Computations of the effects of wind turbines in the close near field of RF installations

The principal effects that have been investigated are the attenuation due to shadowing and the occurrence of false echoes in a radar system. For a communication system, the attenuation due to the presence of the turbine and the modulation due to the movement of the blades and nacelle have been investigated. Some agencies (most Air Navigation Service Providers, for instance) require very large distances to be maintained from their installations, to allow them to follow all air traffic clearly. For special applications however, it is possible to install turbines much closer to the RF installations than one might expect from general rules. The first aspect that has to be noted is that the near-field radar cross section (RCS) of a turbine is much smaller than the far-field formulas. Indeed, the far-field of a turbine is only reached when the turbine is out of view of the installations due to the curvature of the earth. So, the interaction with the turbine happens in the Fresnel zone of the turbine. Approximated formulas for turbines in the far-field of the RF system have already been published in. In this paper, we will investigate a turbine which is even closer namely in the Fresnel zone of an RF system antenna, which has not yet been investigated up to now.