1、 Report ITU-R BT.2142(05/2009)The effect of the scattering of digital television signals from a wind turbine BT SeriesBroadcasting service(television)Rep. ITU-R BT.2142 ii Foreword The role of the Radiocommunication Sector is to ensure the rational, equitable, efficient and economical use of the rad
2、io-frequency spectrum by all radiocommunication services, including satellite services, and carry out studies without limit of frequency range on the basis of which Recommendations are adopted. The regulatory and policy functions of the Radiocommunication Sector are performed by World and Regional R
3、adiocommunication Conferences and Radiocommunication Assemblies supported by Study Groups. Policy on Intellectual Property Right (IPR) ITU-R policy on IPR is described in the Common Patent Policy for ITU-T/ITU-R/ISO/IEC referenced in Annex 1 of Resolution ITU-R 1. Forms to be used for the submission
4、 of patent statements and licensing declarations by patent holders are available from http:/www.itu.int/ITU-R/go/patents/en where the Guidelines for Implementation of the Common Patent Policy for ITU-T/ITU-R/ISO/IEC and the ITU-R patent information database can also be found. Series of ITU-R Reports
5、 (Also available online at http:/www.itu.int/publ/R-REP/en) Series Title BO Satellite delivery BR Recording for production, archival and play-out; film for television BS Broadcasting service (sound) BT Broadcasting service (television) F Fixed service M Mobile, radiodetermination, amateur and relate
6、d satellite services P Radiowave propagation RA Radio astronomy RS Remote sensing systems SA Space applications and meteorology SF Frequency sharing and coordination between fixed-satellite and fixed service systems SM Spectrum management Note: This ITU-R Report was approved in English by the Study
7、Group under the procedure detailed in Resolution ITU-R 1. Electronic Publication Geneva, 2009 ITU 2009 All rights reserved. No part of this publication may be reproduced, by any means whatsoever, without written permission of ITU. Rep. ITU-R BT.2142 1 REPORT ITU-R BT.2142 The effect of the scatterin
8、g of digital television signals from a wind turbine (2009) 1 Introduction This Report is on the topic of performance of television reception in the presence of reflected signals, specifically those from wind turbines as identified in the preliminary draft new Recommendation ITU-R XXX Assessment of i
9、mpairment caused to digital television reception by a wind turbine (Annex 2 to Document 6A/196). It results from studies in Australia. 2 Background Wind turbine farms are proving to be a popular energy source. Due to this growth many administrations are now experiencing interest from developers in c
10、onstructing wind farms. This has raised concerns about the potential impact of wind farms on the reception of broadcasting services. In considering the planning for digital television services within the VHF and UHF broadcasting bands traditionally used for analogue television services, some adminis
11、trations have sought to reference Recommendation ITU-R BT.805 Assessment of impairment caused to television reception by a wind turbine. Recommendation ITU-R BT.805 was approved in 1992 in response to Question ITU-R 6/11. Preliminary draft new Recommendation ITU-R XXX Assessment of impairment caused
12、 to digital television reception by a wind turbine has been developed to address the emergence of digital television and it retains the reference to the Question ITU-R 6/11. However, a current Question ITU-R 69/6 Conditions for a satisfactory television service in the presence of reflected signals d
13、eals with reflections affecting analogue television systems. In 2004 Australia proposed a draft modification to Question ITU-R 69/6 that extended studies to include digital television. The purpose was to encourage eventual modifications to Recommendation ITU-R BT.805 as a result of further study as
14、to whether impairment is caused by wind turbines to digital television, as well as the development of further Recommendations, should they be required, relating to reflections from other objects. In 2006 Australia proposed that a working document towards a modification of Recommendation ITU-R BT.805
15、 be developed based on studies conducted in Australia. The study based on theoretical modelling identified that Recommendation ITU-R BT.805 is not adequate for predicting interference from wind farms for analogue and digital TV signals. In 2007 a further study indicated that the methods to assist in
16、 quality assessment of the coverage and service area for digital television broadcasting in System B in Recommendation ITU-R BT.1735 are not satisfactory for the type of dynamic signal variations from rotating wind turbine blades. From studies undertaken by Australia to date, Recommendation ITU-R BT
