Environmental Engineering Reference
In-Depth Information
Potential Interference with Microwave Communication
Terrestrial microwave communication links, which are widely used for transmitting and
receiving a wide variety of information, operate within the 300 MHz to 300 GHz portion
of the electromagnetic frequency spectrum. Telephone companies employ such links for
long- and short-distance audio communication as well as television and data transmissions
[Bell Laboratories 1971]. These link systems use
microwave repeater stations
at selected
sites where received signals are detected and passed on to local customers or amplified and
transmitted to the next repeater station. A typical link or
TD-system
operates at 4 GHZ and
uses a highly-directional
pencil beam antenna
with a beam width of ± 0.6 deg, an effective
gain (
G
0
)
of 35 db, and side-lobe antenna factors (
F
A
)
less than -30 dB.
A rigorous investigation of any potential interference with such communications by
wind turbines requires detailed knowledge of each specific link, including its function,
method of operations, radiation patterns of transmitting and receiving antennas, and
propagation characteristics in the region around the wind turbines. Problems of interference
in microwave link systems are usually treated statistically [Jakes 1974, Bennett
et al.
1955],
but in the present case it is more convenient to follow a deterministic approach [Sengupta
and Senior 1978]. In a manner similar to that used previously for TV interference, a wind
turbine located in the vicinity of a microwave repeater station is treated as a time-varying
multipath source producing amplitude and phase modulation of the signal that is picked up
by the microwave receiving antenna.
The effect of a rotating blade on the detected signal (and, hence, on the interference
produced) has been qualitatively assessed, by examining the basic detection process in the
TD receiver system [Sengupta and Senior 1978]. It has been found that the interference
effect is a
frequency smearing
of the received base band signal energy, the maximum
amount of which depends on the blade size, rotor orientation, and rotor speed. For the
relatively low rotational speed of a HAWT, this maximum is much less than the total
bandwidth of the signal. The degrading influence of the smear depends on the amplitude
of the scattered signal at the receiver relative to the desired signal,
m
R
,
and it is sufficient
to investigate potential wind turbine interference on the basis of a threshold value of
m
R
.
The flat plate model of a HAWT blade developed for analyzing TV interference can
also be used to estimate the amplitude and phase of the interfering microwave signal,
relative to the desired carrier signal at the receiver. In general, the equations presented
earlier for TVI can be used to evaluate potential interference with microwave signals.
Microwave interference zones differ somewhat from TVI zones because of
lower thresholds
of perception
and
higher antenna directionality.
These two subjects will be discussed
briefly here.
Thresholds of Perception
The region about a microwave link receiver where a wind turbine could produce
unacceptable interference with reception will be called the
forbidden zone.
Within this zone
the amplitude of the blade scattered signal relative to the desired (direct) signal exceeds a
predetermined threshold value, where this threshold depends on the characteristics of the
link system [Curtis 1960, Campbell 1958]. Telephone companies customarily demand a
threshold of -40 dB (
i.e.
,
m
R
= 0.01), but if a -40 dB
margin for fading
is required, the
resulting threshold is -80 dB (
i.e.
,
m
R
= 0.0001).
It is often unnecessary to determine the complete forbidden zone for the purpose of
estimating interference potential. The satisfactory performance of a microwave link system
requires that there be adequate clearance between the
link path
(
i.e.
,
the optical line-of-sight
transmission path between two antennas) and any nearby scattering objects such as a wind
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