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Click here to Order your Radar Equipment Online 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 beamBeam 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. location of the targets. This kind of beam is created with a parabolic reflector, the end of the waveguide being at the focal point of the parabola.Radars display targets on the scope as if the targets are at the center of the beam, even though the target may have been illuminated by energy that has been scattered outside the beam. This means that any object illuminated sufficiently by the radar energy to return to the antenna some of that energy will be shown on the indicators as being directly in front of the antenna, while it actually maybe several degrees to the side. Although this sometimes leads to inaccurate interpretation of the radar-scope information, the problem usually concerns strong targets fairly close to the radar that are masked in the ground clutter.This information is now available on CD in Adobe PDF Printable Format |
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