Figure 12-2.-Flve 1-cm energy waves represented as
concentric circles around the wave emitttcr.
Thus, radar waves can be described either in terms
of frequency or wavelength. The two are related by the
following equation:
FREQUENCY = :
where k is the wavelength and c is the speed of light
(300,000,000 meters per second or 3 x 108).
Wavelengths for Weather Radars
All the wavelengths commonly used for radars will
be reflected by large objects such as buildings and
airplanes, but some waves are so large they are not
affected significantly by small objects such as raindrops,
and, therefore, do not effectively detect their presence.
Other waves are so small that they are completely
scattered and absorbed by raindrops and, therefore,
cannot penetrate beyond the first drops to detect others
far away. Any object that is easily detected by a
particular wave is one that reflects, absorbs, or scatters
that wave, decreasing its potential to detect more distant
objects. The target for weather surveillance radars is the
raindrop, with a diameter usually less than 5 mm. We
must select a wavelength small enough to detect the
raindrop, but large enough not to be completely
reflected and absorbed by a large number of drops. The
choice usually comes to either 5- or 10-cm waves.
Wavelength for Cloud Detection Radars
Wavelengths shorter than 3 cm will detect most
cloud particles.
The optimum wavelength for cloud
detection is about 1 cm (anything shorter is too greatly
attenuated).
RADAR BEAM CHARACTERISTICS
In the following section we will deal with some of
the characteristics of the radar beam.
Beam Height and Width
Although we must always keep in mind that the
radar energy discussed here is emitted in short pulses
rather than in a steady stream, it is useful to think of the
path of the pulses as a beam much like a flashlight beam,
Each pulse has three dimensions within the
beam-height, width, and length-and each pulse is
composed of a number of waves of energy. The beam
is created when the energy waves are directed onto the
reflector, which focuses them in a desired direction and
a predetermined shape. The height and width of each
radar pulse is determined by the radar beam shape and
size, and the beam, in turn, is determined by the shape
and size of the antenna reflector. For weather radars, we
almost always use a narrow cone, or “pencil beam,” so
that we can concentrate the energy in a small spot and
make both vertical and horizontal measurements of the
Figure 12-3.-Energy waves represented as concentric arcs inn a radar beam.
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