Electromagnetic Interference from Wind Turbines - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 556

for an actual blade that has an airfoil contour and may be twisted and coned ( i.e. , inclined
downwind) out of its plane of rotation.

Back-Scatter and Front-Scatter Zone Shapes

Referring to Equation (9-18a), the azimuthal shape of contours of equal scattering inten-
sity around a wind turbine are defined by the product of the sinc and sine terms. The back-
scatter zone is the region in which scattering is specular, with f R = p - f T and p = 0.
Thus

p l p
l

| sin( R )| sinc = cos(f RT /2) Back - scatter

f (9-19a)

where f RT = f R - f T (rad)

In the front-scatter zone f R = p + f T - df, f T = p/2 for maximum effect, and df is a
small angle measured from the direction of the receiver. The zone shape function becomes

p l p
l

p l sin(d f)
l

| sin(f R )| sinc = cos(d f) sinc

which drops rapidly to zero with increasing df, at a rate dependent on the ratio l/ l . As
a result, the front-scatter zone is a narrow spike behind a HAWT aligned with the
transmitter. For our purposes we can approximate the shape of this spike by

|sin(f R )| sinc p l p
l

» cos(2f RT ) , 0.8 p < f R T < 1.2p Front - scatter

(9-19b)

Figure 9-7 shows the azimuthal shapes of the two scatter zones, plotted in accordance with
Equations (9-19). The back-scatter zone, in the shape of a cardioid , is seen to dominate.

Figure 9-7. Idealized interference zone shapes around a HAWT oriented for maxi-
mum signal scatter. Elevations of the transmitter, turbine rotor, and receiver are the same.