Analysis of the Mast Contribution to the Scattering Pattern of Wind Turbines in the UHF Band I. Angulo * , D. de la Vega * , O. Rodríguez * , O. Grande * , D. Guerra * , P. Angueira * * Dept. of Electronics and Telecommunications, University of the Basque Country (UPV/EHU) Alda. Urquijo s/n, 48013 Bilbao, Spain {itziar.angulo, david.delavega}@ehu.es Abstract—The existing models to characterize signal scattering from wind turbines in the UHF band are based on the characteristics of the blades and do not consider the potential contribution from the mast. This paper aims to estimate the Radar Cross Section of a wind turbine and its components in order to determine the relative significance of the mast for the development of future scattering models. I. INTRODUCTION A horizontal axis wind turbine is normally composed of a metallic supporting tower, a metallic nacelle where the fundamental machinery is located and a rotor with three blades made of non-metallic materials. Due to their large size and the continuous movement of their parts, wind turbines may cause different effects on the radioelectric signals. Therefore, if wind farms are located near telecommunication infrastructures, there might be quality degradation of the provided services. One of the potentially affected services is television broadcasting. Several studies for the characterization of the signal scattering from wind turbines were carried out in the 70’s and 80’s. These studies are essentially theoretical, and they aim to provide simple models for estimating the scattering pattern of wind turbines and determine their potential impact on television reception quality [1]-[5]. These scattering models are based on the dimensions and/or materials of the blades, not considering the potential reflected signals from the nacelle or mast. The reason why these models are based on the blades may lie in the evolution of the wind turbines; since most of the above-mentioned studies were carried out, the blades of the wind turbines have evolved from metallic to composite materials, whilst the grid masts have been replaced by tubular metallic masts. The ITU-R has recently asked for studies that propose new models and methods of calculation to determine the signals reflected by the wind turbines and their effect on digital television reception quality [6]. In response, recent empirical studies have been carried out. The obtained results suggest that the mast generates significantly high values of scattered signals [7]. Moreover, the backscattering signals from the wind turbines may be detected in a receiver as a varying multipath channel that might increase the minimum requirements for a satisfactory reception of the digital television service [8]-[9]. These aspects should be revised and the outcomes considered for the development of future scattering models. II. OBJECTIVES The present study is based on the estimation of the Radar Cross Section [10] of a wind turbine by using the Physical Optics method. Simulations aim to determine the relative significance of the mast on the RCS of the whole wind turbine, in order to evaluate the validity of the theoretical models for the UHF band, which only consider the contributions from the blades. For this purpose, a characterization of each element of the wind turbine has been carried out for different incident and scattering directions. Furthermore, different illumination conditions are also analyzed. III. METHODOLOGY The Physical Optics (PO) method [11]-[12] is based on the estimation of the field strength values radiated by the induced currents on the illuminated portions of a target surface. The current values over the illuminated portions are estimated as the current at each point that would be found on a tangent plane of similar material, and set to zero over the shadowed portions. The scattered far field strength is then obtained by integrating the approximate currents. The Physical Optics is a high-frequency approximation method that provides accurate results for electrically large objects (L≥10λ) and for observation points near the specular direction. When the observation point is far from the specular direction or the body is electrically small, the behaviour of the current at the edge of the illuminated surface significantly affects the scattered field [11]. For electrically large targets, one of the most common approaches to estimate the RCS is to decompose its surface into basic geometric shapes and obtain the total target RCS as a sum of the contributions from each of these elements [11]. For instance, any large, smooth surface can be approximated by a collection of small triangles placed edge-to-edge over the surface [13]. POFacets is an implementation of the Physical Optics approximation for bistatic or monostatic RCS estimation of complex targets developed in Matlab [14]. For the study presented in this paper, CAD models of wind turbines composed of multiple triangles have been designed. The size of the triangles is small enough to accurately define the wind turbine surfaces but still fulfilling the condition of being electrically large for the frequencies assigned to digital