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Radar Antennas

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					Antennas fall into two general classes, OMNIDIRECTIONAL and DIRECTIONAL. Omnidirectional
antennas radiate rf energy in all directions simultaneously. They are seldom used with modern
radars, but are commonly used in radio equipment, in iff (identification friend or foe)
equipment, and in countermeasures receivers for the detection of enemy radar signals.
Directional antennas radiate rf energy in patterns of LOBES or BEAMS that extend outward
from the antenna in one direction for a given antenna position. The radiation pattern also
contains minor lobes, but these lobes are weak and normally have little effect on the main
radiation pattern. The main lobe may vary in angular width from one or two degrees in some
radars to 15 to 20 degrees in other radars. The width depends on the system's purpose and the
degree of accuracy required. Directional antennas have two important characteristics,
DIRECTIVITY and POWER GAIN. The directivity of an antenna refers to the degree of sharpness
of its beam. If the beam is narrow in either the horizontal or vertical plane, the antenna is said
to have high directivity in that plane. Conversely, if the beam is broad in either plane, the
directivity of the antenna in that plane is low. Thus, if an antenna has a narrow horizontal beam
and a wide vertical beam, the horizontal directivity is high and the vertical directivity is low.
When the directivity of an antenna is increased, that is, when the beam is narrowed, less power
is required to cover the same range because the power is concentrated. Thus, the other
characteristic of an antenna, power gain, is introduced. This characteristic is directly related to
directivity. Power gain of an antenna is the ratio of its radiated power to that of a reference
(basic) dipole. Both antennas must have been excited or fed in the same manner and each must
have radiated from the same position. A single point of measurement for the power-gain ratio
must lie within the radiation field of each antenna. An antenna with high directivity has a high
power gain, and vice versa. The power gain of a single dipole with no reflector is unity. An array
of several dipoles in the same position as the single dipole and fed from the same line would
have a power gain of more than one; the exact figure would depend on the directivity of the
array. The measurement of the bearing of a target, as detected by the radar, is usually given as
an angular position. The angle may be measured either from true north (true bearing), or with
respect to the bow of a ship or nose of an aircraft containing the radar set (relative bearing).
The angle at which the echo signal returns is measured by using the directional characteristics
of the radar antenna system. Radar antennas consist of radiating elements, reflectors, and
directors to produce a narrow, unidirectional beam of rf energy. A pattern produced in this
manner permits the beaming of maximum energy in a desired direction. The transmitting
pattern of an antenna system is also its receiving pattern. An antenna can therefore be used to
transmit energy, receive energy, or both. The simplest form of antenna for measuring azimuth
(bearing) is a rotating antenna that produces a single-lobe pattern

				
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