Environmental Engineering Reference
In-Depth Information
the
total field
at a receiver then becomes
E
R
(P,
t
) = |
E
R
,
D
(P)|{1 +
m
E
f
m
(
t
) exp[
j
d(
t
)]}
(9-4a)
m
E
= |
E
R
,
S
(P)|
/
|
E
R
,
D
(P)|
(9-4b)
where
d = d
S
- d
D
m
E
= ambient field modulation index
Assuming
m
E
<<
1, Equation (9-4a) can be approximated by
E
R
= |
E
R
,
D
| [1 +
m
E
f
m
cos(d)] exp{
j
[d
D
+
m
E
f
m
sin(d
S
)]}
(9-5)
eliminating the functional notation (Õ,
t
) and (
t
) for simplification. Equation (9-5) shows
that, in general, the total signal is both amplitude and frequency modulated, the former
being dominant. For later use, we write the amplitude of the total field at a receiver R as
|
E
R
| = |
E
R
,
D
| [1 +
m
E
f
m
cos(d)]
(9-6)
displaying the
amplitude modulation
(AM) resulting from rotation of the blades. In the non-
rotating case,
f
m
= 1. Equation (9-7) indicates that any electromagnetic communication
system using AM for transmission could be vulnerable to interference produced by rotating
wind turbine blades. The severity of interference is measured by the modulation index,
m
E
,
and the nature of the interference effects is described by the modulation shape function,
f
m
.
To interpret Equation (9-6) in terms of observable quantities, we note that cos(d)
is a
rapidly-varying function of time compared to
f
m
. It attains its extreme values (± 1) many
times during a single cycle of
f
m
which occurs in the time period 2p/
B
W
.
The
envelope
of
Equation (9-6), therefore, represents the field of the total signal that is actually observed and
is given by
|
E
R
|
envelope
= |
E
R
,
D
| (1 +
m
E
f
m
)
(9-7)
Thus, the modulation shape function represents the time dependence of the envelope of the
scattered signal introduced by the blade rotation.
Signal Power Interference
Up to this point, the discussion of signal interference has centered on the ambient signal
field around the receiver. However, EMI
perception
depends not only on the modulation
of the signal field but also on the degree of modulation of the signal power at the input
terminals of the receiver. This involves the
receiving antenna orientation and response.
Using a TV signal as an example, signal power and signal field are related as follows:
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