Experimental validation

1. IEEE MAGNETICS LETTERS, Volume 10 (2019) 2101905
Electromagnetics
Experimental Validation of a Frequency-Selective Surface-Loaded Hybrid
Metamaterial Absorber With Wide Bandwidth
Atipriya Sharma1
, Ravi Panwar2∗
, and Rajesh Khanna1
1 Department of Electronics and Communication Engineering, Thapar Institute of Engineering and Technology, Patiala 147001, India
2 Discipline of Electronics and Communication Engineering, Indian Institute of Information Technology, Design and Manufacturing,
Jabalpur 482005, India
∗ Member, IEEE
Received 18 Nov 2018, revised 18 Jan 2019, accepted 29 Jan 2019, published 11 Feb 2019, current version 7 Mar 2019.
Abstract—The achievement of wide absorption bandwidth for a single-layer metamaterial absorber remains a challenge.
In this letter, a frequency-selective surface (FSS), single-substrate layer, broadband metamaterial absorber is investi-
gated theoretically, experimentally, and by simulation in the frequency range of 2–18 GHz. Simulations of the reflection
coefficient of the absorber with different substrate dielectric thicknesses, FSS thicknesses, and FSS dimensions indicate
that there exist optimal values for the absorber design. The measured results from a fabricated prototype are in close
agreement with the simulations, suggesting the effectiveness of the structure for actual electromagnetic applications. The
fabricated absorber with thickness 2.0 mm has a minimum reflection coefficient of −29.0 dB at 12.2 GHz. The −10 dB
absorption bandwidth is 7.5 GHz in the range of 8.5–16 GHz. Effective complex electromagnetic parameters are extracted
to quantitatively understand the absorption. A miniaturized structure, single-substrate layer, simple geometry, and wide
bandwidth are some of the key features of the proposed metamaterial absorber.
Index Terms—Electromagnetics, frequency-selective surface, metamaterial, microwave absorber, microwave absorbing material.
I. INTRODUCTION
Microwave absorbers have been widely exploited by radio fre-
quency and microwave scientists for distinct practical applications,
such as stealth and radomes, filters, electromagnetic (EM) interference
shields, cloaks, lenses, etc., in microwave, millimeter, and terahertz
regimes [Chakraborty 2013, Marwaha 2015, Panwar 2015, 2018].
Various microwave absorbing materials and structures are available
in the open literature, on the basis of targeted operating frequency
[Sakran 2008, Li 2012]. Metamaterials-inspired microwave absorbers
(MMAs) are one of the alternating solutions to reduce the thickness
and weight of the structure. The concept of a perfect MMA has been
first introduced by Landy [2008]. Metamaterials (MMs) are artifi-
cial or synthetic materials, engineered to achieve the customized EM
properties, which are not found in the nature [Veselago 1968]. The
condition of impedance matching can be obtained by just manipulat-
ing the resonances of the equivalent material parameters, i.e., complex
dielectric permittivity (εr ) and complex magnetic permeability (μr )
independently. Such an MM technology plays a significant role at
higher frequencies as compared to the conventional materials.
Frequency-selective surfaces (FSSs)-based advanced EM structures
have been introduced to improve the absorption bandwidth of mi-
crowave absorbing structures (MASs) [Costa 2016, Panwar 2017].
Depending on the shape and size of the FSS, it can block or pass EM
waves in a narrowband or wideband spectrum [Azemi 2012, Costa
2016]. Researchers have proposed various structures for MMAs oper-
ating in a single band, dual band, and multiband applications [Landy
2008, Ye 2012, Campbell 2013, Numan 2013, Xin 2017, Zeng 2017].
Corresponding author: Ravi Panwar (e-mail: rpanwar.iitr@gmail.com).