17、.805 does not currently provide adequate advice for predicting interference from wind farms for analogue and digital TV signals. Subsequently Working Party 6A proposed a Preliminary draft new Recommendation ITU-R XXX Assessment of impairment caused to digital television reception by a wind turbine b
18、e developed. 2 Rep. ITU-R BT.2142 Australia also observed that further study is required to review the relationship between the mean MER, slow MER variations, short deep MER notches and the receiver performance. The annexes to this Report contain the results of studies to date: Annex 1 Scattering mo
19、del calculations. Annex 2 The effect of the scattering of digital television signals from wind turbines. Annex 1 Scattering model calculations 1 Introduction This annex analyses the scattering model used in Recommendation ITU-R BT.805, describes its limitations and weaknesses, and suggests improveme
20、nts. The analysis is extended to include the scattering from rotating triangular shaped blades and the wind turbine pylon. 2 Analysis 2.1 Overview The basis for the scattering model in Recommendation ITU-R BT.805 is somewhat unclear, as it apparently calculates backscattering from the turbine blades
21、, although it is referred to as forward scattering. The model in the Recommendation assumes perfect conductors for the blades, although they are typically fibreglass or other composite materials, and a scattering pattern based on vertical blade orientation only. The following analysis assumes that t
22、he dimensions of the scattering object are much greater than a wavelength, which then allows a semi-rigorous analysis based on the physical optics approximation. For the low frequency Band III channels, the wavelength is of the order of 2 m. The maximum blade width is about 3 m for typical turbines,
23、 but the blades taper to a point, so the assumption that the dimensions are greater than a wavelength is clearly invalid for such low frequencies. 2.2 Analysis of Recommendation ITU-R BT.805 mathematics Recommendation ITU-R BT.805 is based on the scattering from a rectangular, metallic wind turbine
24、blade in a vertical orientation, as shown in Fig. 1. Rep. ITU-R BT.2142 3 FIGURE 1 Geometry of wind turbine blade (rectangular, vertical) and incident and scattered signals Report 2142-01y0 rHEWLWind turbine bladexREJsPzIt is assumed the incident signal is horizontally polarized and arrives horizont
25、ally at the turbine, as the transmitter is assumed to be a long distance from the wind turbine. The signal scattered from the blade is received at point R with signal strength Eat distance r from the blade. Because the blade is assumed to be metallic (infinite conductivity), the surface current dens
26、ity Jsis given by: xEHnz,xJxjks0cos0e22)(= (1) where 0is the incident angle relative to the plane of the blade and E0is the incident electric field strength (assumed to be a plane wave). The surface currents re-radiate, resulting in the far field Egiven by the surface integral: ()zxzxJrjkEWWLLzmxnjk
27、srjk=+dde),(4esin2222(2) where k = 2/ and n = cos, m = sin sin . See Fig. 1 for the definition of the angles. Upon substituting equation (1) into equation (2) the resulting field at the receiver is given by: ()()=LmWnnrAEjzxrjEErjkLLzjkmWWxnnjkrjksincsincesindedeesin00222200(3) 4 Rep. ITU-R BT.2142
28、where the blade area A = WL, n0= cos 0and sinc(x) = sin(x)/x. Therefore the scattering coefficient, , is given by: ()() () =sinsinsinsinccoscossinc,00LWggrAEE(4) In this application, the scattered signal of interest will be close to the horizon, so 0, and the scattering function can be approximated
29、by () ()= sincoscossinc)(,0Wgg . For specular reflections 0= , and the scattering function reduces to g() = sin = sin 0. The maximum of the scattering coefficient is given by rAmax= . Recommendation ITU-R BT.805 defines this maximum coefficient at a range of 1 000 m (dB), so that: dB60)/log(20)log(2
30、0 = Amax(5) However, the scattering spatial function g() is different from the Recommendation by the incorporation of the additional sin term, and the additional term associated with the incident angle, which is assumed to be 90 in Recommendation ITU-R BT.805. Note that the Recommendation uses a dif
31、ferent definition of angles, namely relative to the normal to the blade, so that the associated Recommendation ITU-R BT.805 expression is: () 0cossinsinsinc)(0=Wg (6) Therefore Recommendation ITU-R BT.805 applies only when the incident signal is near normal to the blade, whereas equation (4) is vali
32、d for all geometries of incident and scattering angles. 2.3 Numerical example The application of Recommendation ITU-R BT.805 can be illustrated with a numerical example. A wind turbine scatters to a point 1 km from the turbine, as shown in Fig. 2. The wind turbine has three blades, 33 m in length, t
33、riangular in shape, and having a base width of 3.3 m. As the model assumes a rectangular blade, it will be assumed that the average width is 1.65 m, so the total area is about 160 m2. Additional (static) scattering from the tower is ignored in this example. The wind direction is aligned with the vec
