micromachines

Article Toward an Ultra-Wideband Hybrid Metamaterial Based Microwave Absorber

Aicha El Assal 1,2, Hanadi Breiss 1,2 , Ratiba Benzerga 1,*, Ala Sharaiha 1, Akil Jrad 2 and Ali Harmouch 2

1 Univ Rennes, CNRS, IETR—UMR 6164, F-35000 Rennes, France; [email protected] (A.E.A.); [email protected] (H.B.); [email protected] (A.S.) 2 Faculty of Science, LEPA- CRSI, Lebanese University, EDST, Tripoli 1300, Lebanon; [email protected] (A.J.); [email protected] (A.H.) * Correspondence: [email protected]



Received: 4 September 2020; Accepted: 9 October 2020; Published: 13 October 2020


Abstract: In this paper, we propose a novel design of an ultra-wideband hybrid microwave absorber operating in the frequency range between 2 GHz and 18 GHz. This proposed hybrid absorber is composed of two different layers that integrate a multiband metamaterial absorber and a lossy dielectric layer. The metamaterial absorber consists of a periodic pattern that is composed of an arrangement of different scales of coupled resonators and a metallic ground plane, and the dielectric layer is made of epoxy foam composite loaded with low weight percentage (0.075 wt.%) of 12 mm length carbon fibers. The numerical results show a largely expanded absorption bandwidth that ranges from 2.6 GHz to 18 GHz with incident angles between 0◦ and 45◦ and for both transverse electric and transverse magnetic waves. The measurements confirm that absorption of this hybrid based metamaterial absorber exceeds 90% within the above-mentioned frequency range and it may reach an absorption rate of 99% for certain frequency ranges. The proposed idea offers a further step in developing new electromagnetic absorbers, which will impact a broad range of applications.

Keywords: hybrid absorber; metamaterial; dielectric layer; ultra-wideband absorber; anechoic chamber

1. Introduction Microwave absorbing materials are becoming an important topic in most technologies and environments where the main motivation is to reduce the level of electromagnetic fields. They are especially employed in electromagnetic (EM) interference [1], radar signature [2], telecommunication [3], medical systems [4], and in EM characterization environments [5]. Many studies on microwave absorption have been performed in the frequency range 2–18 GHz [6–9], since absorbers operating in the millimeter band are highly required. For this reason, the above-mentioned frequency band will be considered in this paper. Traditional planar microwave absorbers, such as Salisbury screen, Dallenbach layer, and Jaumann absorber, suffer from narrow bandwidth and bulky thickness. The extensive demand of broad operating bandwidth, low profile, low density, and thin thickness absorbers have accelerated the investigation of new absorbers that meet these demands. The idea of a Metamaterial (MM) absorber was firstly proposed by Mosallaei et al. [10], and the first MM was realized by Landy et al. [11], in the gigahertz (GHz) domain. Since then, many studies have been conducted on MM for electromagnetic wave absorption in various frequency ranges, such as microwave [12–16], terahertz [17,18], infrared [19], and visible spectral region [20–22]. Additionally, today, MMs found their applications in different domains, such as radar cross section reduction [23], military target stealth missions [24], human exposure to

Micromachines 2020, 11, 0930; doi:10.3390/mi11100930 www.mdpi.com/journal/micromachines Micromachines 2020, 11, 0930 2 of 16 electromagnetic fields protection [25], frequency tuning [26] telecommunication [27], and antenna gain enhancement [28]. The absorption mechanism of conventional absorbers is based on the impedance matching of lossy medium in order to obtain efficient absorbers either by changing the shape of the material (such as pyramids) to ensure a physical gradient of the impedance [29], or by stacking multiple layers with a decreased impedance, such as Jaumann layers. In this case, the EM energy is completely dissipated inside the used lossy medium [30]. Common electromagnetic MM absorbers consist of arrays of a conductive pattern (a periodic unit cell), a dielectric substrate, and a continuous conductive film at the back. The MMs absorption is mainly due to the local electromagnetic resonance mechanism. For this, the effective impedance of the MM structure must be matched to the impedance of free space [31]; thus, by adjusting the shape and the dimensions of the resonator, and/or the dielectric permittivity and thickness of the substrate. Consequently, at the resonant frequencies, both the transmission and reflection of the waves are minimized and absorption is maximized [31]. According to the absorption frequency range, the absorbers are classified as narrowband, broadband, wideband, single band, multi-band, or ultra-wideband absorbers. Generally, as the absorption of MM only occurs at the resonant frequency, the absorption bandwidth is considered as narrowband. Consequently, many efforts have been made in order to achieve multiple band [32–37], broadband [15,38–40], or ultra-wideband [6,41] microwave MM absorbers. The first method to broaden the absorption bandwidth of MM absorbers is to use a thick substrate. In [40], for example, a 4.2 mm thick FR-4 substrate was used to achieve an absorption higher than 90% in a wide frequency range of 4 GHz (from 4 GHz to 8 GHz), covering the entire C-band. Another proposed structure that consists of an underlined U shape resonator, with FR-4 substrate of 3.2 mm thick [38], results in a 90% absorption in the frequency band from 5.6 GHz to 9.1 GHz. The second method to broaden the bandwidth is based on the combination of various absorption peaks by combining multiple resonating patterns with different sizes or shapes. Consequently, multiple resonances will appear. If these frequency resonances are very close to each other, they form a broadband absorber; if not, they form a multiband absorber. For example, in [42], nine different ring resonators in the same unit cell were used. Like that, exploiting the resonators with different sizes, in order to resonate at different frequencies, results in 2 GHz absorption bandwidth (with absorption higher than 90%) between 10.5 and 12.5 GHz. Viet et al. [16] used the same idea with a dish shape resonator; a total absorption was obtained for 1.8 GHz bandwidth in the microwave region between 13.7 and 15.5 GHz. Open rings and dishes have been combined as one resonator in [43], while using different dimensions; thus, the absorption was increased (higher than 80%) for frequencies between 13.5 and 16.5 GHz. Another method is based on stacking multiple layers in which resonators share the same ground plane [44–49]. In this method, different resonating elements are stacked in sequence as metallic-dielectric pairs forming a two or three dimensional (two-dimensional (2D) or three-dimensional (3D)) absorbers to broaden the absorption bandwidth. In [47], three metallic films, of different dimensions of the same shape, are arranged each one between two substrates forming a 2D absorber. The resulting absorption is higher than 90% in the frequency range of 8.37–21 GHz with an absorption bandwidth of 12.63 GHz. Otherwise, other studies have stacked several layers forming a 3D MM absorber either in the form of pyramid [44] or a cylinder [45]. In the case of pyramidal shape, an absorption bandwidth of 6 GHz is achieved in the range 8–14 GHz and a dual broadband absorption bandwidth, of 2 GHz, is achieved by the cylindrical shape in the ranges of 4–6 GHz and 12–14 GHz. Another approach, by incorporating lumped elements into the MM resonators, was used in order to realize a broadband MM absorber [50–57]. It aims to match the input impedance with free space in a wide frequency range. In [50], for example, a Split Circle Ring (SCR) that was loaded with four lumped resistors, with R = 250 Ω, was proposed for X-band applications. MM without lumped resistors provides two resonances at 8.5 GHz and 12.5 GHz with absorptions of 26.1% and 93.9%, respectively, Micromachines 2020, 11, 0930 3 of 16 while the same MM structure with four lumped resistors exhibits a broadband absorption performance with absorption higher than 90% in the frequency range of 7.8–12.6 GHz [50]. Recently, it has been demonstrated that introducing MM in the design of an absorber may lead to reducing the thickness of a pyramidal absorber and enhancing its absorption performance. For example, in [58], a periodic MM structure composed of two different resonator geometries: an interleaved snake shape and spiral shape which operates in several frequency bands was proposed. The use of this proposed MM at the back of the conventional dielectric pyramidal absorber results in a compactness of this pyramidal absorber, and also an improvement of its absorption performance. This absorption was improved (up to 20 dB) between 8 GHz and 15 GHz for TE mode and between 3 GHz and 6 GHz for TM mode, with thickness being reduced by 21.7 % (total thickness of 90 mm compared to the initial thickness of 115 mm). It should be noted here that the MM absorbers that are based on a single layer offer advantages in term of thickness, weight, and ease of fabrication. However, it is very difficult to broaden the absorption bandwidth, even if these single layer MMs associate several resonators together. As mentioned before, different methods were used to broaden the absorption bandwidth, but most of them failed to meet all of the aforementioned requirements simultaneously. In other words, the design of broadband microwave MM was mostly focused on one or two frequency bands among the C-band (4–8 GHz) [40,59], X-band (8–12 GHz) [15,50,60], Ku-band (12–18 GHz) [41], or K-band (18–27 GHz) [46], whereas only few MM designs for absorption in the S-band (2–4 GHz) [12,52] have been reported so far. For the S-band, as the EM wave has the narrowest beam width, it is considered to be an excellent candidate for radar detection. However, because of the long wavelength of the electromagnetic waves in this band, developing a high efficient microwave absorber, presenting a thin thickness, is still a big challenge. For example, Cheng et al. [52] proposed a broadband MM for the S-band while using lumped elements covering 50% of this band with an absorption higher than 90% from 3.01 to 5.28 GHz. Note that the fabrication of such absorbers is complex, which restricts their practical applications for microwave absorption. The goal of our work is to achieve a broadband absorber with a good absorption at low frequencies, typically in the S-band. For this, first we propose a novel symmetrical MM absorber geometry, called Vshape. Secondly, in order to broaden the MM absorber bandwidth, a hybrid absorber is proposed; it consists on a combination of a thin dielectric layer to this MM to form an ultra-wideband absorber (between 2 GHz and 18 GHz). This article is presented, as follows; first, the Materials and Methods section presents some electromagnetics notions regarding MM absorbers, the design of the novel proposed MM, the preparation of the used dielectric layer, and the characterization technics. In the second section, the properties of the dielectric layer are presented, and then, the simulation of the proposed MM and the hybrid absorber are conducted. Finally, the measurements of the realized prototypes are presented, discussed, and compared with the simulation results.

