UK regulator Ofcom has been looking at the way wind turbines affect microwave radio transmissions and radar signals, and has concluded that we just have no idea if it would be safe to put more turbines near radio connections.
There are around 40,000 microwave links in the UK, generally operating over line-of-sight connections, and used for everything from mobile-phone backhaul to internet connectivity and corporate connections. But while the connections might be line-of-sight, a turbine doesn't have to be anywhere near the direct path to screw up the signal - so at the moment such things are simply not allowed to exist.
Microwave connections, generally operating between 3 and 20GHz, have something called a Fresnel Zone, within which any physical clutter causes problems. The best way to imagine the Fresnel Zone is to consider two dishes, a couple of miles apart, and then imagine an enormous, fat cigar shape with the ends just touching the centre of each dish. The Fresnel Zone isn't quite as fat as you're imagining it - it shouldn't touch the ground at its fattest point - but anything in that zone will cause reflections and refractions that interfere with the intended signal.
There is a lot of maths in calculating the Fresnel Zone, and a certain amount of clutter is admissible - indeed, in urban installations it's unavoidable as the Fresnel Zone scrapes building tops and spires, but those things are largely static and predictable, while a wind turbine is dynamic and ever-changing. The BBC provides a site where developers can estimate how many people are going to suffer TV interference from a proposed wind farm. It creates a rough estimate, based on much lower frequencies than the microwave connections now being examined, but serves to demonstrate how wide an area can be affected by a wind farm.
Not only do wind turbines have an annoying habit of spinning round, but the blades also change pitch depending on the conditions, which is good for efficiency but a pain when trying to calculate the radar cross section - essential in working out how much interference they're going to create.
Ofcom hired Aegis Systems and ERA Technology to do some field experiments and see how big the problem is. The companies chose three wind farms in Cambridgeshire where clumps of turbines could be relied upon to generate a range of profiles and enable decent testing. The finished report (pdf) takes some reading, but for those interested in radio propagation it's worth going through, even if the conclusion is only that more research is needed.
40,000 microwave links, each with its own no-wind-turbine zone, makes a significant chunk of the country in which wind turbines can't be used, but it seems it's going to take more research to establish exactly how effectively microwave communications and wind power can live together, and to decide which we value more. ®