Page 40 - Radar Technology Encyclopedia

30 antenna, dummy antenna gain

not included in signal generator is often called the dummy An antennafier is an antenna with an embedded amplifier.
antenna. SAL
A fixed-beam antenna is one without scanning or beam
Ref.: Johnston (1979), p. 58.
switching. Fixed-beam antennas are primarily used in elec-
tronic warfare systems, and the main types of such antennas
Paraboloidal are broadband dipoles, broadband monopoles, spirals (planar
main
reflector or conical), horns and slots, log periodics, helices, and reflec-
tor types. SAL
Ref.: Schleher (1986), pp. 474–482.
Hyperboloidal
subreflector frequency-independent antenna (see broadband antenna).
A fuselage-mounted antenna is an airborne radar transceiv-
Focus of
ing antenna installed along an aircraft fuselage on the inside
paraboloid
or in a special pod. It may or may not be used to generate a
Plane
synthetic aperture. One without synthetic aperture capability
wavefront
is a long antenna designed for observation of the terrain or
surface objects in accordance with the radiation pattern it
shapes. The antenna is attached in such a way that its beam is
perpendicular to the aircraft flight trajectory and surveillance
Figure A56 Geometry of the Cassegrainian dual-reflector occurs by displacement of the beam during flight. Two anten-
antenna.
nas, one on the right side and the other on the left, often are
used to increase the area of the observed sector. Each antenna
A dynamic antenna is one having parameters periodically has a narrow (1 to 1.5°) beam in the horizontal (azimuth)
changing over time. The changing parameters may be ampli- plane and a relatively wide (50 to 60° beam in the vertical
)
tude or phase distribution of the field in the antenna aperture; (elevation) plane. Each antenna has a waveguide feed and
antenna dimensions; time of activation of an individual usually is a slotted waveguide antenna. AIL
antenna element; and so forth. Dynamic antennas are typi-
Ref.: Kondratenkov (1983), p. 64.
cally arrays. Problems of synthesis of a radiation pattern with
,
)
Antenna gain, G(q,f is “a measure of the ability to concen-
a low sidelobe level may be solved in dynamic antennas by
trate in a particular direction the power accepted by the
switching array elements. They sometimes are called anten-
antenna.” It is related to the antenna directivity, D(q,f in the
)
nas with time-modulated parameters. AIL
direction specified by angles q and f through the radiation
Ref.: Bakhrakh (1989), p. 17; Johnson (1993), Ch. 22.
efficiency, h:
The antenna effective area (in a given direction) is “the ratio
G qf,( ) = h D qf)
(
,
of the power available at the terminals of an antenna to the
incident power density of a plane wave from that direction
and to the effective (aperture) area, A (q,f) by
r
polarized coincident with the polarization that the antenna
would radiate.” (See also APERTURE, effective). 4pA qf,( )
r
=
G qf,( ) ---------------------------
Ref.: IEEE (1993), p. 43. 2
l
antenna efficiency (see APERTURE efficiency). where l is the wavelength.
For aperture-type antennas, A = h A, where h is the
An electrically large antenna is one with aperture greater r a a
aperture efficiency and A is the physical area. In this case,
than 60 wavelengths. They are typically used in radar astron-
omy, antiballistic missile radar, space-surveillance radar, and 4pA
other applications where high gain and narrow beamwidth are G = ----------h a
2
l
required. SAL
(see also aperture-type antenna).
Ref.: Johnson (1993), p. 17.31.
For array antennas, the gain in direction q from normal
0
A fan-beam antenna is one generating a fan beam, typically
to the array surface can be approximately estimated as
used in search radars or surveillance radars with the wide
G(q ) » phNcosq 0
0
dimension in the vertical plane and scanning in azimuth. It is
also used in height-finding radar, where the wide dimension where N is the number of array elements and h is the radia-
is in a horizontal plane and scanning is done in elevation. The tion efficiency for the array antenna (the cosine function may
fan beam can be produced by array antenna or by a reflector be raised to the 3/2 power for many practical arrays, depend-
antenna with the mirror truncated in the plane of the wide ing on the matching of the elements to space).
antenna pattern. SAL Antenna gain is often expressed in decibels:
Ref.: Johnson (1993), p. 1-13; Skolnik (1980), p. 54. G = 10logG
dB
and for a voltage gain pattern with maximum value F
antenna feed (see FEED, antenna).