As one of the potential sources of alternative energy, wind turbines are considered an emerging industry and have been erected in many locations in the US, as well as around the world. Wind turbines work on the principle of conversion of the kinetic energy from wind into mechanical energy, which is then used to generate electricity. Wind turbines in large numbers are called wind farms and could potentially impact radio communications systems, including broadcasting stations, weather radars, airport radars and terrestrial microwave point-to-point systems.
Any time engineers are designing a new microwave link in a desolate area, they should investigate existence and proximity of wind turbines. This brief article will discuss and summarize some of the concerns engineers have to address during the microwave network design, in order to ensure a peaceful coexistence of microwave links and wind turbines. The same conclusions are also applicable to projects that might be considering a construction of a new wind farm in an area with existing microwave links.
Wind Turbines and Wind Farms
1.5 MW turbines can be very large structures, with blades exceeding 67 m (220 feet) in diameter and tower heights exceeding 55 m (180 feet).1 The wind farm concerns include the fact that they are composed of highly reflective materials, they can be up to 400 feet tall and have a large radar cross section (RCS), with blade tips spinning up to 200 mph; wind farms could have hundreds of these high profile structures in a relatively small area.2
Simply speaking, RCS is the measure of a target's ability to reflect radar signals in the direction of the radar receiver (which may or may not be collocated with the radar transmitter). Although it is difficult to actually calculate the RCS of a wind turbine, some measurements show that its value could be close to the RCS of a jumbo-jet. It is easy to conclude that a large wind farm could cause false alarms (or holes in the coverage in case they are ignored by the radar) on the screens of nearby airport radars, so planes flying over the wind farm may or may not be detected.
Wind farms are usually located in elevated and exposed regions, where they can experience an unobstructed exposure to the wind. This type of exposure is also preferred in fixed telecommunication installations, like microwave sites. There has been very little research done on this topic, which is becoming increasingly interesting as more and more wind farms are being constructed. Generally speaking, microwave engineers should carefully consider three criteria that potentially could cause degradation of an RF communications system, such as near-field, diffraction and reflection/scattering.
Near- and Far-Field
The terms far-field and near-field describe the fields around an antenna or, more generally, around any electromagnetic radiation source. The names imply that two regions, with a boundary between them, exist around an antenna. Actually, as many as three regions and two boundaries exist, and it is important to notice that these boundaries are not fixed in space (see Figure 1).
Figure 1 Radiation fields of an antenna.
Usually, two- and three-region models are used. In the near-field, the field strength does not necessarily decrease steadily with distance away from the antenna, but it may exhibit an oscillatory character and, therefore, it is difficult to predict the antenna gain and radiation pattern in that region.3
Engineers perform microwave link engineering, including Fresnel's clearances and path profiles, based on the assumption that microwave antennas are in the far-field region, that is the distance between them is sufficiently large. Any large object (reflective or not), including wind turbines, in the near-field of the antenna may distort the radiation pattern of the antenna and, therefore, should be avoided.
Concept of the Fresnel Zone
The concept of the Fresnel zone is an integral part of the terrestrial microwave point-to-point link design. The most common use of Fresnel zone information on a profile plot is to check for obstructions that penetrate the zone and calculate a possible diffraction. Fresnel zones are specified employing an ordinal number that corresponds to the number of half-wavelength multiples that represents the difference in radio wave propagation path from the direct path. The first Fresnel zone is, therefore, an ellipsoid whose surface corresponds to one half-wavelength path difference and represents the smallest volume of all the other Fresnel zones (see Figure 2).
Figure 2 Fresnel zones.
In microwave engineering, the radius of the first Fresnel zone is the parameter currently employed to establish appropriate clearance of the link from different types of obstacles. The general formula (assuming that Rn << d1 and Rn << d2) to calculate the radius of nth Fresnel zone is approximated by:
In this formula, λ is a wavelength, R is a radius of the Fresnel zone, and d1 and d2 are distances from the antennas to the point of interest, and d is a microwave link length.
