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recurrence rate) determined by the selected operating range.
7.3.3. The generated pulse travels through either coaxial cable or, more frequently, a hollow tube called
the wave guide. The wave guide requires pressurization to ensure the maintenance of conditions for
proper microwave conduction. The energy passes an electronic switching device that directs outgoing
pulses to the antenna and incoming pulses from the antenna to the receiver.
7.3.4. The antenna is a parabolic dish with a protruding wave guide. It is gimbal-mounted to allow
rotation of the dish and, in most cases, to allow stabilization of the dish relative to the earth's surface
when the aircraft turns. Rotation of the antenna could be through 360o or in a sector (either variable or
preset). The 360o rotation, or scan, is usually for mapping; whereas, a sector is used in aircraft with
limited space for the antenna or where the intent is to concentrate energy in a small area.
7.3.5. The antenna assembly will be either permanently locked to the longitudinal axis of the aircraft
(boresighted) or only so aligned when stabilization units are inactive. When not caged, the antenna
stabilization is accomplished by using gyroservo mechanisms. A sensor system that provides
information to a computer keeps the antenna radiation plane parallel to the earth even when the aircraft
is in a climb or a bank.
AFPAM11-216 1 MARCH 2001 175
7.3.5.1. There are two radiation patterns popular in airborne radar design, fan and pencil beams. The fan
beam is a wide pattern that distributes the RF energy across the beam in proportion to the distance it
must travel (Figure 7.2). The fan beam is best for general mapping. To concentrate the energy emitted,
the pencil beam antenna is used. The pencil beam dish allows scanning for weather or aircraft while
eliminating ground clutter. It can be used to put more energy on a section of ground to increase returns.
Figure 7.2. Antenna Radiation Patterns.
7.3.5.2. The antenna can be manipulated to aim the emissions through a control that tilts the dish from
the horizontal plane. At cruising altitudes, in the mapping mode, it is sufficient to slightly tilt the dish
down, but tilt should be constantly adjusted for optimum returns.
7.3.6. After transmission, the reflected energy is directed back to the wave guide where it travels past the
switching device which directs the returns to the receiver. The receiver converts the microwave returns
to electrical signals that are amplified and sent to a CRT called the planned position indicator (PPI). The
amplification of the returns is controllable through a gain circuit. Depending on the type of return
desired on the PPI, the operator adjusts the receiver gain. Other booster circuits, such as sweep intensity
or video gain, are available, but operation of the receiver is most important. If adequate receiver
amplification of weak returns is not applied, no amount of later stage adjustments will put the target on
the scope.
7.3.7. The PPI, or scope, offers both range and azimuth information about targets to the operator. This
information is relative to the aircraft's position which can be referenced at either the center of the scope
or offset to the side of the screen (Figure 7.3). The PPI is a CRT with focusing coils and a deflection
coil. The deflection coil is an electromagnet whose variable field manipulates the electron beam so that
returns can be presented on the scope in their correct position relative to the observer (Figure 7.4).
7.3.8. Applying a polarization to the signals going to the CRT produces the actual presentation of the
return. The null return has a predominantly positive charge; therefore, the trace is suppressed. A
polarization shift is produced in the current to produce a blooming of the trace corresponding to the
strength and position of the received signal.
7.3.9. Range is determined by the travel time of a pulse from and back to the R/T unit. Knowing that RF
energy travels at the constant speed of light, range determination is simple. The synchronizer
coordinates its display on the PPI.
176 AFPAM11-216 1 MARCH 2001
Figure 7.3. Sector Scan Displays.
Figure 7.4. Electromagnetic Cathode Ray Tube.
7.3.10. At the same instant that the timer triggers the transmitter, it also sends a trigger signal to the
indicator. Here, a circuit is actuated which causes the current in the deflection coils to rise at a linear
(uniform) rate. The rising current, in turn, causes the spot to be deflected radially outward from the
center of the scope. The spot thus traces a faint line on the scope; this line is called the sweep. If no echo
is received, the intensity of the sweep remains uniform throughout its entire length. However, if an echo
is returned, it is so applied to the CRT that it intensifies the spot and momentarily brightens a segment of
　
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