## Back Scattering

Scattered signal__/

Transmitter NJ/^^-Receiver

' Direct signal '

Figure 9.14 Interference Mechanisms of Wind Turbines with Radio Systems

Rees, 1989) provide an analysis of the electromagnetic interference caused by a wind turbine.

The useful carried signal received, C, is given by

where PT is the transmitter power (dB), ATR is the attenuation between the transmitter and receiver (dB), and GTR is the receiver antenna gain in the direction of the required signal (dB). The interfering signal, I, is given by f 4na\

where ATW is the attenuation between the transmitter and wind turbine (dB), AWR is the attenuation between the wind turbine and receiver (dB), GWR is the receiver antenna gain in the direction of the reflected signal (dB), 10log10(4^a /°2) is the contribution to scattering of the wind turbine (dB), a is the radar cross section (m2). This may be understood as the effective area of the wind turbine. It is a function of the wind-turbine geometry and its dielectric properties together with the signal wavelength. ° is the wavelength of the signal (m).

It may be seen that the ratio of useful signal to interference is:

Assume the distance between the transmitter and receiver is much greater than the distance from the wind turbine to the receiver denoted as r, then ATW = ATR. Assume the free space loss is AWR = 20log1o(4^r/°) and define the antenna discrimination factor as AG = GTR - GWR. Then (C/I) reduces to

Cj = 10 log1o 4^ + 20 log1o r - 10 logw a + AG (9.25)

Thus the ratio of the useful carrier signal to interference may be improved by:

• increasing the distance from the turbine to the receiver, r,

• reducing the radar cross section, a,

• improving the discrimination factor of the antenna, AG.

The carrier to interference ratio (C/I) defines the quality of a radio link. For example, a fixed microwave link may have a (C/I) requirement of 50-70 dB while for a mobile radio service the requirement maybe only 15-30 dB (ETSU, 1997b).

Hence Equation (9.25) may be rearranged to define a 'forbidden zone' within which a wind turbine may not be located if an adequate carrier to interference ratio is to be maintained.

20 log10 r = I C ) + 10 log10 o — AG — 11 (9.26)

\ 1 required J

It may be seen that the 'forbidden zone' is critically dependent on the radar cross section, o. Determination of the radar cross section of a wind turbine is not straightforward and a number of approaches are described in the literature. Van Kats and van Rees (1989) undertook a comprehensive series of site measurements on a 45 m diameter wind turbine. They estimated a radar cross section in the back-scatter region of 24 dBm2 and a worst case value in the forward-scatter region of 46.5 dBm2 (both values expressed as 10 logi0 o).

Where measured results are not available, simple predictions may be made based on approximating the turbine blades to elementary shapes (Moglia, Trusszi and Orsenigo, 1996). For example, for a metallic cylinder the radar cross section is given by o = ~ar (9.27)

where a is the radius of the cylinder (m), L is the length of the cylinder (m), and X is the signal wavelength (m), while for a rectangular metallic plate

where l is the width of the plate (m).

It is suggested that, in the back-scatter region, reflection is only caused by the metallic parts of the blades and so only these dimensions are used in the simple formulae. However, in the forward-scattering region the entire blades make a contribution although this is reduced because of the blade material (GRP) and shape. Hence, Moglia, Trusszi and Orsenigo (1996) apply the simple formulae but with a —5 dB correction for blade material and a —10 dB correction for blade shape (when using the rectangular approximation).

Hall (1992) quotes a radar cross-section model used by a number of researchers as:

10 log10 o = 20 log10

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