Early experience with wind farms and microwave links 30 years ago showed that windmills could interfere with microwave transmissions, but windmill and radio technology have changed considerably since then. Today, microwave planners may be taking extraordinary measures to avoid windmill farms—increasing CapEx and OpEx in the process —and these measures may be unnecessary.
Since the 1980s, there has been concern about the potential interference that wind turbine farms can cause to wireless communication equipment. The focus has been on TV, civilian and military radar and point-to-point microwave systems. This led to studies by University of Michigan to evaluate the degradation effects that wind turbine farms have on these systems.
The studies concluded that physical propagation effects such as dispersion and diffraction of electromagnetic signals propagating through wind turbine farms produce low-level, long-delay, multipath distortion on wireless telecommunications equipment. As a result, microwave network planners have considered windmill farms to be “exclusion zones,” and have built links around or over them rather than through them.
But windmill and microwave technology have made great strides since the 1980s, and it may no longer be necessary to build around or over windmill farms. In the 1980s, microwave links were analog and more vulnerable to interference and multipath distortion created by wind turbine blades. New digital microwave radios with Forward Error Correction (FEC), Adaptive Coding and Modulation (ACM) and high dispersive fade margin are better equipped to deal with interference and multipath distortion produced by wind turbine blades. These technological advances were not available in the 1980s and 1990s when the most rigorous studies about wind farm interference were completed.
Besides the improvements in digital microwave technology, wind turbines have also changed. In the 1980s they were smaller (compared to contemporary units) and mostly made of metal; today wind turbines are bigger, and the blades are mainly made of reinforced fiberglass, which is transparent to microwaves. Although obstruction due to the wind turbine pole and generator case will create path loss and possible diffraction of the signal, poles and casings are very thin (compared to the Fresnel zone radius of a microwave link) and would have to be in the direct line of sight of the link to produce significant penetration loss.
Although it’s always recommended to take a conservative approach to microwave path planning and necessary to do detailed planning and path surveying for each path, an overzealous design based on outdated studies can lead to unnecessary CapEx and OpEx. More rigorous studies must be undertaken to establish new guidelines based on today’s microwave and windmill technology. What we will likely find is that even if microwave links traverse the orbits of the blades or even hit a couple of towers, the microwave link should still meet good standard performance criteria.
A good starting point can be to check current links that are intentionally or accidentally traversing the orbits of a wind turbine blade. Further analysis of a link in this condition can prove or disprove the hypothesis that modern microwave radios are relatively unaffected by modern fiberglass wind turbine blades and thin wind turbine pole structures. If this hypothesis is confirmed, current guidelines can be relaxed to avoid the use of passive repeaters and bigger towers that represent additional costs.
Eduardo Sanchez is a Technical Marketing Engineer at Aviat Networks