Page 12 - Radar Technology Encyclopedia

2 absorber, dielectric absorber, geometric transition

A dielectric absorber uses dielectric absorbing materials for absorbing wall consisting of individual magnetic rods
its construction. An example of a simple, single-layer dielec- arranged vertically and horizontally. IAM
tric absorber is the Salisbury screen. In practical applications, Ref.: Stepanov (1968), p. 62; Bhattacharyya (1991), pp. 177, 217–218.
multilayer dielectric absorbers are used, such as Jaumann
Frequency-selective surface (FSS) types of absorbers usu-
absorbers and graded dielectric absorbers. Practical graded
ally take the form of a thin metallic patterns etched into or
dielectric absorbers are made of discrete layers with proper-
deposited onto lossless substrates or films. The desired effect
ties changing from layer to layer. SAL
is to pass waves of a given range of frequencies, or all waves
Ref.: Knott (1993), pp. 313–327.
except those in a required band (bandpass or bandstop filter-
ing). Other uses are high-pass or low-pass filtering. Some
configurations used in FSS are shown in Fig. A5. Frequency
selective surfaces find many practical applications: in antenna
reflectors, wave polarizers, RCS control, and so forth. The
Jaumann and circuit analog absorbers are versions of FSS.
SAL
Ref.: Bhattacharyya (1991), pp. 224, 228.

Figure A3 Reflectivity of dominantly electric materials. Solid (a) Rectangular slot (b) Circular slot, (c) Annular slot
circular hole
trace: |e| = 16, |m | = 1, d = 20° , d = 0° ; dashed trace: |e| = 25,
e
r
r
r
m
|m| = 16, d = 30° , d = 20° ; diagonal trace: |e| = |m | = 4, d = d
e
e
m
r
m
r
r
= 15° . e = |e|exp(id) and m = |m|exp(id ) are the complex per-
e
m
r
r
r
r
mittivity and permeability of the material relative to those of free
space (from Knott, 1993, Fig. 8.12, p. 319).
(d) Single loaded slot
(e) Four-legged (f) Three-legged
symmetrically loaded slot
loaded slot
Figure A5 Frequency-selective surfaces (after Knott, 1993,
Fig. 8.22, p. 330).
A geometric transition absorber is based on geometric tran-
sition from free space to the highly lossy medium that pro-
vides an effective dielectric gradient and minimizes
reflections. The major shapes available are convoluted,
wedge-shaped, twisted-wedge-shaped, rectangular, triangular,
conical, and pyramidal. The pyramidal profile is most often
used, usually having the structure of a planar array of pyrami-
Figure A4 Reflectivity of dominantly magnetic materials. Solid dal absorbers (Fig. A6). Geometric transition absorbers are
trace: |m | = 16, |e | = 1, d = 10° , d = 0° ; dashed trace: |m| = 25, used in anechoic chambers to reduce reflection from the
m
e
r
r
r
|e| = 16, d = 20° , d = 30° ; diagonal trace: |e| = |m| = 4, d = d
e m
r
m
r
e
r
= 15° (from Knott, 1993, Fig. 8.13, p. 319).
Ferrite absorbing material provides attenuation of a radio
wave passing through it. Ferrite absorbing coatings are
marked by their low weight and thickness. Usually they are
used for masking the warheads of ballistic missiles and vari-
ous reflective parts of short-range missiles. They provide an
attenuation of 15 to 30 dB. With a thickness of 5 mm, a
square meter of coating has a weight of up to 5 kg. Ferrite
absorbing materials are used for camouflage in a wide wave-
band, from the meter to the centimeter range.
Ferrite material is used for coatings of anechoic cham-
Figure A6 Geometric transition absorber (from Knott, 1993,
bers, taking the form of a layer of tightly placed tiles or an
Fig. 8.18, p. 326).