Identifying and Avoiding Radio Frequency Interference for Wind Turbine Facilities

By Lester E. Polisky

Wind turbine facilities are normally planned for installation in areas that are sparsely populated and on high ground to take advantage of wind flow. However, often there are point-to-point microwave stations and television stations already in operation in these same areas. Because of the nature of radio wave propagation and the desire to obtain optimal placement of wind turbines, the combination of these factors can be disruptive to both microwave operation and television reception. This paper describes how these problems can be identified and avoided during the design and site selection of the wind power facilities through analysis and measurement methods used successfully at various locations throughout the United States.

Introduction

The use of wind energy, one of the oldest forms of harnessing a natural energy source, is now one of the world’s fastest growing alternative energy sources. The United States is now committed to the use of wind energy, and over the next several years billions of dollars will be spent on wind power projects. However, as new wind turbine generators are installed around the country, it is important to note that they may pose an interference threat to existing microwave systems and broadcast stations licensed to operate in the United States.

Wind turbines can interfere with microwave paths by physically blocking the line-of-sight between two microwave transmitters. Additionally, wind turbines have the potential to cause blockage and reflections (“ghosting”) to television reception. Blockage is caused by the physical presence of the turbines between the television station and the reception points. Ghosting is caused by multipath interference that occurs when a broadcast signal reflects off of a large reflective object—in this case a wind turbine—and arrives at a television receiver delayed in time from the signal that arrives via direct path.

Many states and other jurisdictions recognize the need for regulations addressing interference to radio signal transmissions from the wind turbine installations. Specifically, local planning authorities typically require project developers to ensure wind turbines will not cause interference. In some cases they require developers to notify the telecommunication operators in the area of the proposed wind turbine installation. Other factors prompting developers to undertake proactive investigation into potential interference include the need to prevent legal and regulatory problems and the desire to promote goodwill within the community—a good neighbor approach.

Microwave Systems and Television Stations throughout the United States

Comsearch has developed and maintains comprehensive technical databases containing 4’ Panel Antenna, information on licensed microwave networks and 174’ Centerline broadcast television stations throughout the United 6’ dia. Dish, 158’ AGL Centerline States. Microwave bands that may be affected by 8’ dia. High the installation of wind turbine facilities operate Performance Dish, 144’ AGL Centerline over a wide frequency range (900 MHz – 40 GHz). These systems are the telecommunication 6’ dia. High 6’ dia. High Performance Dish, Performance Dish, backbone of the country, providing long-distance 132’ AGL Centerline 135’ AGL Centerline and local telephone service, backhaul for cellular and personal communication service, data 6’ dia. High 4’ dia. Dish, Performance Dish, 110’ AGL Centerline interconnects for mainframe computers and the 112’ AGL Centerline

Internet, network controls for utilities and railroads, 8’ dia. High and various video services. Performance Dish, 100’ AGL Centerline Figure 1 depicts a typical installation of multiple microwave systems on a tower. Figures 2 through 8 show the infrastructure of microwave networks in the 900MHz, 2GHz, 4GHz, 6GHz, 11GHz, 18GHz and 23GHz frequency bands. Figure 9 depicts the locations of television broadcast stations. Figure 10 is an overlay of all licensed microwave networks and broadcast stations in the continental United States. These figures illustrate the need for careful planning and detailed interference analyses when selecting optimal locations for wind turbine facilities that will have Figure 1 Microwave Systems Installed on Tower minimal effects on microwave networks and off-air television coverage.

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Figure 2 Comsearch GeoPlanner Microwave Links, 900 MHz Band

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Figure 3 Comsearch GeoPlanner Microwave Links, 2 GHz Band

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Figure 4 Comsearch GeoPlanner Microwave Links, 4 GHz Band

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Figure 5 Comsearch GeoPlanner Microwave Links, 6 GHz Band

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Figure 6 Comsearch GeoPlanner Microwave Links, 11 GHz Band

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Figure 7 Comsearch GeoPlanner Microwave Links, 18 GHz Band

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Figure 8 Comsearch GeoPlanner Microwave Links, 23 GHz

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Figure 9 US 48 States Television Stations

Figure 10 Comsearch Geoplanner Spectrum Management Services

Microwave Path Analysis for Locating Wind Turbine Facilities

To demonstrate how the analysis is performed, we have created the following example. A ten-wind turbine facility is proposed at a given geographical location with the geographic coordinates of the individual turbines defined by NAD 83 latitude and longitudes. The wind turbines are given hub heights of 60 meters and rotor diameters of 80 meters. Figure 11 shows the wind turbines’ locations depicted as gray circles with blue crosses on a topographic map overlay.

Comsearch’s GeoPlannerTM is a comprehensive package of microwave and broadcast telecom systems and includes technical and regulatory databases, detailed contact information and digital topographic maps for any geographic area in the continental United States. Comsearch uses GeoPlannerTM to analyze potential effects the proposed wind turbine facility will have to the microwave paths in the area. Each turbine is modeled based on its geometric specifications, and the microwave paths are plotted on the map overlay based on data from the Comsearch database and shown as blue lines in Figure 11. These plots show where the potential problems of physical obstruction may occur.

