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1
An Ultra-wideband Absorber Based On Mixed Absorption Mechanisms
Meiling Li, Wenhao Hu, Xue-xia Yang, and Zixuan Yi
1. What is the problem being addressed by the manuscript and why is it important to the Antennas
& Propagation community? (limited to 100 words).
A new ultra-wideband absorber based on mixed absorption mechanisms is proposed. The design provides
a new strategy and more flexibility to design ultra-wideband absorbers. In addition, the combination does not
introduce extra thickness, which ensures the absorber to be low-profile.
2. What is the novelty of your work over the existing work? (limited to 100 words).
Our work provides a new strategy to design ultra-wideband absorbers through combining resonance-based
absorber and spoof surface plasmon polariton absorber without introducing extra thickness. These two
absorbers can be designed separately, which provides more flexibility. The performance has been proved by
simulation and measurement, which shows satisfactory bandwidth and thickness compared with previous
work.
3. Provide up to three references, published or under review, (journal papers, conference papers,
technical reports, etc.) done by the authors/coauthors that are closest to the present work. Upload
them as supporting documents if they are under review or not available in the public domain. Enter
“N.A.” if it is not applicable.
N.A.
4. Provide at least three references (journal papers, conference papers, technical reports, etc.) done
by other authors that are most important to the present work. These references should also be
discussed in the submitted manuscript and listed among its references. Please include the citation
numbers used in the manuscript for easy reference.
[1] J. Yu, W. Jiang and S. Gong, "Wideband Angular Stable Absorber Based on Spoof Surface Plasmon
Polariton for RCS Reduction," IEEE Antennas Wireless Propag. Lett., vol. 19, no. 7, pp. 1058-1062, July
2020.
[2] F. Zhou, Y. Fu, R. Tan, J. Zhou, and P. Chen, "Broadband and wide-angle metamaterial absorber based
on the hybrid of spoof surface plasmonic polariton structure and resistive metasurface," Opt. Express, vol.
29, no. 21, pp. 34735-34747, October, 2021.
[3] T. Shi, L. Jin, L. Han, M. -C. Tang, H. -X. Xu and C. -W. Qiu, "Dispersion-Engineered, Broadband,
Wide-Angle, Polarization-Independent Microwave Metamaterial Absorber," IEEE Trans. Antennas Propag.,
vol. 69, no. 1, pp. 229-238, Jan. 2021.
IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION
3
An Ultra-wideband Absorber Based On Mixed Absorption Mechanisms
Meiling Li, Wenhao Hu, Xue-xia Yang, and Zixuan Yi
Abstract—In this paper, an ultra-wideband (UWB) absorber
(MA) is proposed by the employment of two absorption
mechanisms, including resonance-base absorber and spoof
surface plasmon polariton (SSPP) absorber. The mixed absorber
is a three-dimensional structure which consists of vertically placed
SSPP and resonance-based absorber and horizontally placed
resistive frequency selective surface (FSS). The ultra-wideband
absorption performance mainly originates from the absorption
characteristic of the resonance-based structure in low frequency
range and the SSPP structure in high frequency range. Simulation
shows that the hybrid structure possesses a -10 dB fractional
bandwidth (FBW) of 160.7% from 1.45 GHz to 13.33 GHz with the
thickness of 20.6 mm (0.1 𝝀𝑳 ) under both TE and TM polarization.
A prototype is fabricated and measured. The experimental results
are in good agreement with the simulation results. The present
work provides a new strategy to design ultra-wideband absorber
in a way that does not introduce extra thickness.
Index Terms—Ultra-wideband, microwave absorber,
surface plasmon, dual-polarized.
I.
spoof
INTRODUCTION
BSOBERS can effectively absorb the incident electromagnetic (EM) waves. They play an important role in
microwave applications such as stealth technology and
electromagnetic compatibility (EMC). Nowadays, the
development of detection technology and wireless
communication, absorbers are strongly required throughout the
frequency spectrum and the expansion of absorption bandwidth
has become an essential topic in EM absorbers. An early
attempt to broaden the absorption bandwidth is Jaumann
absorber [1], which uses several layers of resistive sheets and
dielectric layers. However, this design leads to unsatisfactory
thickness and weight. Since broadband, low-profile and
lightweight absorbers are strongly desired, researchers have
proposed a few methods of different mechanisms to meet the
requirement in the past few years.