2. Materials and Methods

2.1. Electromagnetic Absorber Theory A metamaterial absorber is typically a sandwiched structure that consists of an array of periodic metallic patterns, on one side of the substrate, backed with a highly conductive metallic ground plane, on the other side of the same substrate. No transmittance could be found on the other side of the MM due to the presence of the ground plane. The reflection from the MM resides at the air/MM interface; therefore, an impedance matching between the MM and the vacuum is needed in order to limit this reflection of the waves. Concretely, the effective impedance of the MM structure, as a function of the frequency (or pulsation ω) of the EM wave, is matched to the impedance of air by tuning the effective permittivity εeff (ω) and permeability µeff (ω). Moreover, the incident waves, which penetrated the MM, will be trapped inside it thanks to the cavity effect. The latter is formed by the space between the pattern and ground plane (the metallization Micromachines 2020, 11, x 4 of 16 the substrate (so, inside the cavity), due to a multi‐reflection (also named resonance phenomena) and the dielectric loss of this substrate. Note that the absorption frequency depends on the dielectric properties and thickness of the substrate and dimensions and the geometry of the MM. In the case of resonators with different dimensions, the incident EM waves will be trapped in several frequency bands,Micromachines thus forming2020 ,a11 multiband, 0930 absorber. 4 of 16 For a MM absorber, as the metallic film at its back plays the role of perfect reflector and, so, blocks the transmission of the waves; the transmission T(ω) is considered to be equal to zero. Therefore,at the the back absorption of the MM). performance Thereby, the A( energyω) can ofbe the directly incident calculated waves is while dissipated using insideEquations the substrate (1) and (2),(so, where inside S the11 corresponds cavity), due to to the a multi-reflection reflection coefficient; (also named the lower resonance the value phenomena) of S11, the and better the dielectricthe absorptionloss of performance this substrate. of MM Note is. that the absorption frequency depends on the dielectric properties and thickness of the substrate and dimensions and the geometry of the MM. In the case of resonators with 𝐴ω 1𝑅ω (1) different dimensions, the incident EM waves will be trapped in several frequency bands, thus forming a multiband absorber. 𝑅ω |𝑆| (2) For a MM absorber, as the metallic film at its back plays the role of perfect reflector and, so, blocks the transmission of the waves; the transmission T(ω) is considered to be equal to zero. Therefore, 2.2. Design of the Proposed MM the absorption performance A(ω) can be directly calculated while using Equations (1) and (2), where S11 Figurecorresponds 1 shows to thedetails reflection of the coe proposedfficient; theMM lower structure. the value This of MM S11, theconsists better of the three absorption layers (Figure performance 1a,b). ofThe MM middle is. layer is the FR‐4 dielectric substrate with a real part of the permittivity of 4.3, a loss tangent of 0.025 and thickness h of 3.2 mm.A The(ω) top= 1 andR (bottomω) layers are made of copper with (1) an electric conductivity of 5.8 × 107 S/m and thickness of− t = 17 μm. The bottom copper layer is a R(ω) = S 2 (2) continuous film that plays the role of a perfect reflector,| 11while| the top is formed of symmetrical copper resonators with the geometry shown in Figure 1c; it consists of 15 mm × 15 mm unit cell 2.2. Design of the Proposed MM with a periodicity p = 0.04 mm (Figure 1d). Table 1 summarizes the dimensions of the Vshape resonator. Figure1 shows details of the proposed MM structure. This MM consists of three layers (Figure1a,b). TheThe design middle layerand the is the absorption FR-4 dielectric performance substrate of with the a proposed real part of MM the permittivityarray are obtained of 4.3, a losswhile tangent usingof (CST) 0.025 Microwave and thickness Studioh of software. 3.2 mm. TheBecause top andthe bottomunit cell layers is periodic are made in the of copperxy‐plane, with Floquet an electric 7 port boundariesconductivity was of 5.8applied10 inS/ mthe and x and thickness y directions, of t = 17 whereasµm. The an bottom open copperboundary layer is isconsidered a continuous in film × the z thatdirection. plays theHere, role the of awave perfect propagation reflector, while vector the k top is isperpendicular formed of symmetrical to the absorber copper plane, resonators the with magneticthe geometry field is parallel shown to in the Figure x‐axis1c; itand consists the electric of 15 mm field is15 parallel mm unit to cell the with y‐axis a periodicity for the TE pmode,= 0.04 mm × as shown(Figure in Figure1d). Table 1. 1 summarizes the dimensions of the Vshape resonator.