Digital Object Identifier 10.1109/LMAG.2019.2898612
An ultrawideband ultrathin absorber based on MM circular split rings
was proposed with a wide band absorption from 7.85 to 12.25 GHz
[Bhattacharyya 2015]. An MM-inspired broadband absorber was pre-
sented exhibiting high absorption from 6.86 to 15.16 GHz with a total
absorption bandwidth of 8.30 GHz [Costa 2010]. An ultrathin MMA
using a resistively loaded high-impedance surface was propounded,
which shows both narrowband and wideband behavior by properly
selecting FSS shape and substrate material [Bakir 2016].
The development of MMA with wide bandwidth has been always
one of the challenging aspects in front of the scientific community.
Other important issues are related to the electrical thickness, fabrica-
tion, and performance evaluation of such kind of structures. Motivated
by the aforementioned discussions, in this letter, hybrid FSS-based
broadband MMAs have been designed, optimized, fabricated, and
measured in the range of 2–18 GHz. The main contribution of this
letter lies in the achievement of broad absorption bandwidth with the
help of hybrid FSS-based EM structures.
II. DESIGN AND OPTIMIZATION
The unit cell geometry of the proposed MMA is comprised of
a periodic FSS layer stacked with a dielectric substrate backed by a
ground plane. The standard FR4 material has been selected as a dielec-
tric substrate (relative permittivity εr = 4.3 and dielectric loss tangent
tan δ = 0.025) with a substrate layer thickness of 2.0 mm. The metallic
layer and ground plane are made of copper (conductivity σ = 5.8 ×
107
S/m) with a layer thickness of 0.035 mm. Fig. 1 shows the unit cell
geometry of the proposed absorber, which comprises a cross-loaded,
split-ring hybrid structure. Here, the metallic FSS layer (made of cop-
per) is represented by a yellow color, and the rest of the portion shows
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2. 2101905 IEEE MAGNETICS LETTERS, Volume 10 (2019)
Fig. 1. Front view of the of the proposed MAS unit cell with geometrical
dimensions: P = 8.0, S = 0.25, R = 3.14, W = 0.9, G = 0.8, W1 = 1.6,
W2 = 1.6, L1 = 1.7, and L2 = 1.5.
Fig. 2. Frequency-dependent RC characteristics of (a) proposed
MAS. (b) Effect of substrate thickness over RC characteristics of an
optimal sample in the range of 2 to 18 GHz.
the dielectric substrate. These structures have been designed using the
Computer Simulation Technology (CST), Darmstadt, Germany, Mi-
crowave Studio followed by a critical parametric analysis. The floquet
port has been employed to excite the incident wave propagating in the
z-direction, using the periodic boundary conditions along the x- and
y-directions.
III. CRITICAL ANALYSIS AND NUMERICAL
DEMONSTRATION
Fig. 2(a) illustrates the simulated RC-frequency spectra of split ring,
cross, and cross-loaded split-ring FSS-based absorbers in the range of
2–18 GHz. It is noticed that the absorption bandwidth corresponding
to –10 dB RC for a split-ring FSS is 3.7 GHz (8.5–9.8 GHz and 11.4–
13.8 GHz). A split-ring FSS with a radius of 3.14 mm and two split
ends contributes in three RC peaks of –20.7, –15.5, and –27.5 dB,
at 8.8, 9.3, and 12.7 GHz, respectively. A remarkable enhancement
is observed in –10 dB absorption bandwidth with the application of
hybrid FSS geometry, i.e., cross-loaded split-ring FSS, as shown in
Fig. 2(a). The results show that the RC is less than –10 dB from 8.5
to 16 GHz for the hybrid structure, which covers 7.5 GHz bandwidth.
The peak RC values of the hybrid geometry are –20.2, –29.0, and
–21.0 dB at 8.9, 12.2, and 15.5 GHz, respectively.
