Post on 31-Oct-2021
Hybrid Planar Absorber: Towards a compact absorber
A. El Assal1,3, R. Benzerga1, A. Sharaiha1, A. Harmouch2, A. Jrad3
1 Univ Rennes, CNRS, IETR – UMR 6164, F-35000 Rennes, FRANCE 2 CRSI, Université Libanaise, Faculté d’Ingénierie, Tripoli, LIBAN
3 LEPA, Université Libanaise, Faculté des Sciences, EDST, Tripoli, LIBAN
Abstract - This paper presents the design and realization of a
hybrid electromagnetic absorber combining a natural absorbing material, based on epoxy foam loaded with millimetric carbon
fibers, and an artificial resonant absorber, based on metamaterial unit cells. Two studies were conducted in parallel, the first was focused on the optimization of a planar multilayer absorber using
the genetic algorithm and the gradient of impedance principle, and the second was focused on the design and optimization of a metamaterial to broaden the band of absorption. A significant
reduction of the total thickness (by 70%) was obtained for the final hybrid absorber while maintaining the same absorption performance.
I. INTRODUCTION
In recent years, the interest in both absorbing materials and
resonant metamaterial absorbers has highly increased for
various reasons. It is focused on searching for materials with
very good absorption performance over a wide frequency range
while maintaining a compact absorber. At the same time, the
evolution of the regulation to improve the protection of human
health and the environment standards, which are restrictive in
terms of the use of certain materials, encourages the search for
an alternative composition to the current absorbers used for
example in anechoic chambers.
Today, the most commonly used material in the anechoic
chambers is made of polyurethane foam (PU) loaded with fine
carbon particles. This composite has a very good absorption
performance but suffers from weak mechanical properties, due
to the nature of the PU foam matrix, making its machining
inaccurate and non-reproducible. In addition, the particles used
to provide absorption are highly polluting and potentially
harmful to human health [1]. These different limitations
motivated the search for new absorbing materials [2].
In our team, a new absorbing composition was developed [3].
This composition consists of an original combination of an
epoxy foam with millimetric carbon fibers. This composite
responds, on one hand, to the problem of the mechanical
properties of the matrix, and on the other hand, to the problem
of use of fine particles [1]. In addition, the important aspect
ratio (between length and diameter) of the fibers, unlike the
spherical particles, gives the material significant dielectric
losses while keeping a low permittivity (using low percentages
of carbon fibers). This ensures two essential conditions for the
development of a good absorber: high absorption and low
reflection at the surface of the material.
In this work, we propose a planar hybrid absorber which
consists of combining the epoxy foam (in the form of multilayer
loaded with carbon fibers) with an artificial absorber (a
resonant metamaterial (MM) absorber [4,5]) in order to reduce
the bulk of the multilayer absorber while maintaining a good
electromagnetic absorption performance.
This paper is organized as following: the elaboration of the
different composites as well as their properties are first
presented. The choice of the composition and the thickness of
the different layers of the planar absorber is then explained, the
geometry of the MM is detailed; and finally, the absorption
performance of the hybrid material is presented and compared
to that of the reference natural absorber.
II. HYBRID ABSORBER ELABORATION
a. Elaboration of absorbing composites
The method used to produce the composites based on epoxy
foam loaded with carbon fibers is summarized in Figure 1.
Carbon fibers of different lengths (3, 6 and 12mm) and 7 μm in
diameter are used with different weight percentages (0.25, 0.5
and 0.75%). A commercial epoxy resin kit, with its hardener, is
used for the elaboration. The fibers are dispersed in the resin
using ultrasonication dispersion method. The mixture is then
put into a mold for foaming and polymerization steps during 6
hours. Subsequently, the mold is placed in the oven (at 60°C for
6 hours) to finalize the polymerization of the epoxy foam and
thus obtain a rigid composite. The samples are finally cut to the
dimensions needed for the characterization.
Figure 1. Elaboration steps
1) Adding acetone
2) Adding carbon fibers and epoxy
resin
3) Ultrasonication dispersion
5) Foaming and polymerization
7) Characterization
4) Adding the Hardener 6) Cutting
b. Composites characteristics impedance of elaborated
composites
The dielectric characterization of the composites (dimensions
15x15x6 cm3), as well as the measurement of the absorption
performances of the realized prototype, are carried out between
1 and 6 GHz in the anechoic chamber. Extraction of the
complex permittivity of composites is performed using a
method based on the work of Fenner et al, detailed in [6].
After this, the characteristic impedances of the elaborated
composites can be calculated in order to choose the
composition of the different layers to construct the multilayer
absorber. These impedances are presented in Figure 2a.
a) b)
Figure 2. Characteristic impedances of the elaborated
composites a) the topology of the reference absorber b).
III. ABSORBER STRUCTURE
The various simulations (of the multilayer absorber material, the MM as well as that of the hybrid absorber) are carried out using commercial software CST Microwave Studio [9] in the frequency domain.
A. Multilayer absorber
For this study, a number of four layers and a standard
thickness of 250mm were chosen for the realization of the
reference multilayer absorber. Each layer was made by
calculating the characteristic impedance of the composites we
have using the gradient of impedance principle [7]).