Most broadband absorbers are realized by introducing
electric or magnetic resonance by electric or magnetic
resonance by periodic structures. Circuit analog absorbers
(CAAs) are typical electric resonance-based structures.
Through electric resonance of FSS structure and loaded lumped
resistors, wide operating-band and less thickness can be
obtained [2]-[6]. Recently, a three-dimensional electric
A
Manuscript submitted February 10, 2023. This work is supported in part by
the National Natural Science Foundation of China under Grant 51907185 and
52207214, and in part by Natural Science Foundation of Shanghai under Grant
21ZR1423100, Grant 21ZR1423700 and Shanghai Sailing Program Grant No.
21YF1412500. (Corresponding author: Zixuan Yi)
M. Li, W. Hu, X. Yang, Z. Yi are with Shanghai Institute of Advanced
Communication and Data Science, Key laboratory of Specialty Fiber Optics
and Optical Access Networks, Shanghai University, Shanghai, China
(e-mail: meilingli@shu.edu.cn; 2206526055@qq.com; yang.xx@shu.edu.cn;
yizixuan@shu.edu.cn).
resonance-based absorber is proposed to achieve ultrawideband absorption [7]. However, it suffers from the
complexity of structure. Meanwhile, magnetic resonance can
also be introduced to broaden the bandwidth [8]-[10] or achieve
good incident-angle stability [11]-[12].
Spoof surface plasmon polariton (SSPP) structures can also
play the role as absorbers whose mechanism is different with
that of previous resonance-based absorbers [13]-[17]. Via
dispersion engineering of SSPP, broadband and high-efficient
absorption can be easily obtained. Although SSPP has these
advantages, it still meets the difficulty of balance between the
bandwidth and the thickness. In addition, few SSPP absorbers
that works at low frequency ranges has been reported up to now.
Recently, several hybrid structures with different absorption
mechanisms have been proposed [18]-[20]. A wideband
absorber which is a combination of SSPP structure and resistive
FSS is proposed [18]. However, due to the positional
relationship stacked up and down, the thickness of the
combined structure is still relatively large. Another hybrid
structure which takes good advantages of physical space is
proposed, but it’s FBW is relatively narrow [19]. Thus, the
design of ultra-wideband absorbers with different absorption
mechanisms possesses potential research value.
In this paper, an ultra-wideband absorber is proposed, which
is based on two different absorption mechanisms: resonancebased absorption and SSPP absorption. It provides a new
strategy to design ultra-wideband absorbers. In section II,
firstly, a wide-band absorber works at lower frequency range is
designed by SRR structures which introduce magnetic and
electric resonances, another resistive FSS layer is also added on
the top to achieve better absorption. Then a broadband SSPP
absorber that works at higher frequency range is designed
through dispersion engineering. Finally, these two absorbers are
combined in a way which does not introduce extra thickness.
After optimization of SSPP structure, the simulation results
show that the absorption band of these two absorbers is
connected successfully and ultra-wideband absorption is
realized. In section III, the dual-polarized structure is designed.
The simulation results indicate that the proposed structure can
achieve ultra-wide absorption under both TE and TM
polarization. In section IV, a prototype is fabricated and
measured to validate our design concept. Finally, our
conclusions are drawn in section V.
II.
A.
DESIGN AND THEORETICAL ANALYSIS
Design and Analysis of resonance-based absorber
The unit cell of the proposed resonance-based absorber is
depicted in Fig.1 (a), which consists of two combined split ring
resonators (SRRs) with lumped resistors loaded on the gap. The
thickness of the dielectric substrate (𝜀𝑟 = 4.4, 𝑡𝑎𝑛𝛿=0.02) is 2
mm. Here, SRRs are employed to get electric and magnetic
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|S11| (dB)
0
-5
-10
-15
9 11
7
5
Frequency (GHz)
13
15
(b)
Fig. 1. (a)Geometry of the structure. The parameter values are: 𝑡 = 20, ℎ =
20 , 𝑎 = 17.6, 𝑏 = 18.6, 𝑤1 = 3, 𝑤2 = 2.8, 𝑤3 = 1.6, 𝑑𝑟 = 3.2, 𝑑𝑙 = 1.4.
(unit: mm) R1=150 Ω, R2=150 Ω (b)Simulated reflection coefficient |𝑆11 |.
