Editor’s Note: In Dr. Frank Marlowe’s recent article for Broadcast Wind, he discusses the importance of accurately defining potential interference to electromagnetic transmissions caused by the construction of a wind farm. By being proactive and performing a pre-construction impact analysis, a wind farm developer may be able to avoid costly claims for remediation after the fact. His article is re-posted here.
At the permitting stage, a wind farm developer can encounter local resistance based on fear of loss of television or radio reception, and then after construction it can be confronted with costly claims for remediation of loss of service – radio, television or other transmission types, be it real or imagined. But it does not have to be that way. The following are the types of transmissions to consider in a pre-construction phase and the analysis that should be done.
Accurate pre-construction identification and characterization of potential interference to electromagnetic transmissions is vital to the success of a wind-energy project, both for permitting and to avoid post-construction problems. Television and radio broadcasting signals are especially critical because they directly impact the public.
TV station signals
Broadcast Wind specializes in predicting the effects of proposed wind farms on television signals for wind farm developers and other interested stakeholders such as permitting agencies and investors. The company recommends a pre-construction analysis for these types of broadcast transmissions to avoid accidentally placing turbines in obstructing locations.
A computer-based simulation study is used to assess potential signal impairment from a proposed wind farm. The strength of signals from television stations whose coverage areas include the proposed wind farm site are simulated in the regions surrounding the proposed site. Terrain-inclusive electromagnetic wave propagation software simulates signal strength over a 3-dimensional model of the terrain. Individual wind turbines are modeled as additional features in the terrain. Field strengths with and without the turbine models in the terrain are compared.
The simulation uses software specified by the Federal Communications Commission to predict radio field strength at geographic points based on the elevation profile of terrain between the transmitter and each reception point. (FCC/OET Bulletin No. 69). The methodology divides the area within a station’s coverage area into a matrix of square grid cells (0.5 km on a side) and then uses the Longley-Rice propagation model to predict the field strength at each individual cell. The output of the studies are analyzed to determine which cells (if any) show a critical reduction in the predicted field strength as a result of the presence of wind turbines.
Probe data – TV dashboard
Broadcast Wind uses proprietary stationary remotely-monitored RF probes to provide wind-farm developers and stakeholders with long term TV signal strength and signal quality data at critical locations. These probes capture TV-signal metrics through changes in seasons and atmospheric conditions before, during and after wind farm construction. This is much like the way meteorological gear is used by the wind industry to gather wind data over an extended period prior to and following wind farm construction. The probe data provides Broadcast Wind and the wind farm developers with reliable documentation of changes (if any) to signal quality consequential to construction.
The probe data can be displayed on a concise graphical data dashboard and programmed to provide alerts if parameters go out of defined limits. Dashboard data can be viewed remotely by hour, day, month and year. A roll of the mouse lets users see what was going on with atmospheric conditions at the time of the probe reading.
TV probe metrics
The top three graphical metric lines on the dashboard are displayed on a common percent scale for convenience of visualization. The fourth margin is displayed in decibels (dB).
The inset box provides the measurement data.
- The TV signal “SEQ” tells whether the video signal is watchable, or not. An SEQ of 99% would mean that the viewer sees a ”hit” or pixilation to the video, 1% of the time that they are watching.
- Signal strength in dBmV and signal quality (MER) in dB are also captured.
- “Margin” describes how much signal strength buffer there is in dB above the threshold of operation. More margin means a stronger, more stable video signal.
No single metric completely describes the quality of the signal reception. The value of the dashboard is that it presents multiple metrics that can be correlated with each other and with external factors.
Included in predictions for TV signals are estimates of the number of viewers that would be affected, both directly off-air and through cable from providers that pick up local programming off air. Estimation of impairment to cable off-air pick-up is an important, although frequently forgotten, component of an electromagnetic interference analysis (EIA) because of the large number of viewers that may be affected. There are no national databases of cable companies’ off-air receive sites. Local research must be conducted to identify pick-up points for off air cable signals that may be affected by wind farm construction.
AM and FM broadcast radio stations
A wind turbine too close to an AM transmitter antenna or multi antenna array can alter the signal coverage pattern from its FCC authorized pattern. The minimum distance a turbine can be to an antenna without altering the signal coverage pattern, called the exclusion distance, varies depending upon antenna type and broadcast frequency. For an AM station with a non-directional single antenna, the exclusion distance is equal to 1 wavelength of the broadcast signal (e.g. ½ kilometer for a station broadcasting at a frequency of 600 KHz). For an AM station with a directional multi-antenna array, the exclusion distance is the lesser of 10 wavelengths or 3 kilometers.
The coverage of an FM station whose transmitting antenna is at a distance greater than 4 km from one or more wind turbines, is not subject to degradation by the turbines. FM transmitters with antennas closer than 4 km from proposed wind turbines can, under some conditions, experience a compromised signal. This possibility exists when FM antennas and wind turbines are located in close proximity on the same mountain ridge.
When the possibility of signal interference exists, a desktop simulation study similar to the TV signal analysis described above can determine where in the station’s coverage area there is potential for interference and the number or households that may be affected. In areas where simulation indicates the potential for signal interference (predicted field strength reduced to 15 dB or more below the FCC planning factor service threshold), remote monitoring probes with FM signal quality logging capability can provide documentation of changes to the signal, or the absence of changes, as a result of the wind farm construction. The probes are installed before construction and record signal metrics before, during and after construction.
Probe data – FM dashboard
Our proprietary remote stationary probes developed to monitor television signals can also monitor FM radio signals and forward data about signal metrics to the cloud where they are available for review on a dashboard by Broadcast Wind and its clients.
