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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[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