Page 190 - Radar Technology Encyclopedia

feed, space (optical) FENCE 180

Figure F19 Twelve-horn feed (from Barton, 1988, Fig. 4.5.5,
p. 201).

waveguide feed (see linear feed, sinuous feed).

FEEDBACK is “the returning of a fraction of the output to
the input.” A fundamental concept in automatic control sys-
tems, feedback derives its name from a signal “fed back”
from a controlled process to measure its response to an input
command. Feedback is based on comparing the actual
Figure F17 The SA-12 Grill Pan fire control radar, an example response of a controlled output to the desired response. Any
of a space-fed lens array. In the background is the Billboard difference between the two values is used to adjust the ampli-
three-dimensional surveillance radar. tude of the input control signal. When the feedback signal
results in a decrease in the input signal, it is termed negative
absence of both aperture blockage by the feed and of reflec-
feedback; when the input signal is increased (amplified)
tion from the array back to the feed. SAL
through feedback, it is known as positive feedback. Systems
Ref.: Barton (1988), pp. 175–179; Johnson (1984), p. 20.49.
using feedback are referred to as closed-loop systems.
Feed spillover is the effect of the energy losses in reflector The feedback concept is utilized in many areas of radar
antennas due to illumination that extends beyond the edge of systems engineering, among which are the electromechanical
the dish, resulting primarily in formation of spillover lobes servomechanisms used for antenna control, and electronic
(Fig. F18). Typically, spillover reduction is accomplished by systems such as radar frequency control, constant false-alarm
shaping the feed pattern to cut off sharply at the edge of the rate (CFAR) systems, and automatic gain control (AGC).
dish and by maintaining low sidelobes in the feed pattern. PCH, SAL
SAL
Ref.: IEEE (1993), p. 485; Bode (1945), p. 31; Popov (1980), p. 255.
Ref.: Skolnik (1990), p. 6.37.
FEEDER [FEED LINE]. A feeder is a conducting transmis-
Spillover sion line for radio-frequency electromagnetic energy.
region
Depending on the frequency range, either open symmetrical
Diffracted Feed lines of parallel wires or RF cables are used. The cables are
region
spillover symmetrical unshielded or shielded, or coaxial cables (see
transmission line, wire and coaxial). Feeders are used to
channel energy from the transmitter to the antenna and from
the antenna to the receiver, and also to join other high-fre-
quency assemblies of radar up to the long-wave part of the
centimeter-wave range. Ultrawideband video pulses are also
transmitted over high-frequency feeders (usually coaxial).
IAM
Ref.: Popov (1980), p. 455.
Figure F18 Spillover lobes (after Skolnik, 1990, p. 6.39).
FENCE. A fence is (1) a “line of networks of early warning
radars; (2) The locus of the positions of a surveillance radar
A twelve-horn feed is a monopulse horn feed using a twelve- beam that describes the search area covered by space-based
horn cluster (Fig. F19) that permits separate optimization of radar; (3) Concentric conducting barrier erected around a
sum and difference illuminations. SAL ground-based radar to serve as an artificial horizon and sup-
Ref.: Barton (1988), p. 199.