(a) (b)

p p

(c) (d) Figure 1. (a) Perspective view, (b) side view, (c) front view, and (d) periodic and zoom views of the Figure 1. (a) Perspective view, (b) side view, (c) front view, and (d) periodic and zoom views of the Vshape Metamaterial (MM). Vshape Metamaterial (MM).

Micromachines 2020, 11, x 5 of 16

Micromachines 2020, 11, 0930 5 of 16

Table 1. Parameters of the proposed MM. Table 1. Parameters of the proposed MM. Parameters Value (in mm) Parameterst 0.017 Value (in mm) t h 3.2 0.017 h L2 12.8 3.2 L2 12.8 L1 15 L1 15 w1 0.6 w1 0.6 e e 0.5 0.5 w2 w2 0.4 0.4 p p 0.04 0.04

2.3. ElaborationThe design of and the theDielectric absorption Layer performance of the proposed MM array are obtained while using (CST) Microwave Studio software. Because the unit cell is periodic in the xy-plane, Floquet port The dielectric layer used in this article is based on epoxy foam that was loaded with 0.075 wt.% boundaries was applied in the x and y directions, whereas an open boundary is considered in the z of carbon fibers (CFs) of 12 mm length. The choice of this composite is based on our previous works direction. Here, the wave propagation vector k is perpendicular to the absorber plane, the magnetic showing the effect of carbon fiber length and rate [8,61] on the dielectric properties. Here, a low field is parallel to the x-axis and the electric field is parallel to the y-axis for the TE mode, as shown in permittivity and moderate dielectric losses are needed. Figure1. Figure 2 summarizes the different steps of the elaboration method of the composite. First, the 2.3.CFs Elaboration(from Apply of the Carbon Dielectric SA, Layer Languidic, France) are dispersed in the epoxy resin (PB 170 from Sicomin, Châteauneuf‐les‐Martigues, France) using ultrasonication method (Sonics Materials VCX‐ 750‐220,The dielectricSonics & layer Materials, used in Inc., this articleNewtown, is based CT, on USA), epoxy foamas detailed that was in loaded [61]. Subsequently, with 0.075 wt.% the of carbonhardener fibers (DM (CFs) 02 from of 12 Sicomin) mm length. is added The with choice the of proportions this composite Resin: is basedHardener on ourof 10:3.6. previous After works that, showingthe mixture the eisff ectput of into carbon a mold, fiber for length foaming and rate and [ 8polymerization,61] on the dielectric steps, properties. for six hours Here, at aroom low permittivitytemperature. and moderate dielectric losses are needed. FigureSubsequently,2 summarizes the mold the di isff placederent steps in the of theoven elaboration (at 60 °C methodfor six hours) of the composite. in order to First, finalize the CFsthe (frompolymerization Apply Carbon of the SA, epoxy Languidic, foam and, France) thus, areobtaining dispersed the final in the rigid epoxy composite. resin (PB Finally, 170 from the Sicomin, sample Chis cutâteauneuf-les-Martigues, to 15 cm × 15 cm × France)6 cm (L using × l × ultrasonication h), the dimensions method that (Sonics are needed Materials for VCX-750-220,the dielectric Sonicscharacterization & Materials, in the Inc., anechoic Newtown, chamber. CT, USA), The asmeasured detailed density in [61]. Subsequently,for the elaborated the hardenerdielectric (DMlayer 02 is fromabout Sicomin) 0.13 g/cm is3 added. Figure with S1 theshows proportions SEM images Resin: and Hardener optical ofmicrographs 10:3.6. After of that, the theelaborated mixture epoxy is put intofoam a loaded mold, for with foaming 0.075 wt.% and polymerizationof CFs. steps, for six hours at room temperature.

1) Adding carbon fibers 3) Adding the 5) Cutting to epoxy resin Hardener

4) Foaming and 2) Ultrasonication polymerization 6) Characterization

Figure 2.2. Dielectric layer elaboration steps.

2.4. CharacterizationSubsequently, theTechnique mold is placed in the oven (at 60 ◦C for six hours) in order to finalize the polymerization of the epoxy foam and, thus, obtaining the final rigid composite. Finally, the sample is cut The complex permittivity of the elaborated composite has been extracted from the to 15 cm 15 cm 6 cm (L l h), the dimensions that are needed for the dielectric characterization in the measurement× of× the reflection× × coefficient in the anechoic chamber, in the frequency range of 2–18 GHz. For this measurement, a quasi‐monostatic configuration is used (as shown in Figure 3). For Micromachines 2020, 11, 0930 6 of 16 Micromachines 2020, 11, x 6 of 16

anechoic chamber. The measured density for the elaborated dielectric layer is about 0.13 g/cm3. Figure S1 this,shows two SEM horn images antennas and optical (one transmitter micrographs and of the one elaborated receiver) epoxy were foam connected loaded to with a 0.075vector wt.% network of CFs. analyzer; a wave with a power of ‒8 dBm (0.15 mW) was used. 2.4. Characterization Technique For this technique, the measurement of four reflection coefficients is needed: on the material alone, Theon the complex material permittivity that is backed of the with elaborated a metallic composite plate, hason the been metallic extracted plate from alone, the measurement and on the environmentof the reflection of the coe ffianechoiccient in thechamber anechoic (without chamber, the in material the frequency and the range metallic of 2–18 plate). GHz. By For the this combinationmeasurement, of these a quasi-monostatic reflection parameters configuration and using is used the Fenner (as shown et al. in method Figure 3[62],). For the this, extraction two horn of theantennas complex (one permittivity transmitter was and done, one receiver) as detailed were in connected [63]. The to same a vector configuration network analyzer; was used a wave for withthe measurementa power of 8of dBm the reflection (0.15 mW) coefficient was used. of both MM and hybrid absorbers. − Incidence Absorber angle  prototype



