Page 36 - Radar Technology Encyclopedia

26 ANTENNA antenna, Cassegorian

nas, being preferred in military radars where high perfor- (K » 30,000 for q in degrees). More accurate estimates
g
mance under conditions of ECM is a predominant factor. should take into account the aperture illumination and
Reflector antennas feature relative simplicity, high gain, antenna efficiency. For cosecant-squared elevation patterns,
and lower cost, but they are more vulnerable to jamming and about 1.5 dB should be subtracted from the gain as estimated
their scanning capabilities are restricted only to mechanical from the equation above. The obsolete term for aperture
and electromechanical scanning. Nevertheless, in commercial antennas is wavefront antennas. AIL
radar applications where cost factors are very important, Ref.: Sazonov (1988), p. 360; Johnson (1984), pp. 1.6, 1.15; Mailloux
reflector antennas are still the competitive choice. SAL (1994), pp. 267–275.
Ref.: Johnson (1984); Skolnik (1980), pp. 223–277, (1990), pp. 6.1–6.59, antenna array (see ARRAY).
7.1–7.69; Rudge (1982).
antenna axis (see AXIS, antenna).
active antenna (see ARRAY, active).
A backfire antenna is one “consisting of a radiating feed, a
An adaptive antenna is an antenna “having circuit elements
reflector element, and a reflecting surface such that the
associated with its radiating elements such that some of the
antenna functions as an open resonator, with radiation from
antenna properties are controlled by the received signal.”
the open end of the resonator.”
Typically, this type of antenna is used to receive signals from
Ref.: IEEE (1993), p. 83; Johnson (1993), p. 12.14.
desired sources while suppressing those from undesired
(interference) sources. Usually the main component of an Antenna bandwidth is an operating band of frequencies
adaptive antenna system is a phased array (see array, adap- within the limits of which other parameters do not exceed the
tive), a multiple-beam antenna, or a combination of the two. limits of the tolerances called for by the technical require-
The basic configuration of an adaptive antenna (Fig. A49) ments. Usually, the parameter depending the most on fre-
incorporates N ports, N complex weights, a signal-summing quency defines the limits of the operating frequency band.
network, and a weight-determining algorithm. SAL For example, a change in the position of the radiation pattern
maximum, expansion of the beam and a drop in directive
Ref.: IEEE (1993), p. 16; Johnson (1993), Ch. 22.
gain, and so on can stipulate a frequency band limitation. The
e 1 following breakdown of antennas is conditionally accepted
w 1 depending on bandwidth: narrowband (with a ratio of upper
e and lower frequency limits) up to 1.2:1; broadband up to 5:1;
o
S
ultrawideband up to 10:1 and higher. AIL
Ref.: Sazonov (1988), p. 204; Barton (1988), pp. 185–187; Mailloux (1994),
e
n
pp. 34–40.
w
n
e cn e c1 antenna beam (see BEAM, antenna).
Weight determining A broadband antenna is an antenna used for reception and
algorithm transmission of signals in a broad frequency band or on vari-
ous frequencies. This type of antenna is capable of operating
Figure A49 Fundamental adaptive nulling circuit (after with an upper and lower frequency ratio of up to 5:1 and
Johnson, 1993, Fig. 22.1, p. 22.3).
more, with a slight change in radiation pattern shape. Log
antenna aperture (see APERTURE, antenna). periodic and helical antennas fall in this category.
Broadband antennas are finding use mainly in COMINT
An aperture(-type) antenna is one with a flat aperture form-
and ECM systems. These antennas sometimes are called fre-
ing a directional pattern. The aperture is usually a portion of a
quency-independent antennas. AIL
plane surface near the antenna, which is perpendicular to the
direction of maximum radiation and through which most of Ref.: Sazonov (1988), pp. 262–272; Johnson (1993), Chaps. 14, 43.
the radiation flows. The most widely used types of aperture An antenna with built-in functional circuits is one in which
antennas are reflector, lens, and horn antennas. the functions of the antenna and several radar units are com-
Approximate formulas to predict the beamwidth, q, and bined from a design standpoint. This type includes antennav-
gain, G, of an aperture antenna are erters, antennafiers, and antennamitters. Based on functions
l accomplished, an antenna correspondingly is combined with
q = K ---- deg
3 q D a frequency converter, radio-frequency amplifier, and trans-
mitter. Advantages of such devices include better perfor-
K g 4p
G = ----------- = ----------------- mance figures, increased reliability, and compact design. AIL
q q q q L
1 2 1 2 n Ref.: Popov (1980), p. 35.
where l is the wavelength, D is the aperture dimension in the
A Cassegorian antenna is a multiple-reflector antenna with
plane of the pattern, K » 60° is a dimensionless constant, q 1 a subreflector that has a hyperbolic arc in the vertical plane
q
and q are the 3-dB beamwidths in two orthogonal principal (as in the Cassegrainian antenna) and an elliptical arc in the
2
planes, and K » 10, L » 1.3 are constants for q in radians
g n other plane (as in the Gregorian antenna). This type of subre-