Environmental Engineering Reference
In-Depth Information
Radio Interference Effects
Amplitude Modulated (AM) Systems
AM broadcast signal reception is susceptible to interference from various man-made
and natural sources of background noise. Due to the fact that a rotating wind turbine blade
predominantly modulates the amplitude of an electromagnetic signal in its vicinity,
interference with AM radio reception can be anticipated. However, since AM broadcast
frequencies are low and signal wave lengths are very long, any interference will be confined
to the immediate vicinity of a wind turbine. For example, assuming a perfectly conducting
flat earth, an omnidirectional receiving antenna, and equal signal strength at the turbine and
the receiver, Equations (9-15) and (9-26b) give
z
D
=
h
S
B
E
c
2
m
R
l
(9-32)
max
where
c =
mean blade chord (m)
Assuming h
S
=
0.8,
B
E
=
2,
m
R
=
0.1, and h
=
300 m, it is found that (z
/D
)
max
is less than
0.03
c
,
which indicates that the maximum interference distance is only a few meters even for
a large-scale wind turbine.
Frequency Modulated (FM) Systems
Laboratory simulation techniques with an experimental setup similar to that illustrated
in Figure 9-17 have been used to investigate the interference of a wind turbine with FM
broadcast reception [Sengupta and Senior 1978]. In this study, a modulation synthesizer
was attached to the input terminals of an FM test receiver to amplitude-modulate the input
signal. The test receiver was typical of the high-quality FM stereo receivers used in
automobiles. Interference was assessed by evaluating the quality of the audio reproduction
as a function of the ambient signal level and the applied modulation index.
The results of these laboratory simulations indicate that when the ambient level of the
input signal is high (
i.e.
,
a
signal-to-noise ratio >
15 dB), no audio distortion is found until
the modulation index,
m
R
, reaches 0.72, which is equivalent to a modulation range, D, of
16 dB. Even with a weaker signal, there is no significant distortion for
m
R
less than 0.36
(D < 7 dB). As
m
R
is increased beyond this level, however, there is an increasing amount
of audio distortion in the form of a pulsed, high-frequency “hiss” superimposed on the
desired sound.
These results are consistent with the fact that ordinary FM receivers are only
susceptible to noise interference while operating in their threshold regions (
i.e.
,
with signal-
to-noise ratios less than 12 dB) [Peebles 1976]. They also imply that the effects of wind
turbine interference on FM radio reception are negligible, except possibly within a few tens
of meters of a wind turbine located in a region of low signal-to-noise ratio for a particular
FM station.
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