Electromagnetic Interference from Wind Turbines - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 552

of the scattered signal field at the receiver to the direct signal field at the turbine , or

| E R , S |
| E WT , D |

(9-14)

Z =

where

Z = signal scatter ratio

| E WT , D | = amplitude of the direct field at the wind turbine (mV/m)

The ratio Z is a characteristic of the turbine and its location relative to the transmitter
and receiver, and it is independent of the receiver antenna response or the ambient signal
fields at either the receiver or turbine locations. We can use the signal scatter ratio to
predict the receiver input modulation index (the measure of EMI potential) using Equations
(9-4b) and (9-10b), as follows:

F A , W
F A , T

| E WT , D |
| E R , D |

(9-15)

m R = Z

Equation (9-15) shows clearly that EMI potential depends on the combination of turbine and
site characteristics ( Z ), relative antenna characteristics ( F A,W / F A,T ), and relative direct field
strengths (| E WT, D |/| E R, D |) .
Signal scatter ratios actually observed during EMI tests can be determined from records
of signal power versus time, like those shown in Figure 9-4. Data reduction equations,
derived from Equations (9-8), (9-11), (9-12), and (9-14), are as follows:

P R , mean
| P WT , D |

| E R , S | O
| E WT , D |

m R (D)
F A , W

(9-16a)

Z O = =

P R , max
(1 + m R ) 2

(9-16b)

P R , mean =

F A , W = [ P R , mean ] w
P R , mean

(9-16c)

where

Z O = observed signal scatter ratio

|E R , S | O = observed amplitude of the scattered signal at the receiver (mV/m)

|P WT, D | = amplitude of the direct signal power at the wind turbine rotor (mW)

[ P R, mean ] W = average signal power with the antenna beamed at the wind turbine (mW)