Reply #8: Not entirely true. Larger turbines are beginning to poke well up into the air.

[TheMadMonk]

And situating them along ridgelines only raises them higher. Turbines do have the potential to intrude on radar, and in fact I believe they have done so in the past. And while filtering out direct return pulses is almost certainly doable, dealing with scatter from the moving blades is equally certainly not such an easy task.

And no matter how much of the blade is replaced with composite materials, the tip will always be metal to deal with lighning strikes.

Modern radars also see more than metal. They can see flocks of birds, clouds, rain, and in all probability, carbon reinforced epoxy resin.

It is more a military fear than a civil aviation one. Wind turbines can create zones of airspace where the radar return is unreliable. Safety of civil aviation can be ensured by declaring no fly zones or floors on flight levels. The military has to deal with enough bugger factors already to welcome any new ones with open arm. A field of turbines might give an enemy somewhere to play hide and seek from radar guided weapon systems if not actual detection. Yes it's low probability, but that's how the military mind likes to think.

It probably doesn't help that a few early installations ended up close to military ranges with the thinking that they would cause less interference to aviation which was already banned from the surrounding airspace. Early turbines did seriously fuck with low altitude radar and the military has a long memory and doesn't easily let go of an idea once it gets it.