The hybrid geometry has been chosen for further studies, due to
its exceptional microwave-absorbing performance as compared to in-
dividual split-ring and cross FSS geometries. It is well known that
the microwave absorption characteristics are strongly influenced by
the substrate thickness [Panwar 2015]. Therefore, an effort has been
made to study the effect of substrate layer thickness over RC charac-
teristics of the hybrid structure. The corresponding results with varied
substrate layer thickness are presented in Fig. 2(b). The substrate
thickness has been varied from 1.4 to 2.4 mm while keeping the other
parameters (i.e., FSS dimensions, etc.) constant. The degree of res-
onance matching is greatly affected by the corresponding variation
in the substrate thickness. The similar effect has been observed by
Fan [2018]. As the substrate thickness increases, more attenuation
of the wave takes place. Significant microwave absorption properties
have been obtained for the substrate thickness of 2.0 mm. There-
fore, 2.0 mm is considered as an optimal substrate layer thickness of
the proposed absorber. To further investigate the effect of FSS thick-
ness over RC characteristics, conducting geometry thickness has been
varied from 0.015 to 0.055 mm for an optimal substrate layer thick-
ness, i.e., 2.0 mm. Fig. 3(a) shows the FSS-thickness-dependent RC
characteristics of the MMA structure in the range of 2–18 GHz. The
simulation results reveal the FSS-thickness-independent RC charac-
teristics of the proposed MMA. In the similar manner, the outer radius
(r2) of the circle is varied from 3.0 to 3.7 mm, while making the other
parameters constant. When r2 increases, the gap between the outer
circle and inner geometry, i.e., cross sign, increases, which affects the
mutual coupling [Bhattacharyya 2015]. The better microwave absorp-
tion properties in terms of wide absorption bandwidth of 7.5 GHz and
minimum RC of –29.0 dB are noticed with an optimal value of r2,
i.e., 3.14 mm, as clearly shown in Fig. 3(b). The results obtained from
CST have been validated using a high-frequency structure simulator
(HFSS), before the actual fabrication of the absorber. A perfect match
among both HFSS and CST data is observed, as is clear from Fig.
3(c). The optimal structure is found to possess a maximum absorp-
tion of –29.0 dB at 12.2 GHz. The structure is studied under distinct
polarization angles in the range of 0ο
–30ο
at the normal incidence
in order to examine its polarization sensitivity. Fig. 4(a) shows that
absorption decreases gradually with a corresponding increase in the
polarization angle, which may be due to nonsymmetrical geometry
of the structure. Furthermore, an effort has been made to determine
the angular stability of the proposed structure. Fig. 4(b) presents the
simulated RC spectra of proposed absorber with different angle of in-
cidence under transverse electric polarization. It is observed that with
an increase of incident angle, the resonance frequency shifts up under
TE polarization. Overall, the simulation results indicate that for any
chosen dielectric material and FSS, there is always an optimal sub-
strate thickness and optimal FSS design variables, where the reflection
becomes zero, i.e., maximum absorption.

3. IEEE MAGNETICS LETTERS, Volume 10 (2019) 2101905
Fig. 3. Effect of design variables. (a) FSS thickness. (b) Ring radius.
(c) Comparison of CST and HFSS data.
Fig. 4. Simulated RC of proposed absorber with (a) different polariza-
tion angles under normal incidence and (b) different angle of incidence.
IV. COMPARISON WITH EXPERIMENTAL DATA
AND DISCUSSIONS
An optimal sample with an overall size of 200 mm × 200 mm has
been fabricated using standard printed circuit board technology, in or-
der to verify the simulated results. The performance evaluation of the
fabricated structure has been carried out in an anechoic chamber using
Fig. 5. Performance evaluation based on experimental validation
and verification. (a) Free-space microwave measurement setup.
(b) Fabricated prototype of MMA. (c) Measured and simulated spectra.
