Page 49 - Radar Technology Encyclopedia

antenna, superdirective antenna, ultrawideband 39

A superdirective antenna is a linear antenna array of finite A transmitting antenna is one designed for radiation of
length with a large number of radiators making it possible to electromagnetic waves. For transmitting antennas, along with
realize an array factor as an arbitrary function. The capability directivity characteristics, minimum losses of electromag-
of unlimited increase in directive gain of an antenna of finite netic power to heating the antenna conductors and dielectrics,
length is referred to as antenna superdirectivity. Analysis of and high power-handling capability are of important signifi-
the solution demonstrates that, given number of elements, cance. AIL
N ®¥ , element spacing, d ® 0, and constant array length Ref.: Sazonov (1988), p. 201.
Nd = const, the directive gain will increase without limit.
A traveling-wave antenna is one in which an axial radiation
However, computations and research illustrate that the transi-
mode is realized. They are made on the basis of slow-wave
tion to the superdirectivity mode is impractical because the
structures capable of supporting surface waves. A travel-
increase in directive gain compared with normal directional
ing-wave antenna is excited through selection of slow-wave
antennas is extremely small. AIL
structure parameters. They usually have a frequency band
Ref.: Johnson (1993), p. 2.40; Sazonov (1988), p. 358; Mailloux (1994), from several to tens of percent.
pp. 19, 80.
The advantage of such antennas is the slight dimensions
surface-wave antenna (see impedance antenna). of the transverse cross section of the radiating system. This
makes it possible to house them on the flat surface of the bod-
A synthetic aperture antenna is one in which aperture syn-
ies of airborne vehicles. Dielectric rod, helical, impedance,
thesis is achieved based on the effect of the displacement of a
and director antennas are categorized as traveling-wave
physical antenna along the flight trajectory of a moving plat-
antennas. These antennas are also called end-fire antennas,
form, using special signal processing. The physical antenna,
and previously were referred to as surface-wave antennas.
which may take the form of a parabolic cylinder reflector,
AIL
slotted waveguide, or a phased array, is small and has a suffi-
Ref.: Johnson (1993), Ch. 12; Sazonov (1988), p. 302.
ciently wide radiation pattern to illuminate the observed sur-
face over a significant period of platform motion. The essence ultra-low-sidelobe antenna (ULSA) (see low-sidelobe
of the synthesis method is based on a priori information of antenna).
the trajectory of motion of the antenna platform and involves
An ultrawideband antenna is one used for transmission and
the reception of signals during motion, remembering them,
reception of ultrawideband (UWB) signals and operating with
and subsequent coherent addition to form the high-resolution
a signal bandwidth exceeding 50% of the center frequency.
image. The sector of trajectory of the platform in which the
The conventional log periodic and helical broadband anten-
signal is formed and processed is the synthetic aperture. This
nas cannot be used for transmission and reception of UWB
makes it possible to obtain high spatial resolution in the angu-
signals. The reason is the strong dispersion of the phase-fre-
lar coordinate, corresponding to a conventional antenna with
quency response inherent to the aforementioned antennas and
an aperture of hundreds or thousands of wavelengths.
leading to significant distortion of the shape of the ultrawide-
Digital signal processing and an antenna with a digital
band signals.
synthetic aperture (see RADAR, synthetic aperture) are
A plane log periodic dipole after appropriate modifica-
used for signal processing and for obtaining a radar image of
tion can be used to transmit UWB signals (see log-periodic
the earth’s surface directly on board the aircraft. Digital syn-
antenna). This modification involves changing antenna
thetic aperture antennas have several advantages, the main
geometry (selection of dipole length) so the dipole resonant
ones being: responsiveness; ability to synthesize the antenna
frequencies are subordinate to a law of arithmetic progression
aperture during random maneuvers of the airborne platform;
rather than the geometric progression found in the conven-
automation of the detection and measurement of the coordi-
tional log periodic antenna, that is,
nates of objects; and multiple reproduction of the recorded
information. AIL w = w + ( n – 1 )dw n = 1 2 ¼,, N
,
n 1
Ref.: Goryankov (1988), p. 22; Kondratenkov (1983), p. 40; Skolnik (1990),
Ch. 21. where w is the resonant frequency of the first dipole, dw is
1
the step of the progression, and N is the number of dipoles.
antenna temperature (see TEMPERATURE).
Here, dipole lengths must be the following
antenna testing (see TESTING, antenna).
l 1
l = ----------------------------------
A test antenna is one designed for measurement of antenna n ( n – 1 )dw
1 + ------------------------
performance. Horns, dipoles, and paraboloid antennas are w 1
used as measurement antennas. Selection of antenna type
Such a change in the law of progression makes possible
depends on antenna measurement methods, parameters mea-
the transition from a logarithmic dependence of the phase-fre-
sured, and frequency band. AIL
quency response to a linear dependence. The aforementioned
Ref.: Strakhov (1985), p. 10.
departure from logarithmic periodicity leads to an insignifi-
cant deterioration in the band properties of the antenna, but