(a)
(b)
0
0
-10
|G| (dB)
(a)
3
S parameters (dB)
1
|S11|
|S12|,|S21|
|S22|
-20
1
3
5
7
9
11
Frequency (GHz)
13
-10
Simulated by HFSS
Caculated by eq.(1)
-20
15
1
(c)
(a)
(c)
(b)
(d)
Fig. 2. Surface current distribution and power loss density at (a) 1.66 GHz (b)
3.67 GHz (c) 5 GHz (d) 8.54 GHz.
resonances. The electric field parallel to the metal strip would
excite induced current, which generates inductive inductances
and the induced capacitances between the gap, leading to
electric resonances. Meanwhile, magnetic resonances can also
be induced while the magnetic field is perpendicular to the
SRRs [12]. Further, the lumped resistors can not only achieve
good impendence matching over wider band but also enhance
absorption through thermal losses. At the same time, the
interaction between the ground and structure can also introduce
magnetic resonances [9], which also plays a non-negligible role
in achieving broadband absorption. The simulated reflection
coefficient |S11| is shown in Fig. 1(b) and the absorptivity is
absorptivity is determined as 𝐴 = 1 − |𝑆11 |2 . The structure can
achieve over 80% absorptivity from 1.42 to 8.99 GHz. Four
resonance frequencies in the absorption band at 1.66 GHz, 3.67
GHz, 5 GHz and 8.54 GHz can be discovered. From the surface
current and the power loss density of the resistor in Fig. 2,
electric and magnetic resonances can be observed at different
resonating frequencies. At 1.66 GHz, the surface current on the
lowest strip and the ground plane are antiparallel, which can be
3
5
7
9
11
Frequency (GHz)
13
15
(d)
Fig. 3. (a) Unit cell of the combined structure, the parameter values are (unit:
mm): 𝑐 = 10.6 . (b) A schematic showing the flow diagram of EM waves in
the combined structure. (c) Simulated S parameters of FSS layer. (d)
Comparison of the simulated and calculated reflection coefficient |Γ| of the
dual-layer absorber.
regarded as a magnetic resonance. The current forms a loop on
the upper SRR at 3.67 GHz, which also provides another
magnetic resonance. Meanwhile, the current parallel with the E
field of the incident wave provides an electric resonance at 5
GHz and 8 GHz. Therefore, these magnetic resonances and
electric resonance and the lumped resistors are the key to realize
wideband absorption. To enhance the absorption, an extra FSS
layer is added on top of the former absorber. The combined
structure can be regarded as a two-layer system, and its
reflection coefficient Γ can be calculated by the following
formula [19]:
𝛤 = 𝑆11 +
𝑆21 𝑆12 Γ𝑙
1−𝑆22 Γ𝑙
(1)
𝑆11 , 𝑆12 , 𝑆21 , 𝑆22 are the S parameters of the FSS layer, and Γ𝑙
is the reflection coefficient of the lower layer. It can be inferred
from (1) that small 𝑆11 and 𝑆22 as well as large 𝑆12 and 𝑆21 are
required to enhance absorption [20]. Meanwhile, taking the
energy conservation relationship into account, the upper layer
must be lossy to get better absorption. Thus, we introduce an
resistive FSS layer, which consists of resistive square patches
with sheet resistance 𝑅𝑠 = 335 Ω/sq printed on the 0.6 mm
FR4 substrate. To minimize the interaction between these two
layers, the resistive film is located at the edges of the unit cell.
The 3-D view of the combined Simulated S parameters of the
FSS layer are presented in Fig. 3(c). It is obvious that the
condition of small 𝑆11 and 𝑆22 as well as large 𝑆12 and 𝑆21 is
satisfied in the absorption band of the lower layer. It can be
predicted that the combined absorber can achieve better
absorption than that of the structure without resistive FSS layer.
To verify our analysis further, the absorptivity of the two-layer
structure calculated by (1) and full-wave simulation result are
compared in Fig. 3(d). It can be observed that they are in good
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0
|S11| (dB)
-10
-20
-30
Without resistors
With R=50W
With R=100W
-40
1
(a)）
(a)
Frequency (GHz)
9
Light line
w=6mm, without resistor
w=6mm, with R=50W
w=8.5mm, without resistor
w=8.5mm, with R=50W
w=11mm, without resistor
w=11mm, with R=50W
6
3
0.2
0.4
0.6
bp/p
0.8
1.0
(b)）
Fig. 4. (a) Geometry of the SSPP structure with different w and R. The
parameter values are (unit: mm): 𝑠ℎ = 0.5, 𝑠𝑝 = 1 (b) Simulated dispersion
relationship.
agreement which indicates that the introduction of the resistive
FSS layer is a feasible way to get better absorption.
