Wind Turbines and Interference to Broadcast Signals


TV Interference


 Digital TV uses 8-level amplitude modulation (8-VSB). Reception is sensitive to interference caused by wind turbines, which can alter the signal amplitude when it passes through the spinning blades (chopped) or when it is reflected from the blades (multipath). The graphs to the right show the impact of a UHF TV signal passing through spinning blades. The interference is strongest when the signal-receiver line of site passes through the turbine hub.
Multipath interference is caused by a signal bouncing off a turbine and creating a delayed signal (echo)that arrives at the receiving antenna after the primary signal and adds to the amplitude of the primary signal at the receiver. The “ghosts” seen on old analog TV sets were caused by echoes adding to the primary signal at the receiver. Multipath signals reflected from spinning blades introduce additional distortion due to Doppler shift of the echo. Today’s digital television receivers contain circuitry that can identify and cancel echoes – up to a point.
Weak signals from distant broadcasters are especially susceptible to multipath interference. The Longley-Rice TV signal propagation map to the left illustrates an example of a town to the east of a mountaintop wind farm that lost reception of TV stations that were more than 35 miles to the southwest. The developer had to restore lost reception by building a tower with translators to serve the affected area.

A pre-build TV RF survey can advise the wind farm developer of the number of Nielsen TV households (by census tract and block) within the area of potential impact and the locations of the cable and satellite pickup facilities (head-ends) that rely on local broadcast signals. It can also indicate mitigation options in advance of construction that will enable the developer to budget for their costs. The map to the left shows an example of the data collected and reported in a pre-build survey.
Broadcast Wind has received funding from the USDA to research and deploy advanced DTV / wind turbine interference computer modeling for the Broadcast and Wind Energy Industries.
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AM Interference

Turbines within 1-3 km of AM radio transmitters can alter the AM signal coverage from its FCC prescribed pattern, which can lead to FCC fines levied upon the station, lost audience, and post construction lawsuits. When needed to preserve the pattern, tower mitigation is accomplished by “de-tuning” the mast with wires and metal rods so that it becomes “invisible to” the AM antenna array.
The alteration in signal pattern due to a wind farm can be predicted by RF modeling and simulation. The AM broadcast antenna elements induce currents in nearby wind turbine masts, which in turn become passive radiating elements. The model enables the alteration in antenna pattern caused by these passive elements to be calculated as functions of turbine locations. The figure at right shows an example of the change in pattern calculated by a pre-construction model of a proposed 50-turbine wind farm near a 5-element AM antenna array. The smooth curve (blue) is the FCC authorized pattern; the jagged one (red) is the simulated coverage pattern. The model enabled the developer to adjust the siting of the turbines to minimize pattern alteration and to identify which turbines needed mitigation.

Spark Hazard During Construction

An AM radio station produces a strong electromagnetic field. The loop made by the crane boom and cable forms a receiving antenna that produces an RF voltage at the crane hook. If the crane hook comes into contact with a worker or a grounded object, an electrical arc will be produced. This arc can cause a painful burn, or startle-and-fall hazard to a worker on an elevated work area. If near combustible materials or petroleum products the electrical arc can be a source of ignition.
RF modeling can determine site-specific measures needed to maintain a safe construction site.
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FM Interference

Analog FM is generally insensitive to interference caused by wind turbines. Multipath and chopping affect signal amplitude; the FM
receiver detects frequency modulation. As long as the amplitude perturbation is sufficiently lower than the direct signal, the FM receiver will ignore it.
Digital radio is being transmitted by many FM broadcasters. The FCC authorized “HD radio” is a hybrid system proprietary to iBiquity. Low amplitude digital signals are included in-band. Audio compression enables low data rate with good signal-to-noise. Its modulation method is OFDM (like European DVB-T), so theoretically HD radio should be robust to multipath interference caused by wind turbines. More study is needed to characterize its robustness to multipath from turbines.
For much more:  www.BroadcastWind.com

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