Spun up: Planning for Microwave Paths Near Wind Turbines
The impact of wind turbines on microwave radio paths is generally a bit of a mystery and in some cases misunderstood. I’ll provide a couple planning tips along with some excellent reference material to assist you in making good decisions about how to plan for a peaceful coexistence of microwave paths and wind turbines.
On several occasions I have been involved in the planning of paths near wind farms and very near individual wind turbines. I have also participated in the investigation of performance issues caused by the presence of these spinning behemoths near microwave paths. I’ve learned and researched enough to know that if I’m planning a path over rural, flat terrain I need to consider that wind turbines could be present and that allow for additional consideration and planning if they are in the vicinity of the proposed radio sites. My first reaction in every case was to panic a bit, but upon regaining confidence and composure I found that following the recommendations below made it a straightforward planning exercise.
The presence of wind farms is obvious if you’re familiar with an area or personally involved in surveying sites. However, much of our preliminary work is done from a distance and often without a feel for the landscape. An excellent resource for identifying wind turbines and wind farms in the wild is the USGS website and the Google Earth .KMZ file they make available at, http://energy.usgs.gov/OtherEnergy/WindEnergy.aspx#4312358-data. You can quickly review Google Earth with this file to identify the potential for wind turbines in the path.
There are three main considerations to ensure a successful outcome when deploying microwave in the presence of wind turbines:
- Effects of a wind turbines in the near-field of the antenna, where the expected radiation pattern of the antenna could be altered.
- Line of sight obstruction caused by any part of the wind turbine; the tower, blade or the top rotor enclosure.
- Signal reflections caused by the wind turbine(s) in the proximity of the RF path.
The impact of the first two criteria is generally manifested as reduced received signal level (RSL) from the designed plan. This generally degrades throughput performance and sometimes causes bit errors. The third item, reflections caused by the moving blades, will generally be manifested by increased bit errors and perhaps varying RSL. Don’t let the fact that the blades are seemingly non-conductive fool you. They likely contain trace amounts of copper to assist in lightning protection.
Over the past twenty years a fairly substantial amount of empirical data was collected to quantify the impact of wind turbines on analog wireless systems in the TV broadcast and radar bands. Comparatively less work has been done to collect similar data for digital transmissions in the microwave bands. However, paths can be planned with confidence using the guidance indicated in these documents; Wind Farms and Microwave Links – Harvey Lehpamer and Fixed Link Wind-Turbine Exclusion Zone Method – D F Bacon. These documents describe the general rules for exclusion zones around the path and distill it down to simple parameters. Harvey Lehpamer’s recommendation form his paper entitled Wind Farms and Microwave Links generally recommends the following:
- 0.6 mile buffer between the radio site and any wind turbines.
- Approximately 300 feet between the LOS path and any part of a wind turbine.
These are approximations that vary with the path distance, frequency of the radios in the path and size of the wind turbines.
Have you ever suspected interference from a wind turbine? Please take a second and tell us what you did to resolve the problem below. Don’t forget to sign up for notification of future posts.