A more practical formula to calculate the radius of the first Fresnel zone in feet, which uses distances in miles, frequency in gigahertz, is given by:
Diffraction theory indicates that the direct path between the transmitter and the receiver needs a clearance of at least 60 percent of the radius of the first Fresnel zone to achieve free-space propagation conditions.
If the geometry of the path is such that an even-numbered Fresnel zone happens to be tangential to a good reflecting surface (such as a lake, highway, or smooth desert area, or wind turbines in this case), signal cancellation will occur as a result of interference between the direct and indirect (reflected) signal paths.
What is not included in this discussion is that the refractive properties of the atmosphere are not constant and the variations of the index of refraction in the atmosphere (expressed by the Earth-radius factor k) may force terrain irregularities to totally or partially intercept the Fresnel zone. A much more detailed discussion of the effects of climate on clearance requirements is given in reference 3.
Microwave Link Engineering in the Proximity of Wind Turbines
Perhaps the most dangerous situation is if the wind turbine is blocking or impinging the first Fresnel zone. Diffraction (obstruction) or reflection of radio waves by a wind turbine can degrade the performance of a point-to-point microwave link due to the effect of large blades rotating at approximately 32 rpm (typically there are two or three blades). Thus any significant interfering signal, such as a delayed multipath component, will fluctuate in signal level approximately 1.0 to 1.5 Hz.4
Based on some measurements, a single turbine can cause fades of 2 to 3 dB on microwave links with frequencies up to 18 GHz. A wind farm with only 17 turbines can produce up to 20 dB of fading if it is inside the Fresnel zone. Considering that the microwave link fade margin is typically 30 to 35 dB, this is a very significant loss of signal.5
It is important to remember that the horizontal axis of blade rotation varies in azimuth according to the wind direction, so this is not a static obstacle, like a tree or a building. Although typically 60 percent clearance of the first Fresnel zone is sufficient to guarantee undisturbed performance of the microwave link, in this situation the recommendation is to keep the first Fresnel zone completely, 100 percent clear. In addition, even a clear first Fresnel zone may not be sufficient, so a more stringent requirement of also keeping the second Fresnel zone clear should be implemented.
Signal reflection from the physical structure of a turbine propagating into the microwave receiver can potentially result in receiver's threshold degradation, resulting in a critical increase in the carrier-to-interference (C/I) ratio, usually expressed in decibels, depending on the modulation and coding schemes) requirement for the link. Care should be taken with respect to a possible multiple reflections from the individual turbines of a wind farm. A long string of wind turbines running in parallel with the microwave link could be especially detrimental.
Conclusion
The process of designing a microwave link in the vicinity of wind turbines follows the usual good practices of microwave engineering – avoid obstacles in the near-field of the antennas, keep first (and second, in this case) Fresnel zone clear of obstacles, and pay special attention to reflections when running microwave link in parallel to the long string of wind turbines. In some special cases, space diversity or some other method of reducing effects of multipath and improving reliability of the microwave link may be required.
References
- M. Ragheb, "Radar Signatures of Wind Turbines," 2009.
- S. Baron, "Wind Turbines in the National Airspace System," FAA, Presented to BLM Renewable Energy Summit, 2009.
- H. Lehpamer, "Microwave Transmission Networks – Planning, Design, and Deployment," 2nd Edition, McGraw-Hill, New York, NY, 2010.
- D.F. Bacon, "Fixed-Link Wind-Turbine Exclusion Zone Method," 2002 www.
ofcom.org.uk. - B.S. Randhawa and R. Rudd, "RF Measurement Assessment of Potential Wind Farm Interference to Fixed Links and Scanning Telemetry Devices," ERA Technology Ltd., Surrey, UK, 2008,
www.ofcom.org.uk.
Harvey Lehpamer, PhD, is a licensed Professional Engineer in the Province of Ontario, Canada. He is an owner and the Principal Engineer of the HL Telecom Consulting, company in San Diego, CA. Lehpamer is an author of a number of technical articles and books: "Transmission Systems Design Handbook for Wireless Networks," Artech House, 2002, and "Microwave Transmission Networks - Planning, Design and Deployment," McGraw-Hill, 2004, and a Second Edition in 2010. The third book, "RFID Design Principles," was published by Artech House in 2008.
