RADAR BEAM CHARACTERISTICS

Figure 2-10.—Half-power points and beamwidth. it occupies along the beam at any point in time. Unlike pulse length, volume does not remain constant. While the amount of power within a pulse is determined by its length and remains constant, power density decreases with distance. This occurs because the pulse’s fixed amount of energy is spread over a greater area (pulse volume) as the beam broadens. The further a pulse travels, the weaker and less effective it becomes due to increased pulse volume. RADAR BEAM CHARACTERISTICS The characteristics of a radar beam refer to beamwidth, beam broadening, and the presence of sidelobes. Beamwidth Since EM energy contains properties similar to light, it can be pointed and controlled much like a flashlight. A suitable antenna can easily focus it into a beam and direct its movement. A radar beam is the path that guides a pulse’s travel. Energy emitted into the atmosphere remains concentrated along the beam axis. As you move outward at right angles to this axis, power density gradually decreases. At some point, power density equals one-half of that found at the beam axis. These half-power points wrap completely around the beam and define its shape in terms of height and width, or more appropriately, its circumference. The area within these half-power points is defined as the beam, and it contains nearly 80 percent of all energy (fig. 2-10). The angular distance between half-powerpoints in a plane passing through the beam centerline is the beamwidth. Beamwidth varies directly with wavelength and inversely with antenna size. Radar systems that produce relatively small beam widths generally provide greater target resolution. Beam Broadening As pulses travel away from the antenna, the beam takes on a cone-like appearance and expands in all directions. This expansion or beam broadening increases pulse volume, resulting in decreased signal strength (fig. 2-11). Distant targets appear distorted, in fact, they may not be seen at all. Beam broadening also causes "partial beam filling," which implies that distant targets occupy proportionally less of an expanded beam. Thus, the true characteristics of a target may be hidden or altered during display. Beam broadening reduces azimuthal resolution and produces a form of radar nearsightedness. As the beam diameter increases with distance, closely spaced targets may occupy the beam simultaneously and Figure 2-11.—Radar beam broadening. 2-8