Environmental Engineering Reference
In-Depth Information
Unattended Testing
A TV signal measuring system that will operate unattended is coupled to the same
antenna as before and has a
programmable computer
controlling a spectrum analyzer and
a
plotter/printer.
For a given orientation of the antenna, the total signal can be sampled at
incremental frequencies spanning the band of frequencies within the TV channel of interest,
and the results can be plotted as a function of frequency. Peak signals exceeding a
predetermined strength level can be printed out, along with the frequencies at which they
occurred. Video and audio carrier signal strengths can be found from these data. The
process is repeated for different antenna azimuths, to determine maximum signal strengths
for each TV channel.
An unattended system of this type typically updates the printed information about every
15 minutes. It has been observed that peak audio frequencies are often missed on some
channels because of the incremental frequency steps. Nevertheless, a computer-based,
automatic monitoring system is a useful supplement to an attended system.
Observed TVI Effects Caused by Wind Turbines
For a receiver in the backward scattering zone of the wind turbine (Fig. 9-7) the
delayed multipath signal leads to a
ghost image
on the TV screen that pulsates (
jitters
)
horizontally at the blade passage frequency. If the ghost image is sufficiently strong, the
resulting interference can be objectionable. As the interference increases, the entire picture
(desired plus ghost) also shows a pulsed brightening. Still larger levels of interference can
disrupt the vertical synchronization of the receiver, causing the picture to roll (
flip
)
or even
break up. This type of interference occurs as a result of scattering signals off the broad
faces of the rotor blades, which is primarily a specular (
i.e.
,
mirror-like) scattering.
As the angle f
RT
increases, the separation between the desired and ghost images
decreases, and a somewhat stronger scattered signal is required to produce the same amount
of video distortion. Video distortion in the backward scattering zone shows no significant
dependence on either the ambient level of the signal (provided this is well above the noise
level of the receiver) or on the specific receiver used.
In the forward scattering region (Fig. 9-7), where the wind turbine is almost on a line
between the transmitter and the receiver, there is virtually no difference in the time of
arrival of the direct and scattered signals. The ghost image is then superimposed on the
desired picture, and any video distortion appears as a fluctuation in the brightness (
intensity
)
of the picture in synchronism with the blade passage.
In both the backward- and forward-scattering cases, noticeable distortion occurs only
when the modulation waveform is of a pulsed nature. Other things being equal, the
modulation index and the resulting video distortion increase with increasing TV channel
numbers (
i.e.
,
increasing frequency and decreasing wave length) and with decreasing
distance from the wind turbine. No audio distortion has been observed in any of the field
testing to date.
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