Metallic plate (Reflective plane)

Antennas

Figure 3. Configuration used in the anechoic chamber for the reflection coefficient measurement. Figure 3. Configuration used in the anechoic chamber for the reflection coefficient measurement. For this technique, the measurement of four reflection coefficients is needed: on the material 3. Results and Discussion alone, on the material that is backed with a metallic plate, on the metallic plate alone, and on the environment of the anechoic chamber (without the material and the metallic plate). By the combination 3.1. Properties of the Dielectric Layer of these reflection parameters and using the Fenner et al. method [62], the extraction of the complex permittivityThe composite was done, that aswas detailed based on in [epoxy63]. The foam same loaded configuration with 0.075 was wt.% used of for12mm the‐ measurementlength CF was of preparedthe reflection and measured coefficient in of the both anechoic MM and chamber. hybrid absorbers.Figure 4 shows the extracted dielectric properties. The real part of the permittivity 𝜀’ and the dielectric losses tan𝛿 both decrease with respect to frequency.3. Results Indeed, and Discussion these properties decrease from the values of 𝜀’ = 2.20 associated with tan𝛿 = 0.64 (at 2 GHz) to the values of 𝜀’ = 1.20 associated with tan𝛿 = 0.15 (at 18 GHz). Figure S2 shows 3.1. Properties of the Dielectric Layer dielectric properties of composites loaded with various weight percentages of CFs. These figures show, Theas expected, composite an thatincrease was basedof the properties on epoxy foam as a function loaded with of the 0.075 CFs wt.%rate. of 12mm-length CF was prepared and measured in the anechoic chamber. Figure4 shows the extracted dielectric properties. The real part of the permittivity3 ε’ and the dielectric losses tanδ both decrease with respect to frequency. Indeed, these properties decrease from the values of ε’ = 2.20 associatedεʹ with tanδ = 0.64 (at 2 GHz) to the values of ε’ = 1.20 associated with tanδ = 0.15 (at 18 GHz). Figure S2 shows dielectric properties tan δ of composites loaded2 with various weight percentages of CFs. These figures show, as expected, an increase of the properties as a function of the CFs rate.

1

0 2 4 6 8 1012141618 Frequency (GHz)

Figure 4. Dielectric properties of the carbon fibers (CFs) loaded epoxy foam layer (12 mm‐0.075 wt.%). Micromachines 2020, 11, x 6 of 16

this, two horn antennas (one transmitter and one receiver) were connected to a vector network analyzer; a wave with a power of ‒8 dBm (0.15 mW) was used. For this technique, the measurement of four reflection coefficients is needed: on the material alone, on the material that is backed with a metallic plate, on the metallic plate alone, and on the environment of the anechoic chamber (without the material and the metallic plate). By the combination of these reflection parameters and using the Fenner et al. method [62], the extraction of the complex permittivity was done, as detailed in [63]. The same configuration was used for the measurement of the reflection coefficient of both MM and hybrid absorbers.

Incidence Absorber angle  prototype



Metallic plate (Reflective plane)

Antennas

Figure 3. Configuration used in the anechoic chamber for the reflection coefficient measurement.

3. Results and Discussion

3.1. Properties of the Dielectric Layer The composite that was based on epoxy foam loaded with 0.075 wt.% of 12mm‐length CF was prepared and measured in the anechoic chamber. Figure 4 shows the extracted dielectric properties. The real part of the permittivity 𝜀’ and the dielectric losses tan𝛿 both decrease with respect to frequency. Indeed, these properties decrease from the values of 𝜀’ = 2.20 associated with tan𝛿 = 0.64 (at 2 GHz) to the values of 𝜀’ = 1.20 associated with tan𝛿 = 0.15 (at 18 GHz). Figure S2 shows dielectric properties of composites loaded with various weight percentages of CFs. These figures Micromachines 2020, 11, 0930 7 of 16 show, as expected, an increase of the properties as a function of the CFs rate.

Micromachines 2020, 11, x 3 7 of 16 εʹ tan δ 3.2. EM Wave Absorption Performance:2 Simulations The proposed MM structure has been studied under normal and oblique incidences for TE and TM polarizations; Figure S3 illustrates the detailed setup of these configurations. The simulated absorption performance 1of the proposed multiband MM at normal incidence (θ = 0°) for both TE and TM polarizations is illustrated in Figure 5a. The proposed MM exhibits several distinct absorption peaks, the highest among them (with absorption > 90%) occurs at 2.2 GHz, 5.63 GHz, 6.94 GHz, 8.99 GHz, 12.80 GHz, and 15.01 GHz, corresponding to an absorption of 91.29%, 99.9%, 96.09%, 94.96%, 94.2%, and 299.2%, 4 respectively. 6 8 Furthermore, 1012141618 the absorption peaks and intensity of the proposed MM are unchanged for bothFrequency TE and (GHz)TM polarizations, as expected, due to its symmetrical shape. Figure 4. Dielectric properties of the carbon fibers (CFs) loaded epoxy foam layer (12 mm-0.075 wt.%). FigureFigure 5b 4. illustrates Dielectric propertiesthe absorption of the performancecarbon fibers (CFs)of the loaded proposed epoxy multiband foam layer MM (12 mmat oblique‐0.075 incidence of θ = 30° for both TE and TM polarizations. It exhibits several distinct absorption peaks 3.2. EM Wavewt.%). Absorption Performance: Simulations that differ between TE and TM. For the TE mode, the highest peaks (with absorption > 90%) occur at 5.7The GHz, proposed 6.9 GHz, MM 10.5 structureGHz, 12.7 hasGHz, been and studied 14.92 GHz, under corresponding normal and to oblique an absorption incidences of 99.7%, for TE and TM93.31%, polarizations; 96.74%, 97.39%, Figure and S3 94.74%, illustrates respectively. the detailed For TM setup mode, of the these highest configurations. peaks (with absorption The simulated > 90%) occur at 5.6 GHz, 6.9 GHz, 8.7 GHz, 10.6 GHz, 12.8 GHz, and 15.27 GHz, corresponding to absorption performance of the proposed multiband MM at normal incidence (θ = 0◦) for both TE and an absorption of 99.4%, 97.29%, 98.4%, 98.81%, 99.64%, and 99.96%, respectively. TM polarizations is illustrated in Figure5a. The proposed MM exhibits several distinct absorption It can be noticed that the absorption rate is almost similar between the TE and TM modes when peaks, the highest among them (with absorption > 90%) occurs at 2.2 GHz, 5.63 GHz, 6.94 GHz, dealing with normal incidence. However, when it comes to oblique incidence a clear difference, in 8.99the GHz, absorption 12.8 GHz, rate, andbetween 15.01 the GHz, TE and corresponding TM polarizations, to an is absorption observed (see of 91.29%,Figure 5). 99.9%, In brief, 96.09%, the TE 94.96%, 94.2%,and TM and results 99.2%, of respectively. any symmetric Furthermore, structure will the be absorption the same for peaks normal and incidence intensity and of different the proposed for MM areoblique unchanged incidences. for both TE and TM polarizations, as expected, due to its symmetrical shape.