From the calculated impedances in Figure 2, the unloaded
epoxy foam (0%) was chosen to make the first layer of the
absorber because it will ensure the impedance matching
between air ( = 377Ω) and this first layer of the absorber ( =
344Ω). The composite loaded with 0.75% of fibers of 6 mm
length, having the lowest impedance, was chosen to make the
last layer of the absorber. The composites loaded with 0.25% of
3mm and 12mm carbon fibers (with intermediate values) were
chosen for the two intermediate layers.
Figure 2b. shows the topology of the absorber of reference
(250 mm). For this reference absorber, a thickness of 70mm
was used for the first adaptation layer, then 60mm respectively
for the other three epoxy loaded foam layers, with an overall
thickness of 250mm often used in commercial multilayer
absorbents.
The simulation results of the reflection coefficient for the
normal incidence ( = 0°) of the reference absorber is presented
in Figure 3 (Black line). We can note we have a reflectivity less
than -10 dB in all the bandwidth.
The genetic algorithm (GA), implemented in the CST
software, was subsequently used to optimize the thicknesses of
the different layers of the multilayer absorber. The reflectivity
is defined as the goal of the genetic algorithm with Γ<-10 dB.
The parameters that the GA optimize to obtain the best solution
are the different thicknesses of the layers composing the
absorber. These thicknesses are modified at each iteration of the
GA until achieving the requested reflectivity.
Optimum thicknesses of 43mm, 18mm, 9mm and 7mm have
been obtained for the four layers of composition 0%, 3mm-
0.25%, 12mm-0.25% and 6mm-0.75%, respectively. In fact,
the thickness of the matching layer (0%) responds to the
quarter-wave principle [8], which makes it possible to ensure
the impedance transition of all the wavelengths of the range of
frequency studied (from 1 GHz). For the next layer (3 mm to
0.25%), it must also ensure the transition between the first and
the third layer, the thickness must be sufficiently large (18 mm
proposed by the GA) to allow the absorption of maximum
wavelengths. For the last two layers that provide absorption,
thinner thicknesses (9mm and 7mm) have been proposed by
GA. The total thickness is now equal to 77mm which represent
a height reduction of almost 70%
A reflectivity less than -10 dB is obtained for the optimized
absorber MLA77 between 2 and 10 GHz (see figure 4).
However, the performance is deteriorated below 2 GHz for this
optimized absorber.
Figure 3. Simulation of reflection coefficient of the
absorber of reference and the optimized absorber ( = 0°).
B. Metamaterial geometry and properties
The proposed MM here consists of a unit cell size of 10x10 mm²
(figure 4), where the resonant pattern, made of copper of 35μm
thickness, is deposited on a commercial dielectric substrate FR-
4 of 3.2 mm thickness ( '= 4.3, tan = 0.025), itself metallized
at the back and considered as a ground plane. It has the shape
of X encountered by two L shape and that resonates at 6 and
10 GHz. The final Metamaterial Array can operate in several
frequency bands as can be seen in figure 5 where we present the
simulated reflection coefficient of the proposed MM.
0
100
200
300
400
500
600
1 2 3 4 5 6 7 8 9 10
(Ω
)
Frequency (GHz)
3mm-0.25% Air
12mm-0.25% 3mm-0.5%
6mm-0.5% 12mm-0.5%
3mm-0.75% 6mm-0.75%
0%
-40
-30
-20
-10
0
1 2 3 4 5 6 7 8 9 10
Γ(d
B)
Frequency (GHz)
MLA 250 / i= 0°
MLA 77 + MP / i=0°
Figure 4. Unit cells and metamaterial array
Figure 5. Simulation of the reflection coefficient of the MM
array at normal incidence.
Now, for the hybrid absorber, the proposed metamaterial is
associated to the optimized multilayer absorber in order to
ameliorate the absorption performance.
In figure 6, we show the simulated reflection coefficient for
the MLA77 with or without the MM. We observe that we
improve the absorption behavior between 1 and 3 GHz with a
gain up to 20 dB and approaching the performances of the
MLA250.
Figure 7. Simulation of reflection coefficient of the
absorber of reference, the optimized absorber and the
hybrid absorber for normal incidence ( = 0°) wave
IV. CONCLUSION
This work shows the possibility of realization of a compact
planar hybrid absorber by combining the natural and artificial
absorbers. A planar absorber of 70% of reduction in thickness
was achieved, while maintaining the same absorption
performance as the absorber of reference thanks to the
metamaterial design incorporated on the back of the optimized
planar absorber. The incorporation of the metamaterial
increases the performance of the classical absorber.
ACKNOWLEDGEMENTS
The authors wish to thank Jérome Sol, the European Union
through the European Regional Development Funds (ERDF),
the Ministry of Higher Education and Research, the Brittany
Region, the Department of Côtes d'Armor and Saint-Brieuc
Armor Agglomération through CPER projects 2015-2020
MATECOM and SOPHIE / STIC & Waves
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425102 (5pp)
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[9] https://www.cst.com/2018
-15
-10
-5
0
1 2 3 4 5 6 7 8 9 10
Γ(d
B)
Frequency (GHz)
X shape / i=0°
-40
-30
-20
-10
0
1 2 3 4 5 6 7 8 9 10
Γ(d
B)
Frequency (GHz)
MLA 250 / i= 0°
MLA 77 + MP / i=0°
MLA77 + MM Xshape / i=0