Module 18—Radar Principles
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Index-1 to 3
Figure 3-21.—Horn radiators.
A waveguide horn, called a FEEDHORN, may be used to feed energy into a
parabolic dish. The directivity of this feedhorn is added to that of the parabolic dish. The resulting pattern is
a very narrow and concentrated beam. In most radars, the feedhorn is covered with a window of polystyrene
fiberglass to prevent moisture and dirt from entering the open end of the waveguide.
associated with feedhorns is the SHADOW introduced by the feedhorn if it is in the path of the beam. (The shadow
is a dead spot directly in front of the feedhorn.) To solve this problem the feedhorn can be offset from center.
This location change takes the feedhorn out of the path of the RF beam and eliminates the shadow. An offset
feedhorn is shown in figure 3-22.
Figure 3-22.—Offset feedhorn.
AIRBORNE RADAR ANTENNAS
Airborne radar equipment is used for several specific purposes. Some of these are bombing, navigation, and
search. Radar antennas for this equipment are invariably housed inside nonconducting radomes, not only for
protection but also to preserve aerodynamic design. Some of these radomes are carried outside the fuselage, while
others are flush with the skin of the fuselage. In the latter case, the radar antenna itself is carried inside the
fuselage, and a section of the metallic skin is replaced by the nonconducting radome. The radar antenna and its
radome must operate under a wide variety of temperature, humidity, and pressure conditions. As a result,
mechanical construction and design must minimize any possibility of failure. Transmission lines are usually
hermetically sealed to prevent moisture
accumulation inside them. Such accumulation would introduce losses. Because the low air pressures
encountered at high elevations are very conducive to arcing, pressurization of equipment is widely used (the
pressure is maintained by a small air pump). In some airborne radar equipments, practically all of the equipment
is sealed in an airtight housing, along with the antenna and transmission line. The antenna radome forms a portion
of the housing.
Airborne radar antennas are constructed to withstand large amounts of vibration and shock; the radar antennas are
rigidly attached to the airframe. The weight of the radar antenna, including the rotating mechanism required for
scanning, is kept to a minimum. In addition, the shape of the radome is constructed so as not to impair the
operation of the aircraft.
The airborne radar antenna must have an unobstructed view for most useful
operation. Frequently, the antenna must be able to scan the ground directly under the aircraft and out toward the
horizon. To meet this requirement, the antenna must be mounted below the fuselage. If scanning toward the rear is
not required, the antenna is mounted behind and below the nose of the aircraft. If only forward scanning is
needed, the antenna is mounted in the nose. When an external site is required, a location at the wing tip is
common. A fire-control radar antenna is frequently located near the turret guns or in a special nacelle, where it
can scan toward the rear or sides of the aircraft.
Q14. How many major lobes are produced by a paraboloid reflector?
Q15. What type of radiator
normally drives a corner reflector?
Q16. The broadside array consists of a flat reflector and what other
Q17. Horn radiators serve what purpose other than being directional radiators?
The following is a brief summary of the important points of this
A radar INDICATOR presents the information (video) from the radar receiver in a usable
manner. The display usually consists of one or more of the coordinates of range, bearing, and altitude.
CATHODE-RAY TUBE (CRT) is the best available device for displaying the two-dimensional
relationship produced by radar coordinates. The most commonly used CRT displays are the A-SCOPE, the RHI, and the
PPI. The A-scope presents range information only. The RHI displays range and height information. The PPI is the
most widely used radar display indicator and presents range and bearing.
The range of a radar contact is determined by special ANGING CIRCUITS. The
basic types of ranging circuits are used.
produce a movable gate that measures range based on elapsed time and can be used on A-scope and PPI displays.
RANGE-MARKER GENERATORS produce fixed interval range marks that can be used to estimate the range to a
detected target. Range marks appear as an intensified series of vertical dots on an rhi and as concentric circles
on a PPI.
The RANGE-STEP GENERATOR produces a movable step that is displayed on an A-scope
RADAR ANTENNAS are usually directional antennas that radiate energy in a one directional
lobe or beam. The two most important characteristics of directional antennas are directivity and power gain. Radar
antennas often use parabolic reflectors in several different variations to focus the radiated energy into a
desired beam pattern. Other types of antennas used with radar systems are the corner reflector, the broadside
array, and horn radiators.
ANSWERS TO QUESTIONS Q1. THROUGH Q17.
A1. Range, bearing, and elevation.
A2. Triggers, video, and antenna information.
A3. Range and
A4. Range and bearing.
A7. Range gate
or range step.
A9. The radar mile (12.36 microseconds).
A10. The A scope.
A16. Two or more half-wave dipoles.
A17. Waveguide impedance matching devices.
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