1 θ = 0° 0.8

0.6

0.4 Absorption 0.2 TE TM 0 24681012141618 Frequency (GHz) (a) 1 θ = 30° 0.8

0.6

0.4 Absorption 0.2 TE TM 0 24681012141618

Frequency (GHz) (b)

FigureFigure 5. 5.Simulated Simulated absorption absorption performance performance ofof the the MM MM for for TE TE and and TM TM modes modes at (ata )( normala) normal incidence

θ =incidence0◦ and ( θ b)= oblique0° and (b incidence) oblique incidence of θ = 30 of◦. θ = 30°.

Figure5b illustrates the absorption performance of the proposed multiband MM at oblique incidence of θ = 30◦ for both TE and TM polarizations. It exhibits several distinct absorption peaks that Micromachines 2020, 11, 0930 8 of 16

differ between TE and TM. For the TE mode, the highest peaks (with absorption > 90%) occur at 5.7 GHz, 6.9 GHz, 10.5 GHz, 12.7 GHz, and 14.92 GHz, corresponding to an absorption of 99.7%, 93.31%, 96.74%,

Micromachines97.39%, and 2020 94.74%,, 11, x respectively. For TM mode, the highest peaks (with absorption > 90%) occur8 of 16 at 5.6 GHz, 6.9 GHz, 8.7 GHz, 10.6 GHz, 12.8 GHz, and 15.27 GHz, corresponding to an absorption of 99.4%, 97.29%, 98.4%, 98.81%, 99.64%, and 99.96%, respectively. FigureIt can S4 be presents noticed that a comparison the absorption of the rate absorption is almost similar resonances between between the TE normal and TM and modes oblique when incidences.dealing with Almost normal all incidence.of the obtained However, peaks when for both it comes angles to of oblique incidence incidence are the a clearsame, di butfference, with two in the newabsorption resonances rate, that between appeared the for TE the and oblique TM polarizations, incidence of is30° observed at 13.8 GHz (see and Figure 15.45 ).GHz In brief,with 85.6% the TE andand 88.8% TM resultsabsorption of any rates, symmetric respectively. structure will be the same for normal incidence and different for obliqueThe dielectric incidences. composite based on CF loaded epoxy foam is used to broaden the absorption bandwidthFigure of S4the presents proposed a comparisonMM. The advantage of the absorption of this dielectric resonances layer between lies in both normal the and used oblique low loadincidences. of the CFs Almost and the all used of the thin obtained thickness peaks of forthis both composite angles to of form incidence the hybrid are the absorber. same, but Figure with two 6 showsnew resonancesthe proposed that structure appeared of forthe the ultra oblique‐wideband incidence absorber. of 30 ◦Itat is 13.8 composed GHz and of 15.4the proposed GHz with MM 85.6% ofand 3.2 mm 88.8% thickness absorption and rates, of the respectively. epoxy foam composite (loaded with 12 mm CF of 0.075 wt.%), with 13 mmThe thickness, dielectric mounted composite on the based top of on the CF MM. loaded The epoxytotal thickness foam is usedof the to proposed broaden planar the absorption hybrid absorberbandwidth is of of 16.2 the mm. proposed MM. The advantage of this dielectric layer lies in both the used low load of theIt CFs should and thebe noted used thin here thickness that, for ofthis this hybrid composite material, to form EM theabsorption hybrid absorber. will be provided Figure6 showsnot only the byproposed the MM structurestructure, ofbut the also ultra-wideband by the composite absorber. layer. It Indeed, is composed the latter of the ensures, proposed on the MM one of hand, 3.2 mm anthickness impedance and matching of the epoxy at air/absorber foam composite interface (loaded (thanks with to its 12 mmlow permittivity) CF of 0.075 wt.%), that allows with 13for mm a smooththickness, penetration mounted of on the the EM top waves of the in MM. the The absorber total thickness and, on ofthe the other proposed hand, planaran absorption hybrid absorber of the EMis waves of 16.2 thanks mm. to its moderate dielectric losses.

Figure 6. The proposed hybrid absorber. Figure 6. The proposed hybrid absorber. It should be noted here that, for this hybrid material, EM absorption will be provided not only Before presenting the simulation of the absorption performance of the chosen hybrid structure by the MM structure, but also by the composite layer. Indeed, the latter ensures, on the one hand, (with a composite loaded with 0.075 wt.% of CFs), Figure S5 shows the simulation of the reflection an impedance matching at air/absorber interface (thanks to its low permittivity) that allows for a coefficient of the hybrid absorber that was carried out using the dielectric properties of different smooth penetration of the EM waves in the absorber and, on the other hand, an absorption of the EM composites with different CFs loads (dielectric properties shown in Figure S2). These simulations waves thanks to its moderate dielectric losses. are compared to the one of the MM absorber alone. Figure S5 shows that a compromise must be Before presenting the simulation of the absorption performance of the chosen hybrid structure made in order to obtain the best absorption performance and the largest bandwidth. Indeed, Figure (with a composite loaded with 0.075 wt.% of CFs), Figure S5 shows the simulation of the reflection S5a shows that, when lower CF rates are used (0.025 wt.% or 0.05 wt.%), the reflection of the MM is coefficient of the hybrid absorber that was carried out using the dielectric properties of different improved, but the losses are too low to broaden the bandwidth, and an uninterrupted large composites with different CFs loads (dielectric properties shown in Figure S2). These simulations bandwidth ( < ‒10 dB) could not be obtained. Contrary to that, when higher CFs loads are used are compared to the one of the MM absorber alone. Figure S5 shows that a compromise must (0.1 wt.% or 0.2 wt.%), the complex permittivity is higher. This high permittivity no longer ensures be made in order to obtain the best absorption performance and the largest bandwidth. Indeed, the impedance matching and, so, induces a higher reflection (Figure S5b) at the air/hybrid absorber Figure S5a shows that, when lower CF rates are used (0.025 wt.% or 0.05 wt.%), the reflection of the interface (especially in the frequency range between 3 GHz and 8 GHz), which affect the absorption MM is improved, but the losses are too low to broaden the bandwidth, and an uninterrupted large performance. These simulations confirm that, for the hybrid absorber, a low 𝜀’ that is near to that of bandwidth (Γ < 10 dB) could not be obtained. Contrary to that, when higher CFs loads are used air (ensuring an impedance− matching at the air/material interface and so, the lowest reflection) and (0.1 wt.% or 0.2 wt.%), the complex permittivity is higher. This high permittivity no longer ensures moderate dielectric losses (that ameliorate the absorption performance) are needed. Figure 7 shows the simulated absorption performance, at normal incidence for both TE and TM polarizations, of the chosen hybrid absorber (using the 0.075 wt.% CFs loaded composite). An important absorption rate higher than 90% is obtained in an ultra‐wide frequency range from 2.6 GHz to 18 GHz, covering 70% of the S‐band and the entire C‐band, X‐band, and Ku‐band. Figure S6 shows the simulated absorption performance at oblique incidence of 30° for both TE and TM polarizations of the proposed hybrid absorber. For both TE and TM polarizations, a Micromachines 2020, 11, 0930 9 of 16 the impedance matching and, so, induces a higher reflection (Figure S5b) at the air/hybrid absorber interface (especially in the frequency range between 3 GHz and 8 GHz), which affect the absorption