Point-to-point microwave systems
Point-to-point microwaves that could be affected by the construction of wind farms span the range of frequencies from 900 MHz to 23 GHz. These microwave systems provide telecommunication backhaul and other support for essential services throughout the country including land based telephone services, cellular networks and personal communication services; data and internet interconnects, emergency and disaster communications, television and radio broadcasting, network controls for utilities, railroads and many others.
A thorough point-to-point interconnect analysis must begin with accurate antenna geolocation information. A field survey is done to validate and correct, as necessary, tower coordinate data found on FCC, FAA and other government registered data bases. Once the accuracy of tower position data is assured, a 2D analysis determines whether any microwave signals intersect a proposed wind turbine’s footprint.
After a 2D analysis is completed, a 3D analysis (pictured below) lets us determine whether the signal can safely pass under or above the turbine blades without interference from them.
Point-to-multipoint microwave systems
Wireless Internet Service Providers (WISPs) deliver Internet and other data services through microwave transmission to business and residential subscribers. WISPs can use frequency bands in licensed and unlicensed spectra. Many rural community WISPs operate in the unlicensed spectrum because of low initial capital outlay and ongoing operating costs. The most common unlicensed bands used for this purpose are the 900 MHz, 2.4 GHz and 5.8 GHz bands. Because there are no government databases containing local WISP information, site surveys and local town business research is needed to identify WISP operators and their antenna locations.
Very often wind farm developers can avoid permitting objections and post construction claims of interference by working with WISP system operators in advance of construction. 2D and 3D analysis like that used for point-to-point microwave signals can relieve concern about blade intersection, and where necessary small adjustments to turbine location (“micro siting”) can avoid signal intersections with turbine blades.
Land mobile & emergency services
Evaluation is needed for first responders, such as police, fire, emergency medical services and other state, county and municipal agencies. Generally land mobile and emergency radio systems are designed with multiple transmitters to provide redundancy so the service will not be interrupted as the receiving radio moves into and out of areas of signal blockage. Never-the-less, a thorough assessment of the effect of a proposed wind farm on emergency services radio signals is an important, yet frequently overlooked aspect of an electromagnetic impact analysis. Building a densely populated wind farm can be analogous to placing a city with hundreds of tall structures between an emergency transmitter and the emergency responder. If signal levels are attenuated following construction, higher powered transmitters, repeaters, or signal boosters may be employed to fill the compromised area. The key to success in this area is the performance of a thorough RF field analysis prior to construction. An analysis of this type will identify the industrial and business land mobile radio systems and commercial E911 operators near the proposed wind energy facility.
Mobile phone systems
Modern mobile phones support a wide variety of personal communication services including telephony, text messaging, email and internet access. The major U.S. mobile phone service providers currently support three digital technologies: legacy 2G (voice and limited data), main stream 3G and newer faster 4G. Mobile phone services are divided into three categories, each operating in its own frequency bands. Advanced Wireless Service (AWS), Personal Communication Service (PCS) and Cellular (CLR). They hold licenses on an area-wide basis which are typically comprised of several counties.
Wind turbines present no significant threat to mobile phone services. The mobile phone system architecture is based on low-latency packet switching and redundant cellular geographic coverage. Packets are dynamically routed among cells as mobile phones change location and as network traffic changes. A given mobile phone conversation is typically made up of packets that travel different routes and are assembled seamlessly at their destination. If a given cellular link is unavailable for any reason – interference from a wind turbine or other – the packet is automatically switched to another cell without service interruption. The user of the phone is unaware of cellular transitions and so will not be affected by any that may be triggered by a wind turbine.
Government radar systems
Wind farm siting can potentially affect government radar systems. The Department of Defense offers a Preliminary Screening Tool that can be used by wind farm developers to determine if there will be interference to air defense and homeland security radars (long range radars), to weather surveillance radar – Doppler radar (NEXRAD), or to military operations radars.
The FAA requires that all developers proposing a wind farm with turbines that exceed 200 feet above ground level file a Notice of Proposed Construction or Alteration form. For each turbine in the farm, the form must specify the turbine ID number, latitude and longitude in degrees, minutes and seconds, site elevation, height above ground level, overall height above mean sea level and preferred marking and lighting. Upon approval of the proposal, the FAA will issue a Determination of No Hazard…
Commercial Doppler radar
Commercial Doppler radars are located on television towers to take advantage of their height. They are either operated by the broadcaster to support the station’s local weather forecasting service or leased to other commercial parties for private weather monitoring or forecasting. A wind developer must know if his turbines will be in the line of sight of any Doppler radars. The curvature of the earth determines the minimum separation distance, below which a wind turbine is in the line of sight of a distant radar. If the separation distance is less than the sum of the distances to the horizon of each object, the turbine will be in the radar’s line of sight.
The separation distance below which a turbine is in the line of sight is given by:
Dseparation = 3.57 x [(Hturbine)1/2 + (Hradar)1/2] x 1,000
All dimensions are in the same units.
For example, a turbine with blade tip height 150 meters above ground will be within the line of sight of a radar on a tower 300 meters above ground when the distance between them is less than 106 kilometers.
A comprehensive survey of all communication towers within range of a proposed wind farm can provide a valuable check for completeness of the possible impact of a farm on the individual telecommunication services discussed in the previous sections of this article. Data obtained from FAA and FCC data bases, from county and township planning and zoning boards, and from other sources, is compiled and analyzed to ensure that all RF signals that may be affected are accounted for and identified.
The survey can be extended to on-site verification of tower locations in case there are errors in the location information on file in registered data bases. This verification will ensure the accuracy of the predicted impact of the wind turbines on microwave, television and radio signals.
By Dr. Frank Marlowe of Broadcast Wind