IEEE 1260-2018 pdf free.IEEE Guide on the Prediction, Measurement, and Analysis of AM Broadcast Reradiation by Power Lines.
An AM broadcast array is carefully constructed to radiate strongly towards local listening areas and weakly in directions where interference to other stations could result. The strong signals are contained in a major lobe 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 other stations operating at the same frequency (co-channel), at the next highest or lowest frequency (adjacent channel). orat frequencies two channels away (second adjacent channel). The signal shall be weak towards these stations so as to limit the interference to acceptable levels.
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 rrradiuiion. 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 efFective at reradiation when they are close to a quarter wavelength (A.4) tall. The AM broadcast band of 535—I 705 kI lz results in corresponding A14 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 1oop length of 500 m, which is 2A fora 1.2 tlIz signal. The 1oop 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. The electrical height of a tower is typically 15% higher than the physical height due to the top-loading effect of the conductor crossarms. As quarter-wavelengths oIAM stations are 44 m to 140 m, there is great potential for resonant towers.
The effective radius of a structure strongly atkcts the radiation resistance and, therefore, the efficiency of’ the tower as an antenna. For steel towers, the effective radius is typically 3 m to 4 m. For wood pole lines, the effective radius of the grounding wire is as little as 0.01 m, resulting in a higher radiation resistance, lower parasitic current, and less reradiation.
Computer programs can be used to predict the effect ofreradiation on AM broadcast antenna patterns. Moment- method programs are the most rigorous, with structures modeled as collections of wire segments. The current in each segment is approximated by a set of current distributions, which are solved to satisfr the boundary conditions. The number of towers and skywires that can be simulated is limited due to the complexity of the problem and limitations in minimum wire separation, computing speed. and computer memory. Transmission- line programs are simpler. The skywire and its image in ground are treated as a transmission line. Current is induced into this transmission line by the incident signal and redistributed according to the tower and skywire impedances. 1any more towers and power lines can be modeled using the transmission line method.
Analysis using moment method computer modeling does not require the potentially affected AM station to be operating at the time of analysis. Accordingly, even if an AM station is silent, a tower proponent can and should utilize moment method modeling to assess whether the proposed construction or modification will affect the AM station’s pattern. Both field strength measurements and detuning, however, can only be done when an AM station is operating with its licensed parameters.
Standards and procedures have been issued and developed by the countries that signed the North American Regional Broadcasting Agreement (NARBA). Computer programs are helpful in indicating which situations might cause interference and which structures would be ideal candidates for remedial measures.
Parasitic current ollen is seen during the construction phase of a new power line near a high-power antenna array. The construction crane, tower segments. and workers can create a resonant loop, causing high levels of radio frequency (RF) current to flow. This can cause serious RF burns to a worker. Likewise, the presence of detuning stubs on a tower can cause high levels of RF current to flow, creating a safety hazard.
3.2 Proof of performance
When an AM radio station applies for a license, the proposed radiation pattern shall be provided. Afler the application has been accepted and the installation completed. the results of a proof of performance shall be submitted. with field strength measurements included to ensure that the measured pattern agrees with the proposed one.
The ratio method is a common method radio stations use to establish the shape of a pattern for a proof-of-performance submission. The ratio method involves taking field strength measurements around the station in two conditions. The first involves the normal connection of the transmitter to the array. The second. called an onsnidirrcfitrnal, involves connecting the transmitter to one antenna tower only. Assuming there are no significant reradiators, the omnidirectional pattern should be circular in shape. Therefore, the ratio between the first directional pattern to the second omnidirectional pattern yields the true shape of the directional pattern and is independent of ground conductivity.
Ratio measurements usually are made about 150 apart at a sufficient distance to be in the far field. Since the pattern to quantify usually is a smooth and regular shape, the l5 spacing usually is sufficient, Ratio tests are usually completed in a few days or weeks, ensuring that changes in ground conductivity and nearby construction are kept to a minimum.IEEE 1260 pdf downlaod.