34、tor between the transmitter and the wind turbine, so the incident signal on the turbine blade is close to normal (the worst case). The frequency is 600 MHz. As the transmitter is assumed to be remote, the direction of the signal at the receiving site and at the wind turbine is assumed to be the same
35、. Rep. ITU-R BT.2142 5 FIGURE 2 Geometry for the example. The transmitter is remote, so the drawing is not to scale Report 2142-02The reflection coefficient based on the Recommendation ITU-R BT.805 model is thus: dB 10log20)log(20 =rAmax(7) With an omnidirectional antenna, the signal-to-interference
36、 ratio (SIR) will therefore be 10 dB. For DTV, the SIR can be equated approximately to the receiver C/N due to the randomization processing in the receiver, and so the C/N is well below the typical 20 dB required for DTV planning. It should be noted that this example assumes flat metal blades, which
37、 is not typical. Additionally, if a directional antenna were pointed at the DTV transmitter and away from the reflected signal, the directivity of the antenna would be added to the 10 dB; for a typical television antenna with directivity of 12 to 20 dB, this would provide sufficient margin. It is im
38、portant to note that the interference signal is due to backscattering, rather than forward scattering as stated in Recommendation ITU-R BT.805. The practical significance is that the directivity of a correctly oriented antenna reduces the observed effects of the scattering from the wind turbine. Thi
39、s is considered in more detail in 2.4.5. 2.4 Extension of the analysis This section suggests the issues that should be addressed in modifying Recommendation ITU-R BT.805 model to improve the accuracy of the predictions of the scattered signals. These corrections would apply equally to both analogue
40、and digital television signals. The issues in the following subsections are components that affect the prediction of the scattered signal. The individual examples are not necessarily related, and the combination of these individual effects is not discussed here. The examples are intended only to ill
41、ustrate the particular aspects, and are not intended as a comprehensive statement of potential interference from wind farms in general. 2.4.1 Non-metallic turbine blades Recommendation ITU-R BT.805 assumes metallic (perfectly conducting) wind turbine blades, but actual blades are typically made of f
42、ibreglass. As a consequence the Recommendation ITU-R BT.805 model over predicts the level of the scattered signal. The rigorous calculation of scattering from a non-conducting material can be simplified by an extension of the physical optics principle used in calculating the scattering from a metall
43、ic surface. As the surface becomes large relative to the wavelength, the physical optics solution approaches the simple ray optics solution, with the angle of incidence equal to the angle of reflection. For an infinite non-conducting surface with a relative dielectric constant r, the reflection coef
44、ficient can be calculated as a function of the angle of incidence. Therefore the scattering from 6 Rep. ITU-R BT.2142 a finite dielectric surface can be obtained by multiplying the solution from a metallic surface by the reflection coefficient calculated for an infinite surface. The reflection coeff
45、icient from a non-conductor depends on the polarization of the signal. For these calculations it is assumed that the polarization is horizontal. The reflection coefficient for horizontal polarization is given by: +=22sincossincosrrrrE(8) where is the incident angle relative to the normal to the surf
46、ace, and ris the relative dielectric constant and is greater than 1. For fibreglass typically used in wind turbine blades the relative dielectric constant is about 4. The main practical interest when the incident angle is near the normal ( is small), so that equation (8) becomes: rrrrE+ (9) For r= 4
47、, this gives a reflection coefficient of 1/3, or 10 dB. The reflection coefficient is not very sensitive to the exact value of the dielectric; for a value of r= 9, which results in E= 1/2 or 6 dB. Thus the inclusion of this electric field reflection coefficient is important in assessing the overall
48、scattered signal from wind turbines. In particular, based on the above analysis, Recommendation ITU-R BT.805 overestimates the scattered signal by about 6 to 10 dB. 2.4.2 Triangular turbine blades Recommendation ITU-R BT.805 assumes that the wind turbine blades are rectangular, while actual turbine
49、blades are close to triangular. Therefore the calculations need to use a triangular shape for the surface integral in equation (3). The only modification is that the limits in the x-coordinate are replaced by: =LzWzw 12)( (10) so that surface integral becomes: ()zxrjEEzwzwLLzjkmxnnjkrjk=ddeeesin)()(2200(11) Again the assump
