Page 48 - Radar Technology Encyclopedia

38 antenna, reflector antenna, stacked-beam

ola, and in the orthogonal plane it is a circle. The combination slot varies approximately quadratically, having a zero value at
of focusing capabilities of the parabolic mirror and wide- the edges and the maximum in the center. Multislot antennas,
angle capabilities of the spherical mirror provides scanning which have a series of slots cut in the wall of the waveguide
angles up to 120° when the displacement of the feed along the at equal spacings, are used to create highly directional radia-
focal arc is employed. Such reflectors can be used to provide tion. It is best to cut longitudinal slots in the narrow wall of a
wide-angle coverage in very-large-aperture radar antennas, waveguide and transverse slots in the wide wall. Such slot
such as antenna systems of early-warning radars (e.g., the placement leads to significant radiation pattern sidelobes. For
AN/FPS-50). this reason, it is most advisable to use a multislot antenna
A shaped-reflector antenna uses a reflector shaped to with longitudinal slots on the wide wall of the waveguide.
produce the desired pattern and scan characteristics. Among The distance between slots and their tilt angle are selected so
the shapes used for radar applications, the spherical reflector the radiation of all slots is cophasal. AIL
has the shape of a sector of a sphere. It can provide wide- Ref.: Fradin (1977), p. 118; Johnson (1993), Chaps. 8, 9.
angle electromechanical scanning by feed displacement, but
A space-time antenna is one having one or several parame-
the construction is rather bulky and this antenna is used pri-
ters that are a function of time. This category includes, for
marily when very large apertures are required (e.g., in radar
instance, the synthetic aperture antenna. SAL
astronomy). The Gregorian antenna configuration is the most
Ref.: Bakhrakh (1989), p. 17.
typical for such antennas. A conical reflector antenna uses the
reflector antenna of conical shape. It requires a special struc- spiral antenna (see log-spiral antenna).
ture of feed to produce a conical wave illuminating the reflec-
A stacked-beam antenna forms a multiple-beam pattern that
tor that blocks the center of cone and gives large sidelobes,
covers the entire elevation search sector. Up to 1975 it was
and typically are not used in radar applications. The patterns
usually a doubly-curved reflector with an array of feed horns
formed are typically fan or cosecant-squared beams, the latter
of front of it (Fig. A76). Modern stacked-beam antennas use
using symmetrical or asymmetrical cut-outs from the parabo-
loidal reflector (truncated paraboloids) to form the required
pattern. Doubly-curved reflectors are the most common con-
figurations for cosecant-squared beams.
SAL, AIL
Ref.: Rusch (1970); Johnson (1984), pp. 17.1–17.54; Skolnik (1990), pp.
6.1–6.63; Sazonov (1988), pp. 371–394; Sletten (1988, 1991).
antenna scanner (see SCANNER).
A signal-processing antenna forms a radiation pattern in the
receive mode by nonlinear signal processing or in both
receive and transmit modes by means of multifrequency exci-
tation or modulation of parameters. Signal-processing anten-
nas typically include monopulse phased-array antennas with
phase scanning and adaptation, logical synthesis antennas,
and space-time antennas. AIL
Ref.: Bakhrakh (1989), p. 24; Fradin (1977), p. 345; Steinberg (1976), Chaps.
11, 12.
Figure A76 AN/TPS-43 stacked-beam reflector antenna.
A slot antenna comprises one or several parallel narrow slots
cut in current-carrying surfaces of differing shape. Many vari-
planar arrays in which each row of elements is fed from an
eties of slot antennas have been designed. They are distin-
elevation beam-forming matrix. Mechanical rotation in azi-
guished by their shape, size, positioning, and the shape of the
muth is still used, especially when 360° coverage is required.
surface in which they are cut. A slot system’s radiation maxi-
The transmitter is normally connected through circulators to
mum is directed perpendicular to the plane in which the slot is
all beam ports, with coupling coefficients such as to produce
cut. Several slots may be used to intensify the directional
a single cosecant-squared beam pattern.
characteristics of the antenna. The ability to fabricate slot
The elevation beam-forming matrix can operate at RF,
antennas flush to a metallic surface makes them convenient to
using a Rotman lens, a Butler matrix, or equivalent network;
use on airborne vehicles. They also are used as the elements
at intermediate frequency; or it can be a digital beam-former.
of antenna arrays and as feeds for other complex antennas.
SAL
AIL
Ref.: Barton (1988), pp. 196–198.
Ref.: Johnson (1993), Ch. 9; Sazonov (1988), p. 253.
strip(line) antenna (see microstrip antenna).
A slotted-waveguide antenna has the form of slots in the
walls of a rectangular waveguide. The slots have a length superbroadband antenna (see ultrawideband antenna).
close to l/2, where l is the wavelength. The voltage in the