Environmental Engineering Reference
In-Depth Information
the antenna angle for the direct signal is -134 deg, for which the site response factor is -13
Db (Fig. 9-3). This greatly increases the
relative size
of the modulation compared to the
direct signal and, therefore, the interference experienced by a viewer.
Figure 9-4(b) shows that the opposite is true when the antenna is aimed at the
transmitter. Here the direct signal is received at full strength and the scattered signal is
reduced by -12.5 dB. Thus, the modulation-to-direct signal ratio is about 25 dB lower when
the antenna is aimed at the transmitter than when it is aimed at the turbine. These figures
illustrate how potential electromagnetic interference can be avoided by the use of a
properly-oriented directional antenna.
Combining Equations (9-7) and (9-8) gives
|
P
R
|
envelope
= |
P
R
,
D
| (1 +
m
R
f
m
)
2
(9-10a)
m
R
=
F
A
,
W
/
F
A
,
T
m
E
(9-10b)
where
|
P
R,D
|
=
amplitude of direct signal power input at the receiver location (mW)
m
R
= receiver input modulation index
F
A
,
W
=
antenna response factor for a signal from the wind turbine (mW/mW)
F
A,T
= antenna response factor for a signal from the transmitter (mW/mW)
Because the maximum magnitude of
f
m
is always unity, the maximum and minimum depar-
tures (in dBm) from the level of the direct signal are
(9-11a)
D
1
= 20 log
10
(1 +
m
R
)
(9-11b)
D
2
= 20 log
10
(1 -
m
R
)
giving rise to
1 +
m
R
1 -
m
R
D = D
1
-
D
2
= 20 log
10
(9-11c)
where
D
=
P
R, max
-
P
R, min
=
signal power modulation range (dBm)
D
1
=
P
R, max
-
P
R, mean
(dBm)
D
2
=
P
R, min
-
P
R, mean
(dBm)
P
R, mean
= |
P
R, D
| (dBm)
Figure 9-5 is a graphical solution of this equation which shows the receiver input modulation
index as a function of the signal power modulation range. A convenient empirical equation
for this relationship is
m
R
= 0.0620D (1 - 0.0169D)
(9-12)
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