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