Micromachinesperformance. 2020 These, 11, x simulations confirm that, for the hybrid absorber, a low ε’ that is near to that9 of 16 of air (ensuring an impedance matching at the air/material interface and so, the lowest reflection) and moderate dielectric losses (that ameliorate the absorption performance) are needed. significantFigure absorption7 shows the rate simulated higher absorptionthan 90% is performance, obtained in atan normal ultra‐wide incidence frequency for both range TE from and TM 2.6 GHzpolarizations, to 18 GHz, of the covering chosen 70% hybrid of the absorber S‐band (using and thethe 0.075 entire wt.% C‐band, CFs loaded X‐band, composite). and Ku‐band. An important A better Micromachinesabsorption 2020 rateperformance, higher11, x than for 90% the isTM obtained polarization, in an ultra-wide as compared frequency to the range TE polarization, from 2.6 GHz is to expected 189 GHz,of 16 (Figurecovering S6). 70% of the S-band and the entire C-band, X-band, and Ku-band. significant absorption rate higher1 than 90% is obtained in an ultra‐wide frequency range from 2.6 GHz to 18 GHz, covering 70% of the S‐band and the entire C‐band, X‐band, and Ku‐band. A better absorption performance for0.8 the TM polarization, as compared to the TE polarization, is expected (Figure S6). 0.6 1 0.4 Absorption 0.8 θ = 0° 0.2 MM Vshape 0.6 Hybrid Absorber 0 0.4 2 4 6 8 10 12 14 16 18 Frequency (GHz) Absorption θ = 0° Figure 7. Simulation resultsresults0.2 of of the the absorption absorption performance performance ofMM the of the hybrid Vshape hybrid absorber absorber compared compared to that to of that the ofMM the for MM the for normal the normal incidence incidenceθ = 0◦ (the θ = blue0° (the dashed blue linedashed correspondsHybrid line corresponds Absorber to the absorption to the absorption value of 90%). value 0 of 90%). 2 4 6 8 10 12 14 16 18 Figure S6 shows the simulated absorption performance at oblique incidence of 30◦ for both TE Frequency (GHz) 3.3.and EM TM Wave polarizations Absorption of thePerformance: proposed Measurements hybrid absorber. For both TE and TM polarizations, a significant absorptionFigure 7. rate Simulation higher than results 90% of isthe obtained absorption in anperformance ultra-wide of frequency the hybrid range absorber from compared 2.6 GHz to to that 18 GHz, coveringofThe the proposed 70%MM for of the the MM normal S-band absorber incidence and is the fabricated θ entire= 0° (the C-band, bluewith dashed 10 X-band, × 10 line unit corresponds and cells Ku-band. with to a the total absorption A betterdimension absorption value of 150 mmperformance of× 15090%). mm for × the3.2 mm TM polarization,while using laser as compared printing to technique the TE polarization, with LPKF islaser expected machine (Figure of the S6). IETR laboratory. Figure 8a shows the top view of the fabricated structure. The elaborated dielectric layer 3.3.that3.3. EM EMis made Wave Wave ofAbsorption Absorption epoxy loaded Performance: Performance: foam is Measurements Measurementscut to the desired thickness, 13 mm. This latter is glued on the top layer of the elaborated MM to compose the proposed hybrid absorber (Figure 8b). TheThe proposed proposed MM MM absorber absorber is is fabricated fabricated with with 10 10× 10 unit10 unit cells cells with with a total a total dimension dimension of 150 of × mm150 mm× 150 mm150 mm× 3.2 mm3.2 mm while while using using laser laser printing printing technique technique with with LPKF LPKF laser laser machine machine of of the the IETR IETR × × laboratory.laboratory. Figure Figure 8a8a shows the top view of thethe fabricatedfabricated structure.structure. The elaborated dielectric layer thatthat is is made made of of epoxy epoxy loaded loaded foam foam is is cut cut to to the the desired desired thickness, thickness, 13 13 mm. mm. This This latter latter is is glued glued on on the the toptop layer layer of of the the elaborated elaborated MM MM to to compose compose the the proposed proposed hybrid hybrid absorber absorber (Figure (Figure 88b).b). mm

150 mm

15

15 mm mm

150

150 mm mm

(a) 15 (b)

Figure 8. (a) Elaborated 15Vshape mm MM and (b) the final hybrid absorber.

