a 200 m span give a loop length of 500 m, which is 2λ fora 1.2 MHz signal. The loop distance to the second or third tower over can also be of concern. For power lines without skywires, the tower height in wavelengths and the shape become the prime factors.