Page 38 - Radar Technology Encyclopedia

28 antenna, conformal antenna, dipole

practical applications as conformal arrays, typically in cylin- used. The reflectors in such antennas are formed by a flexible
drical, conical, ring, and spherical shapes. AIL, SAL material that is stretched over a frame made of radial ribs,
Ref.: Johnson (1993), Ch. 21; Voskresenskiy (1981), p. 150. attached to a central bushing, that fold like an umbrella.
Antennas with rigid ribs are stable against wind loads and are
A conical-scan antenna is a tracking radar antenna that uses
highly reliable. Umbrella antennas with folding ribs are used
the conical scan technique. One of the simplest conical-scan
both in ground-based and in space radars. Umbrella antennas
antennas is a parabola with an offset feed rotated about the
with flexible ribs are used only in space due to their lack of
axis of the reflector. If the feed maintains the plane of polar-
rigidity. AIL
ization fixed as it rotates, it is called a nutating feed, and if the
feed causes polarization to rotate, it is called a rotating feed. Ref.: Gryannik (1987), pp. 4, 7; Cantafio (1989), pp. 499–524.
The tracking bandwidth of a radar employing conical-scan A dielectric [polyrod] antenna (Fig. A54) is one consisting
antennas is low, and this technique was used primarily in of a dielectric rod having circular or rectangular cross section,
early radar development. DKB with a length of a few wavelengths. The rod is fed by a sec-
Ref.: Skolnik (1980), p. 155. tion of circular or rectangular waveguide. In the dielectric
rod, the lowest hybrid slowed electromagnetic wave, HE , is
11
A cosecant-squared antenna is one forming a cosecant-
used. The slow-wave factor of phase velocity along the
squared beam in elevation (see BEAM, cosecant-squared).
dielectric rod is defined by the diameter and taper of the rod.
This beam can be generated with an array or shaped reflector
The half-power beamwidth of antenna, q , and its direc-
3
antenna. In the latter case, either the true parabola with a
tivity, D, may be evaluated by using expressions
properly designed set of multiple feed horns, or distorted sec-
tion of parabola may be used. Doubly-curved reflectors are
–
q = ( 107° 61° ) l L ¤
3
typically used to generate cosecant-squared beams in search
radars (see REFLECTOR, doubly-curved). An example of D = ( 4.0 – 7.2 ) L l¤
a cosecant-squared antenna is a doubly-curved reflector
where L is the length of the rod.
antenna used in the Raytheon ASR10SS radar (Fig. A53).
Dielectric rod antennas are used as independent antennas
SAL
and as the array elements with broad patterns. AIL
Ref: Johnson (1984), p. 17–19; Skolnik (1980), p. 55.
Ref.: Johnson (1993), pp. 12.21–12.22; Sazonov (1988), p. 303; Wolff
(1988), pp. 394–405.
d
L
Input (a)

d d
1 2
L

Input (b)

Figure A54 Dielectric antennas: (a) cylindrical rod;
(b) conical rod.
Figure A53 Cosecant-squared antenna for ASR10SS radar.
A diffraction antenna is one in which, besides a feed and a
A deployable antenna is one that changes its dimensions and
radiator, there is a diffraction element shaping the radiation
shape during transition from travel to operating configuration.
pattern. A horn, lens, mirror, slot, or dielectric rod usually is
These antennas are typically categorized as self-deploying or
employed as the diffraction element. AIL
manually deploying, and as inflatable or mechanical, based
Ref.: Popov (1980), p. 115.
on how the reflector is packed in travel configuration. Inflat-
able antennas may be solid or tubular. Mechanical antennas A dipole antenna is one in which one or several dipoles are
are categorized as umbrella, truss, circular, prefabricated, or used. Individual dipoles are rarely used as independent
sectoral. Deployable antennas are used in mobile antenna systems. Multidipole antennas may comprise dipoles
ground-based radars and in space-based radars. assembled in several tiers with the corresponding distances
One widely used configuration is the deployable between tiers. A metallic reflector is located at a specific dis-
umbrella antenna, in which rigid, folding, or flexible ribs are