Figure 11 Proposed Wind Turbine Locations and Microwave Paths

Comsearch then performs an obstruction analysis that takes into account the Fresnel Zone of the microwave path. The Fresnel Zone is the defined volume between two microwave stations wherein an obstacle can substantially degrade the performance of the communication links. The following formula defines the dimensions of the zone.

n d1d2 Rn ≅ 17.3 √ FGHz d1+d2 Where,

Rn = Fresnel Zone radius at a specific point in the microwave path, meters n = Fresnel Zone number, 1

FGHz = Frequency of microwave system, GHz

d1 = Distance from antenna 1 to a specific point in the microwave path, kilometers

d2 = Distance from antenna 2 to a specific point in the microwave path, kilometers

Before microwave links are designed, detailed survey and analysis establishes that no obstacles exist in the 1st Fresnel Zone. Antenna heights are selected taking into account tree growth over at least a ten-year period assuring the 1st Fresnel Zone will be clear for that period of time. Figure 12 is a diagram of a surveyed microwave path showing its Fresnel Zone clearance. Table 1 gives the design characteristics of the path and the expected availability.

Figure 12 Diagram of a Surveyed Microwave Path Showing its Fresnel Zone Clearance

Table 1 Microwave Path Design Data

Site 1 Site 2 Elevation (ft) 38.06 36.02 Latitude 33 44 52.30 N 33 45 19.10 N Longitude 078 59 19.70 W 078 49 19.60 W True azimuth (deg) 86.89 266.99 Vertical angle (deg) -0.12 0.02 Antenna model PL10-57W PL10-57W Antenna height (ft) 175.01 112.97 Antenna gain (dBi) 42.90 42.90 Radome loss (dB) 0.00 0.00 TX line type HJ4-50 HJ4-50 TX line length (ft) 215.00 135.00 TX line unit loss (dB /100 ft) 7.84 7.84 TX line loss (dB) 16.86 10.58 Connector loss (dB) 1.00 1.00 Miscellaneous loss (dB) 1.00 1.00 Frequency (MHz) 5800.00 Polarization Vertical Path length (mi) 9.61 Free space loss (dB) 131.53 Atmospheric absorption loss (dB) 0.13 Net path loss (dB) 77.29 77.29 Radio model WM31600 WM31600 TX power (watts) 0.10 0.10 TX power (dBm) 20.00 20.00 EIRP (dBm) 44.04 50.32 RX threshold criteria 10E-6 10E-6 RX threshold level (dBm) -90.00 -90.00 Maximum receive signal (dBm) -5.00 -5.00 RX signal (dBm) -57.29 -57.29 Thermal fade margin (dB) 32.71 32.71 Dispersive fade margin (dB) 58.00 58.00 Dispersive fade occurrence factor 1.00 Effective fade margin (dB) 32.69 32.69 Climatic factor 1.00 Terrain roughness (ft) 20.00 C factor 3.29 Average annual temperature (°F) 50.00 Worst month - multipath (%) 99.99772 99.99772 (sec) 59.92 59.92 Annual - multipath (%) 99.99943 99.99943 (sec) 179.77 179.77 (% - sec) 99.99886 - 359.54

Without the proper Fresnel Zone clearance, the design objectives of the microwave link cannot be realized.

For a wind turbine facility, the obstruction analysis insures that no part of the wind turbine assemblies will enter the 1st Fresnel Zone of any current microwave path. The maximum Fresnel Zone diameter is used to perform the initial, worst-case analysis. The maximum diameter occurs at the mid-path point of the microwave link. Although the actual Fresnel Zone volume would be an ellipsoid, the initial analysis resembles a cylinder. The maximum Fresnel Zones are depicted in Figure 13 as the area bounded by yellow lines on either side of each microwave path. As shown in Figure 13 there are four turbines, in red, obstructing three microwave paths in the area.

Figure 13 Microwave Paths Fresnel Zones and Affected Wind Turbines

At this point, a more detailed obstruction analysis is performed to determine whether using the actual Fresnel Zone, rather than the maximum zone, would provide clearance for the obstructed microwave paths. Upon further analysis, two of the paths are found to be clear of the actual 1st Fresnel Zone leaving only two of the wind turbines to be relocated to provide clearance. Based on data from the obstruction analysis, the facility developer can confidently change the locations of the obstructing wind turbines to a location that will not obstruct microwave paths in the area.

Television Station Reception Analysis and Measurement

The television reception evaluation in the presence of a proposed wind turbine facility for an area is performed as a two step process. First, we identify the licensed television stations within a 200-mile radius of the area from the Comsearch database and define their locations and coverage areas. Then we examine the communities within the immediate area of the proposed wind turbine facility to see which stations provide programming services to the area.