(d) Measured spectra at distinct incidence angles.
a single-port, free-space microwave measurement setup, comprising
a vector network analyzer (Keysight E5063A), and a broadband horn
antenna, as depicted in Fig. 5(a). More details of the single-port, free-
space measurement setup are provided in Panwar [2018]. Fig. 5(b)
presents the fabricated prototype, and a corresponding enlarged view
of the unit cell is depicted in the inset. There are 25 unit cells in each
row. The results obtained from CST are validated using both HFSS
and experimental measurements. Fig. 5(c) shows both the simulated
and measured RC-frequency spectra of the proposed structure under
normal incidence in the range of 2–18 GHz. Fig. 5(d) presents the
measured RC-frequency spectra of the proposed absorber with dif-
ferent incidence angle. A close agreement has been observed among
simulated and measured data. It has been noticed that the proposed
absorber shows minimum RC from 8 to 16 GHz both in simulated
and measured plots. Moreover, the –10 dB absorption bandwidth is
7.5 GHz in the range of 8.5–16 GHz. There is a slight difference in sim-
ulated and measured results, which might be due to finite prototype
dimensions, and fabrication tolerances. Going one step further, the
frequency-dependent complex dielectric permittivity, complex mag-
netic permeability, and normalized impedance of the proposed MMA
are extracted using the mathematical formulations reported in Bhat-
tacharyya [2014].
Fig. 6(a)–(c) shows the extracted real and imaginary parts of the
complex dielectric permittivity, complex magnetic permeability, and
normalized impedance of the proposed structure, respectively. The
negative values clearly depict the MM nature of the proposed structure
and are responsible for maximum absorption of the wave. Fig. 7(a)–
(f) illustrates the electric and magnetic field distributions at three
resonant peaks (viz., 8.9, 12.2, and 15.5 GHz) in order to understand
the absorption mechanism. After analyzing the result, it is noticed
that hybrid geometry is the primary contributor to the more localized

4. 2101905 IEEE MAGNETICS LETTERS, Volume 10 (2019)
Fig. 6. Frequency-dependent EM properties. (a) Complex dielec-
tric permittivity. (b) Complex magnetic permeability. (c) Normalized
impedance for an optimal sample.
Fig. 7. Electric field distributions at (a) 8.9 GHz, (b) 12.2 GHz, and
(c) 15.5 GHz. Magnetic field distributions at (d) 8.9 GHz, (e) 12.2 GHz,
and (f) 15.5 GHz.
field. The contribution of the split ring is more at lower frequency. On
the other hand, at higher frequency, fields are more influenced by the
cross geometry. The top surface current distributions of the proposed
structure at the resonant frequencies, i.e., 8.9, 12.2, and 15.5 GHz, are
illustrated in Fig. 8. It is shown that at the lower resonant frequency,
i.e., 8.9 GHz, the surface current distribution is more in the outer
ring, whereas a small amount of current flows in the cross. A quite
good matching of surface current is noticed for both the split ring
as well as cross at 12.2 GHz. The major contribution of the surface
current at 15.5 GHz is due to cross; however, the outer ring is also
found to be contributing to the same. At the resonant frequencies, high
absorption has been observed, because with the top metallic patch
the electrical resonance is coupled and the surface current makes the
circulating loops around the incident magnetic field, therefore creating
Fig. 8. Surface current distribution of the proposed structure at
(a) 8.9 GHz, (b) 12.2 GHz, and (c) 15.5 GHz.
Table 1. Comparison of proposed structure with earlier reported
structures.
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strong magnetic resonance. The absorption mechanism in MMAs can
also be explained on the basis of interference theory [Chen 2015].
Table 1 presents a systematic comparison of microwave absorption
properties of the proposed absorber with other reported works. The
comparison has been made on the basis of absorber geometry, unit
cell dimensions, absorber layer thickness, and relative bandwidth. The
absorption properties of the proposed structure are in good agreement
with the other reported works.
ACKNOWLEDGMENT
This work was supported by the Science and Engineering Research Board, Department
of Science and Technology, Government of India under Early Career Research under Grant
ECR/2017/000676.
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