Page 13 - Radar Technology Encyclopedia

absorber, geometric transition absorber, magnetic 3

walls. This type of absorber can provide reflectivity reduction Interference materials are made either with metal or non-
in excess of 50 dB and bandwidth from 100 MHz to 100 GHz. metal substrate having a high relative dielectric constant (100
SAL to 200), the latter simplifying the attachment of the coating to
Ref.: Knott (1993), pp. 326, 528–532; Bhattacharyya (1991), p. 219. the masked structure.
Multilayer interference materials provide signal attenua-
A graded absorber is constructed from discrete layers with
tion from 20 to 40 dB at X-band, and from about 7 to 12 dB at
properties changing from layer to layer. The most common
C-band (Fig. A8). IAM
use layers of dielectric materials. One commercial example is
Ref.: Stepanov (1968), p. 55; U.S. Patent no. 3,568,195, cl. 343-18, 3-2-71.
a three-layer graded dielectric absorber about 1 cm thick with
properties shown in Fig. A7. In the commercial productions
of graded dielectric absorbers, five or more layers have been
used. Commercial graded magnetic absorbers appear to have
been limited to three layers. SAL
Ref.: Knott, (1993), p. 324.

Figure A8 Reflection coefficient as a function of frequency for
Jaumann, graded dielectric, and hybrid absorbers (from Knott,
1993, Fig. 8.28, p. 342).
Jaumann absorber is a wideband multilayer structure. It is
made from alternating layers of lossy film and relatively thick
layers of low-loss materials. The cascade process used in
Figure A7 Measured reflectivity of a three-layer graded dielec-
multilayer absorbers considerably improves the bandwidth of
tric absorber (from Knott, 1993, Fig. 8.17, p. 325).
the absorption. Figure A9 shows the calculated reflected
A hybrid absorber combines different types of absorbers to
provide broader bandwidth or improved performance within
the same band. For example, magnetic and circuit analog
absorbers, or Jaumann and graded dielectric absorbers can be
combined. Reflection coefficients as a function of frequency
for a three-layer Jaumann, a graded dielectric, and a hybrid
absorber are shown in Fig. A8. SAL
Ref.: Knott (1993), pp. 339–343.
Interference absorbing materials, when used as coatings,
constitute resonant absorbers, consisting of one layer of
dielectric applied to the metal surface that is to be protected.
The thickness d and the constants e he permittivity) and m
(t
(the permeability) of the material are selected for a given
1/2 Figure A9 Jaumann absorber (from Knott, 1993, Fig. 8.15,
wavelength, l, to meet the condition d = l/4(em) .
p. 322).
The coating is usually made of plastic or rubber, filled
with graphite powder or carbonyl iron. Such materials are power versus frequency for Jaumann absorbers containing
narrowband absorbers and operate well only at angles of inci- variable numbers of resistive sheets. SAL
dence close to normal. Materials of the interference type can Ref.: Skolnik (1990), p. 11.48; Bhattacharyya (1991), p. 215; Knott (1993),
also be used for effective absorption over a broad frequency pp. 320–323.
band, with several layers having thickness and structure opti-
A magnetic absorber uses magnetic radar absorbing mate-
mized for different wave lengths. This is achieved through a
rial such as ferrite slabs. It has an advantage over dielectric
specific combination of dielectric and magnetic constants of
absorbers, because usually it requires only 1/10 of the thick-
the absorber. The material can also contain dipoles made from
ness of dielectric absorbers to cause the same RCS reduction.
metal fiber, filamentary crystals, or fibers made from plastic
As an example, absorption characteristics of a two-layer mag-
with a metal coating.
netic absorber, constructed from a ferrite-resin mixture