From the measured150 mm reflection coefficient, in Figure 9 we present the deduced absorption performance for TE and TM polarizations(a) at normal incidence (Figure 9a) and (atb) oblique incidence of 30° (Figure 9b,c). These measurements are compared to the simulation results. It can be noted here that good agreementFigureFigure 8 8.. (a( a) between)Elaborated Elaborated measurements Vshape Vshape MM MM and and and (b (b) ) the thesimulations final final hybrid hybrid isabsorber. absorber. obtained and the same resonances are observed, even if the absorption level is slightly different. For example, for the From the measured reflection coefficient, in Figure 9 we present the deduced absorption normal incidence (Figure 9a), a lower intensity is obtained at 2.3 GHz, with an expected absorption performance for TE and TM polarizations at normal incidence (Figure 9a) and at oblique incidence of 90% by simulation and an obtained absorption of 63% by measurement. For the other resonances, of 30° (Figure 9b,c). These measurements are compared to the simulation results. It can be noted the same or even better absorption is obtained; for example, at 4.2 GHz, the expected absorption by here that good agreement between measurements and simulations is obtained and the same simulation is 70.9% while an absorption of 77% is achieved by measurement. The weak difference resonances are observed, even if the absorption level is slightly different. For example, for the observed between the simulation and measurement results is probably due to a geometric deviation normal incidence (Figure 9a), a lower intensity is obtained at 2.3 GHz, with an expected absorption between the perfect simulated structure of the MM and the produced one. of 90% by simulation and an obtained absorption of 63% by measurement. For the other resonances, the same or even better absorption is obtained; for example, at 4.2 GHz, the expected absorption by simulation is 70.9% while an absorption of 77% is achieved by measurement. The weak difference observed between the simulation and measurement results is probably due to a geometric deviation between the perfect simulated structure of the MM and the produced one. Micromachines 2020, 11, 0930 10 of 16

FromMicromachines the 2020 measured, 11, x reflection coefficient, in Figure9 we present the deduced absorption10 of 16 performance for TE and TM polarizations at normal incidence (Figure9a) and at oblique incidence of 30◦ (FigureLikewise,9b,c). These the agreement measurements between are simulation compared and to measurement the simulation can results.be noted It for can both be TE noted and here that goodTM modes agreement at an oblique between incidence measurements of 30°. For and TE simulations (Figure 9b), isthe obtained same resonances and the sameare observed resonances are observed,with a lower even intensity if the absorption only at 2.3 GHz, level iswith slightly an expected different. absorption For example, of 93% forby simulation the normal and incidence an (Figureobtained9a), a lower absorption intensity of 76% is obtained by measurement. at 2.3 GHz, However, with an the expected same or absorptioneven better absorption of 90% by simulationfor the and another obtained resonances absorption is obtained. of 63% For TM by measurement.(Figure 9c), the same For the resonances other resonances, are also observed the same with or a even betterlower absorption intensity is obtained;at 2.3 GHz, for with example, an expected at 4.2 GHz,absorption the expected of 93% by absorption simulation by and simulation an obtained is 70.9% absorption of 76.9 % by measurement; the same or even better absorption performance for the other while an absorption of 77% is achieved by measurement. The weak difference observed between the resonances is confirmed. However, the most obtained resonances have absorption rate higher than simulation90% as andpredicted measurement with simulation. results This is probablyMM is considered due to aas geometric a good multiband deviation absorber, between despite the the perfect simulatedslight structuredifference ofbetween the MM simulation and the and produced measurement one. results.

1

0.8

0.6

0.4 Absorption 0.2 MM‐ Sim TE / θ = 0° MM‐ Meas 0 24681012141618 Frequency (GHz) (a) 1

0.8

0.6

0.4 Absorption 0.2 MM‐ Sim TE / θ = 30° MM‐ Meas 0 2 4 6 8 10 12 14 16 18 Frequency (GHz) (b) 1

0.8

0.6

0.4 Absorption 0.2 MM ‐Sim TM / θ = 30° MM ‐Meas 0 24681012141618 Frequency (GHz) (c)

FigureFigure 9. Comparison 9. Comparison between between measured measured and and simulated simulated absorption absorption performance performance ofof thethe proposedproposed MM MM at (a) normal incidence θ = 0° and (b) for TE and (c) TM modes at oblique incidence of θ = 30°. at (a) normal incidence θ = 0◦ and (b) for TE and (c) TM modes at oblique incidence of θ = 30◦.

Likewise,The measured the agreement absorption between performance simulation of the andhybrid measurement absorber at normal can be incidence, noted for as both well TEas, and at an oblique incidence of 30° for TE and TM polarizations are presented in Figure 10a‒c, TM modes at an oblique incidence of 30◦. For TE (Figure9b), the same resonances are observed withrespectively. a lower intensity These onlymeasurements at 2.3 GHz, are withalso compared an expected to the absorption simulation of results. 93% byNote simulation that, a good and an obtained absorption of 76% by measurement. However, the same or even better absorption for the Micromachines 2020, 11, 0930 11 of 16 other resonances is obtained. For TM (Figure9c), the same resonances are also observed with a lower intensityMicromachines at 2.3 GHz, 2020, 11 with, x an expected absorption of 93% by simulation and an obtained absorption11 of 16 of 76.9 % by measurement; the same or even better absorption performance for the other resonances is confirmed.agreement However, between the measurements most obtained and resonances simulations have is obtained absorption and rate it confirms higher thana high 90% absorption as predicted with simulation.performance Thisgreater MM than is considered 90% for frequencies as a good between multiband 2.6 absorber,GHz and despite18 GHz, the excluding slight di fftheerence betweenfrequency simulation range and between measurement 4.10 GHz results.and 4.45 GHz. The decrease of the absorption level in this Thenarrowband measured is due absorption to the relatively performance high coupling, of the hybrid in this absorberfrequency atband, normal and incidence,between the as horn well as, antennas used for the measurements in the anechoic chamber. at an oblique incidence of 30◦ for TE and TM polarizations are presented in Figure 10a-c, respectively. Otherwise, a better performance is obtained through measurements for the entire studied These measurements are also compared to the simulation results. Note that, a good agreement between frequency band. For example, at 13 GHz, an absorption of 99.5% is obtained by measurement, while measurementsthe predicted and absorption simulations by issimulation obtained is and 90% it at confirms normal incidence. a high absorption At oblique performance incidence of greater 30°, an than 90% forabsorption frequencies rate of between 99.9% is 2.6 obtained GHz and by measurement 18 GHz, excluding while the the predicted frequency absorption range between by simulation 4.10 GHz and 4.45is 92.2% GHz. for The TE decrease polarization. of the The absorption uncertainty level on the in thisdielectric narrowband properties, is dueas extracted to the relatively from the high coupling,measurement in this frequency in anechoic band, chamber and and between used for the the horn absorption antennas simulation, used for may the be measurements at the origin of in the anechoicthis slight chamber. observed difference between simulation and measurement results.