In most cases, the areas that wind turbine facilities are constructed are rural, and there are no more than five, and as few as one, off-air television stations providing services to the communities. In these rural areas it is not unusual for a home to have both an off-air television antenna and a direct broadcast satellite (DBS) antenna on the same rooftop. The off-air television service is important to the resident because it provides local content (news and weather for the area) whereas the DBS service most often will not.

The initial television reception analysis determines which stations can provide off-air reception in the communities. With this information a Comsearch field engineer visits the area to measure signal strength and video quality of the off-air television signals. This is done using a calibrated television antenna mounted on a 20-foot mast and connected to a spectrum analyzer through calibrated coaxial cable. After the signal strength is measured, the spectrum analyzer is replaced with a video recorder and a recording of the video signal is made. These measurements establish the baseline conditions of off-air television reception at well-defined test points in the various communities. After the wind turbine facility is installed the same measurements can be repeated to determine if the off-air television reception in the area has been degraded and by how much by comparing the results of both measurements.

Two criteria are used to describe the quality of the off-air television reception measured. One is qualitative and the other is quantitative. The qualitative criterion applies to the video quality. The field engineer applies it based on observation of the video. Table 2 contains the video evaluation scale applied by the field engineer.

Table 2 – Video Quality Evaluation

Quality Rating Picture Condition 1 Perfect cable quality 2 Good picture with some noise 3 Serviceable picture but some rolling video and noisy picture 4 Trace of picture but unfit for watching 5 No video or audio discernable N/R No TV signal received

The quantitative measure of the television signal is based on the signal level measured. Figure 14 shows the spectrum analyzer display of a measured television signal. The FCC has established various signal levels for grades of signals. Table 3 provides a comparison of video quality versus

Date: November 5, 2002 Azimuth: 38° Ant. Centerline: 15 ft

Television Channel: Channel 6 Highest Recorded Signal

MHz Level (dBm1) 83.25 –54.2

Figure 14 Television Channel Spectrum Characteristics received signal for low VHF, High VHF and UHF television signals. In practice, the subjective video quality evaluation does not correlate directly to signal level, an objective evaluation, or the FCC quality rankings of “City Grade”, “Grade A” and “Grade B”. Many times, a video quality rating of 1 or City Grade will occur at signal levels well below those in the table. Conversely, excellent video is sometimes possible beyond the Grade B contour of a television station.

Table 3 – Television Reception Grade Levels

Channels City Grade Grade A Grade B

Quality Rating 1 Quality Rating 1 or 2 Quality Rating 2 or 3 DBm dBm dBm 2 through 6 -34.7 -40.7 -61.7 7 through 13 -31.7 -37.7 -52.7 14 through 69 -28.7 -34.7 -44.7

The greatest number of television reception problems occur when the wind turbine facility is between the television station and the point of reception. Measured data at installed wind turbine facilities

indicate that a blockage loss of up to 8 dB of signal (equivalent to a drop of 84.2% of signal strength) will occur. Variation in signal can occur because of the motion of the turbine propellers. This may occur even with non-conductive propeller blades because of the dielectric characteristics of the material used in the propellers. Multipath caused by reflections from the turbine towers, hubs and propellers may cause degraded signal levels and multiple images in the picture. Electromagnetic noise produced from wind turbine generators is more prominent at low VHF channels and its effect is a function of the separation distance between the television receiving antenna and the wind turbine. This means that channels 2 through 6 will be more affected by the noise generated than channels 7 and above. Also, at distances greater than 0.5 miles from a wind turbine, the effect of the electromagnetic noise generated is negligible even to the low VHF channels.

Degraded Television Reception Mitigation Strategies

In those areas that may have their television reception affected by a wind turbine project, there are some actions that can be taken to mitigate the effects. The most cost effective to implement would be to provide direct broadcast satellite (DBS) television reception systems to the residents who have degraded off-air television reception. Satellite television reception is unaffected by the wind turbines. However, the downside to DBS television is that it often will not have local content for the rural areas affected. Another method of mitigating the effects of interference is to install wireless or cable television distribution systems that can rebroadcast the television channels to the communities whose off- air television reception is affected by the wind turbine facility. Using a wireless cable television distribution system method is the easiest of the two options due to the physical nature of the environment and the large separation distances between service points (homes) in the rural areas that the wind turbine installations are normally installed.

Through careful planning, analysis and mitigation techniques, wind turbine facilities can be successfully installed without interference to microwave networks or degraded television reception. This will ultimately result in fewer complaints from local residents, carriers, broadcasters and local governments who may have prior concerns before the wind turbine facility installation.

About the Author Les Polisky is Director of Field Services at Comsearch. Les has over 38 years of experience in the field of electromagnetic interference and the evaluation of radiation hazards. He initially joined Comsearch in 1980 after serving in the U.S. Air Force and spending 17 years with Atlantic Research Corporation. He pioneered the radiation safety services program at Comsearch and is the lead instructor in the radiation hazard training course. He is frequently published in the areas of interference issues and electromagnetic radiation hazards and can be reached at [email protected].