Now I aint an engineer but this is the way it was explained to me.
A pure radar picks up on anything solid that is within the path of the radio beams.
The majority of what a Radar picks up is not normally required for air traffic control (buildings, bridges,weather etc).
The pure Radar picture is therefore filtered by a processor which picks up particularly on angular velocity ( ie how quickly a target moves within its scope.)
This means that an Air traffic controller can sit at their Radar screen and be confident that the returns that they view are aircraft moving around the airfield.
The angular velocity of a wind turbine confuses the processor into believing it might possibly be an aircraft and so produces a radar return, it cannot be disregarded because the return received might actually be an aircraft.
Anyone still following me???
The effect, I believe is not just around where the wind farm is situated but due to reflections could be for many miles around.
This is the most likely reason for an objection to a wind farm being placed close to an airport.
My apologies to the engineers on this thread but this is the way the teccies I work with explained it to a simpleton like myself.
Ps. as a result of this sort of thing, Glasgow airport is having to get a second radar to "fill in" the bits its normal radar is now missing.