1

0.8

0.6

0.4

Absorption Hybrid Absorber ‐ Sim 0.2 Hybrid Absorber ‐ Meas θ = 0° 0 2 4 6 8 10 12 14 16 18 Frequency (GHz) (a) 1

0.8

0.6

0.4 Hybrid Absorber ‐ Sim Absorption 0.2 Hybrid Absorber ‐ Meas TE / θ = 30° 0 24681012141618 Frequency (GHz) (b) 1

0.8

0.6

0.4 Hybrid Absorber ‐ Sim Absorption Hybrid Absorber‐Meas 0.2 TM / θ = 30° 0 24681012141618 Frequency (GHz) (c)

FigureFigure 10. Comparison10. Comparison between between simulated simulated andand measured absorption absorption performance, performance, of the of thehybrid hybrid absorber at (a) normal incidence θ = 0° and (b) TE and (c) TM modes for oblique incidence of θ = 30°. absorber at (a) normal incidence θ = 0◦ and (b) TE and (c) TM modes for oblique incidence of θ = 30◦. Micromachines 2020, 11, 0930 12 of 16

Otherwise, a better performance is obtained through measurements for the entire studied frequency band. For example, at 13 GHz, an absorption of 99.5% is obtained by measurement, while the predicted absorption by simulation is 90% at normal incidence. At oblique incidence of 30◦, an absorption rate of 99.9% is obtained by measurement while the predicted absorption by simulation is 92.2% for TE polarization. The uncertainty on the dielectric properties, as extracted from the measurement in anechoic chamber and used for the absorption simulation, may be at the origin of this slight observed difference between simulation and measurement results. Simulations of the absorption performance for other incidence angles were conducted, and these simulations are shown in Figure S7; the measurement for these angles of incidence was not carried out because of the space restrictions in the anechoic chamber used. Figure S7 presents the simulation results for TE (Figure S7a) and TM (Figure S7b) polarizations for θ varying between 0◦ and 60◦; Figure S7c and Figure S7d show the same simulations for only incidence angles of 0◦, 15◦, 30◦, 45◦, and 60◦, for TE and TM polarizations, respectively. These simulations predict an absorption performance that is higher than 90% between 2.6 GHz and 18 GHz, for the different incidence angles. However, for the TE mode and highest angles (θ 45 ), the absorption performance ≥ ◦ is slightly affected, in low (<4.5 GHz) and high (>16 GHz) frequencies (Figure S7c). Finally, the absorption performance of the proposed hybrid absorber is compared with the other reported structures of microwave MM absorbers. Table2 presents the MM characteristics in terms of the frequency range with an absorption higher than 90%, absorption bandwidth, covered bandwidth, and the method of broadening bandwidth. It can be observed that the proposed structure has an absorption higher than 90%, between 2.6 GHz and 18 GHz, forming an ultra-wideband hybrid absorber that covers 70 % of the S-band (2.6–4 GHz) and the entire C-band (4–8 GHz), X-band (8–12 GHz), and Ku-band (12–18 GHz); whereas, other achieved MMs only cover one or two bands. As a conclusion, the hypothesis of broadening the absorption bandwidth of the proposed multiband MM by associating a thin, light, and slightly CFs loaded dielectric layer is confirmed.

Table2. Comparison between the proposed structure of this work and other presented MMs of literature.

Reference Absorption > 90% Absorption Bandwidth Covered BW Method of Broadening the BW [52] 3.01–5.28 GHz 2.27 GHz 50% of the S-band incorporating lumped elements [38] 5.6–9.1 GHz 3.5 GHz 87.5% of the C-band Thick substrate [40] 4–8 GHz 4 GHz C-band Thick substrate Combining multiple resonating [16] 13.7–15.5 GHz 1.8 GHz 30% of the Ku-band structures Combining multiple resonating [42] 10.5–12.5 GHz 2 GHz 50% of the X-band structures Combining multiple resonating [43] 13.5–16.5 GHz (>80%) 3 GHz 50% of the Ku-band structures X-band and 33.3% of the [44] 8–14 GHz 6 GHz stacking multiple layers in 3 D Ku-band 50% of the C-band and 33.3% [45] 4–6 GHz and 12–14 GHz 2 GHz for each BW stacking multiple layers in 3 D of the Ku-band X-band [47] 8.37–21 GHz 12.63 GHz stacking multiple layers in 2 D And Ku-band [50] 7.8–12.6 GHz 4.8 GHz X-band incorporating lumped elements 70 % of the S-band This Associating a thin dielectric layer 2.6–18 GHz 15.4 GHz Entire of C-band, X-band and work to the MM Ku-band

4. Conclusions In this work, a new geometry of multiband MM absorber is proposed. A thin, light, and slightly charged dielectric layer, made of 0.075 wt.% CFs loaded epoxy foam, is associated to the front of this proposed MM absorber, in order to broaden the absorption performance. The choice of this composite is based on the necessary compromise between a low permittivity (to ensure the impedance matching at the air/absorber interface) and a moderate dielectric loss in order to improve the absorption performance. The dielectric layer has a thickness of 13 mm and the proposed MM has a thickness of 3.2 mm. The hybrid absorber presents a total thickness of 16.2 mm with an absorption performance Micromachines 2020, 11, 0930 13 of 16

that is greater than 90% from 2.6 GHz to 18 GHz, for TE and TM polarizations of normal (θ = 0◦) and oblique (θ = 30◦) incidences. As a result, an ultra-wideband, light, and compact absorber that covers 70 % of the S-band (2.6–4 GHz) and the entire C-band (4–8 GHz), X-band (8–12 GHz), and Ku-band (12–18 GHz) is achieved. A good agreement between measurement and simulation is obtained for both normal (θ = 0◦) and oblique incidence (θ = 30◦) for TE and TM modes, which validates this proposed hybrid absorber.

Supplementary Materials: The following are available online at http://www.mdpi.com/2072-666X/11/10/0930/s1, Figure S1: SEM images of the epoxy foam loaded with 0.075 wt.% of CFs, Figure S2: Real part of the permittivity and dielectric losses of epoxy foams loaded with different CFs weight percentages, Figure S3: Setup under TE and TM polarizations, Figure S4: Simulated absorption performance of the MM, Figure S5: Simulation of the reflection coefficient of the MM alone and the hybrid absorber based on composites loaded with CFs loads 0.075 wt% and CFs loads 0.075 wt%., Figure S6: Simulated absorption performance of the hybrid absorber compared≤ to that ≥ of the MM at oblique incidence of θ =30◦ for the TE and for the TM polarizations, Figure S7: Simulation of the absorption performance of the hybrid absorber with different incidence angles. Author Contributions: Modeling, simulation and experimental investigations, A.E.A. and H.B. writing—original draft preparation, A.E.A.; R.B. and A.S.; writing—review and editing, A.E.A.; R.B.; supervision and funding acquisition, R.B.; A.S.; A.J.; A.H. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: This work was supported by the European Union through the European Regional Development Fund (ERDF) and by the Ministry of Higher Education and Research, Brittany Région, Côtes d’Armor Département and Saint Brieuc Armor Agglomération through the CPER Projects MATECOM and SOPHIE-STICC. This work was also supported by the Syndicat de Gestion du Pole Universitaire de Saint Brieuc (France). The authors want to thank J. Sol for its technical support for the prototypes measurement. Conflicts of Interest: The authors declare no conflict of interest.

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