IEEE 1260-1997 pdf download IEEE Guide on the Prediction, Measurement, and Analysis of AM Broadcast Reradiation by Power Lines
4.1 Description
An AM broadcast array is carefully constructed to radiate strongly towards local listening areas and weaklyin directions where interference to other stations could result. The strong signals are contained in a majoilobe directed toward the local listening area. Other smaller listening areas can be serviced with minor lobes.The directions of weak signals, called nulls, generally are towards areas without listeners, or towards otherstations operating at the same frequency (co-channel), at the next highest or lowest frequency (adjacentchannel), or at frequencies two channels away (second adjacent channel). The signal shall be weak towardsthese stations so as to limit the interference to acceptable levels.
As the clectromagnetic waves emanating from the radio station travel outward from the antenna array, theycontaining metal. The passing waves induce electric currents to flowmay meet various manmade structuresin the metal. These induced currents radiate their own electromagnctic waves at the same frequency as theradio station. The waves produced by the induced currents are called reradiation. The reradiating waves mayalter 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 effective at reradiation when they are close to a quarter wavelength (N4) tallThe AM broadcast band of 535-1705 kHz results in corresponding /4 heights of 140 m to 44 m. This rangcof heights includes many buildings, transmission line structures, antenna towers, wood poles with verticalground wires, and even down guys without series insulators. Horizontal structures, such as power lines, alsocan 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 tow.ers and overhead ground wires. These currents are dependent on the wavelength, tower design, and towerspans. If a loop consisting of two towers, the span between them (provided at least one overhead ground wireis present), and the ground image is a multiple of the wavelength of the AM station, then a resonance may beset up that causes a high current to flow. As an example, 50 m tall towers with a 200 m span give a looplength of 600 m, which is 2> for a 1 MHz signal. The loop distance to the second or third tower over can alsobe of concern.
For power lines without skywires, the tower height in wavelengths and the shape become the prime factorsThe electrical height of a tower is typically 15% higher than the physical height due to the top-loading effectof the conductor crossarms. As quarter-wavelengths of AM stations are 44 m to 140 m, there is great potential for resonant towers.
The effective radius of a structure strongly affects the radiation resistance and, therefore, the efficiency of thetower as an antenna. For steel towers, the effective radius is typically 3-4 m. For wood pole lines. the effec.tive radius of the grounding wire is as little as 0.0l m, resulting in a higher radiation resistance, lower parasitic currentand less reradiation.
IEEE 1260-1997 pdf download
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