Page 57 - Radar Technology Encyclopedia

array, monopulse array, phased 47

for one frequency band are dispersed between the radiators of
the other bands.
(2) Convex multifrequency arrays with distributed multi-
frequency radiators positioned on convex, curvilinear sur-
faces.
(3) Multifrequency arrays constructed using multifre-
quency or wideband radiators and frequency separation fil-
ters.
(4) Multifrequency combined arrays that operate as ordi-
nary arrays in higher frequency bands, while at lower fre-
quencies the array aperture forms a shaped impedance
structure that is illuminated by a feed positioned at the periph-
ery of the aperture. AIL
Ref.: Bakhrakh (1989), p. 113.
Array mutual coupling is the effect of coupling among adja-
cent radiators. The degree of coupling depends on the dis-
tance between elements, the pattern of the individual
elements (element factor), and the structure in the vicinity of
the element. The main effect of mutual coupling is the change
of impedance and pattern of the element, which vary as func-
tions of scan angle. The coupling is typically significant for
Figure A86 AN/SPY-1 Aegis monopulse phased-array
elements spaced up to several wavelengths from each other. It
antenna viewed from the rear, during assembly.
can be described by a mutual-coupling coefficient which
relates the voltage induced in the mnth element to the voltage
A multibeam array supports the generation of several beams at the pqth element. SAL
that can be used simultaneously for surveillance of a given Ref.: Johnson (1984), pp. 20–25; Skolnik (1990), p. 7.23.
sector. Each beam has its corresponding separate input chan-
A passive (antenna) array is one in which all elements are
nel. The basic element supporting generation of several
excited from a common oscillator or connected to a common
beams is the multiple beam-forming network.
receiver. Therefore, an immutable part of a passive array is
The essential elements of the beam-forming networks are
the feed network connecting the elements. Passive antenna
quadrature directional couplers and fixed phase shifters. The
arrays are categorized as receiving, transmitting, and trans-
general shortcomings of multibeam arrays using beam-form-
ceiving. They are finding wide use in variable-purpose radars.
ing networks are the large numbers of connecting structures,
AIL
phase shifters, and complicated branching feeds. AIL
Ref.: Sazonov (1988), p. 397; Mailloux (1994), p. 40.
Ref.: Bakhrakh (1989), p. 142; Sazonov (1988), p. 405; Mailloux (1994),
pp. 167–182, 423–446. array pattern (see PATTERN, array).
A multifaced array is a system of planar arrays. It can be A phased array is an “array antenna whose beam direction
either a set of two to four independent planar arrays covering or radiation pattern is controlled primarily by the relative
different azimuth sectors, or a set of subarrays arranged on phases of the excitation coefficients of the radiating ele-
the faces of a convex polyhedron and interconnected to pro- ments.” Physically it is composed of a group of individual
duce a single scanning beam. If the number of subarrays is ten elements that are arranged in a linear or two-dimensional
or fewer, they are usually positioned on the faces of a regular (typically planar) spatial configuration. Usually the following
or truncated pyramid. For larger numbers of subarrays, they basic features are used to classify phased arrays:
are located on the faces of regular polyhedra. The patterns of (1) Scanning methods,
pyramidal phased arrays are similar to those of planar arrays. (2) Radiator feed methods;,
Multifaced phased arrays of this type are quasiconformal (3) Positioning of radiators in the array.
arrays. AIL The main scanning methods are phase scanning and fre-
Ref.: Voskresenskiy (1981), p. 146; Mailloux (1994), pp. 231–233. quency scanning. Typically, in radar literature, it is the phase-
A multifrequency array is one that operates over several fre- scanned arrays that are referred to as phased arrays. From the
quency bands. There are several approaches to achieving viewpoint of feed methods, arrays are divided into con-
multifrequency operation: strained-feed arrays and space-fed arrays, the latter taking the
(1) Multifrequency, overlapping arrays formed by the form of reflectarrays or transmission arrays (see array feed
merging of one array into another in such a way that all the networks). With regard to element positioning, phased arrays
radiators are located in the same aperture, and the radiators are divided into uniformly spaced and unequally spaced
arrays.