Page 50 - Radar Technology Encyclopedia

40 antenna, ultrawideband aperture illumination

makes it possible to transmit a UWB signal without distor- antenna, the spacing d between discrete elements must be
tions. AIL such as to meet the criterion for avoidance of grating lobes at
Ref.: Yatskevich, V. A., and Fedosenko, L. L., Antennas for Radiation of the maximum scan angle q:
Ultrawideband Signals, Moscow, Electronics, vol. 29, 1986, p. 69 (in
Russian); Taylor (1995), Ch. 5. l
d £ -----------------------
1 + sin q
An antennaverter is an antenna with an embedded frequency
converter. DKB
wave-channel antenna (see ARRAY, Yagi-Uda). Ref.: Johnson (1993), p. 2.31.

wave-front antenna (see aperture-type antenna). aperture distribution (see aperture illumination).
A waveguide antenna consists of a waveguide fed in the The effective aperture is an important concept in radar
dominant mode and opening onto a conducting ground plane. antenna theory; it may be regarded as a measure of the effec-
In radar applications it is mostly used as a feed for reflector tive area (see antenna gain) presented by the antenna to the
antennas or frequency-scanning arrays. SAL incident wave. If G is the radar receiving antenna gain, and l
r
the wavelength of the radiation, then the effective aperture (or
Ref.: Fink (1982), pp. 18.23–18.26; Johnson (1993), Ch. 9; Barton (1988), p. 2
167. effective area) is A = G l /4p. For a uniformly illuminated
r
r
aperture, an ideal (lossless) antenna will have maximum
Yagi antenna (see ARRAY, Yagi-Uda).
directivity or gain; that is, the antenna takes full advantage of
APERTURE, antenna. The antenna aperture is “a surface, the physical area A and the antenna gain is at its maximum
2
near or on an antenna, on which it is convenient to make value, G . Then A = G l /4p. PCH
0
0
assumptions regarding field values for the purpose of com- Ref.: Johnson (1993), p. 1.6.
puting the field at external points.” More generally, the aper-
Aperture efficiency, h , relates the effective aperture area,
a
ture of an antenna is its physical area projected on a plane
A , to the physical antenna area A; that is, A = h A. Aperture
r
r
a
perpendicular to the mainbeam direction. Radar antennas are
efficiency is also equal to the ratio of the actual directivity of
sometimes classified by the geometrical shape of their aper-
an antenna to the maximum possible directivity of that
tures (e.g., circular, elliptical, and rectangular). SAL
antenna: h = G /G . In a rectangular aperture with separable
a
0
r
Ref.: IEEE (1993), p. 46.
illuminations, the aperture efficiency will have two compo-
An active aperture is one that incorporates active transmitter nents, h = h h . PCH
a
x y
elements within the antenna structure itself, as opposed to a Ref.: IEEE (1993), p. 47; Johnson (1993), p. 1.6.
passive aperture, which focuses energy received from a
An elliptical aperture can be a parabolic reflector, a lens, or
source (transmitter or target) outside the antenna (see
an array having an active area that is elliptical in shape. The
ARRAY, active). PCH
analysis is performed as with a circular aperture, but the
Aperture blockage occurs when a source of physical inter- beamwidth and sidelobe level in each principal plane will
ference, usually the antenna feed structure of a reflector depend on the dimension D and the illumination function in
antenna, blocks a portion of the aperture from taking part in that plane. DKB
the exchange of radar energy. The blocked portion of the Ref.: Barton (1988), p. 155.
aperture is said to be shadowed and therefore will not contrib-
aperture excitation (see aperture illumination).
ute constructively to the field. The far-field antenna pattern
will then consist of the sum of the intended pattern, plus three Aperture illumination is the electric field distribution across
unintended components: blockage, feed scattering, and spill- an antenna aperture. It is also called the aperture distribution,
over. PCH excitation, taper, or weighting function. If this function is
known, the radiation pattern, or electric field intensity, can be
Ref.: Johnson (1993), pp. 17.32, 30.34–30.40.
found as a function of x, y, and z coordinates relative to the
A circular aperture can be a parabolic reflector, a lens, or an
antenna centroid. The far-field (Fraunhofer) electric field
array having an active area that is circular in shape. The
intensity for a one-dimensional aperture of width a in the
beamwidth of a circular aperture with uniform illumination
z dimension (perpendicular to the direction of the main
over diameter D is q = 1.02l/D, and the first sidelobe level is beam), where a >> lcan be expressed by the Fourier trans-
3
,

-17.6 dB. Tapered illuminations are more commonly used,
form
with beamwidth constants near 1.2 and sidelobes -20 to -
25 dB. DKB a 2 ¤
2p z
æ
ö
Ref.: Skolnik (1980), p. 233; Johnson (1993), p. 2.19. E f() = ò Az () exp j--------- sin f dz
è
ø
l
A continuous aperture is one over which the illumination – a 2 ¤
function is smooth, producing no grating sidelobes. The mesh where A(z) is the aperture illumination, or current at distance
of a reflector must be fine enough to prevent significant leak- z, flowing in the x-direction, and f is the angle off beam cen-
age of the wave through its surface. In the case of an array