B.
5
7
9
11
Frequency (GHz)
13
15
(b)
Fig. 5. (a) Schematic of the SSPP absorber with linearly varied length. the
parameter values are (unit: mm): 𝑠ℎ = 0.5, 𝑠𝑝 = 1, ℎ𝑠 = 3. (b) Simulated
reflection coefficient of the SSPP with and without resistors.
12
0
0.0
3
Design and Analysis of SSPP absorber
To achieve ultra-wideband absorption, an SSPP absorber that
works at high frequency range should be designed. The
absorption mechanism of SSPP structure originates from its
slow-wave characteristic. To explain in detail, as frequency
approaches the asymptotic frequency of SSPP, the group
velocity of the surface wave tends to zero and the energy can be
effectively absorbed by the lossy substrate.
Typical SSPP absorbers structures consist of corrugated
metal strips [16], parallel metal wire arrays or meandered-wireshaped metal strips [14]. The dielectric loss and ohmic loss
would lead to the absorption, hence, high density and large
amounts of metal wires are required to achieve broadband and
high-efficient absorption. Recent researches indicate that
loading lumped resistors is an effective way to reduce density
and numbers of the metal strips and get better absorption [21][22]. In consideration of the combination of SSPP structure and
resonance-based absorber, the interaction between them should
be as weak as possible. Thus, metal strips array with loaded
resistors is a better choice to be employed as broadband
absorber at higher frequency ranges. The unit element of the
SSPP structure is depicted in Fig. 4(a). Periodic boundaries are
set along z direction and the dispersion relationships with
respect to different w are calculated by simulation software. The
comparison between the dispersion relationship of SSPP with
and without resistors are shown in Fig. 4(b). It can be
discovered that the introduction of resistors takes little effect on
the dispersion relationships of SSPP. Therefore, the
introduction of lumped resistors According to the mentioned
above, an absorption band near the asymptotic frequency can
be formed and the application of resistors can strengthen the
absorption. To achieve broadband absorption, the dimension of
SSPP structure should be designed with gradient length. As the
resonance-based shows poor absorption performance in the
frequency ranges of 9 GHz -11 GHz and 13 GHz - 15 GHz, the
SSPP absorber should take effect at these frequency ranges. In
addition, taking the influence of the FSS layer into
consideration, the beginning and ending frequency should be
set at lower frequencies, which is chosen as 8 GHz and 13 GHz.
Since the absorption frequency range is settled, the dispersion
relation in Fig. 4(b) can help us to determine the initial and
terminal length w. Therefore, the length of the metal strips is
chosen to be linearly varying from 5 mm to 10 mm. The
configuration of the SSPP absorber is presented in Fig. 5(a) and
the absorption performance comparison between the structure
with and without resistors are shown in Fig. 5(b). It can be
observed from Fig. 5(b) that the absorption band starts at 7.5
GHz and ends at about 12 GHz, which is consistent with the
analysis through dispersion relationship. By optimizing those
resistors, the final SSPP absorber with the resistors of 100 Ω
can achieve a broadband absorption over -10 dB from 7.2 GHz
-13 GHz.
Design and Analysis of the hybrid structure
Since the resonance-based absorber works at lower
frequency range while the SSPP absorber operates at higher
frequency range, a possible way to achieve ultra-wideband
absorption is by placing the two absorbers on either side of the
dielectric substrate. The schematic of the hybrid absorber is
depicted in Fig. 6(a). Due to the unignorable interaction
between the two absorbers, the original hybrid absorber formed
by directly combining these two absorbers without changing the
parameters has poor absorption performance in the interim
frequency band, as shown the results in Fig. 6(b). Therefore, the
length and location of SSPP structure should also be adjusted
and the reflection coefficient of the optimized hybrid structure
is presented in Fig. 6(b) which indicates that the absorber can
achieve -10dB reflectivity from 1.44 GHz to 13.5 GHz.
From the surface power loss density of lumped resistors
shown in Fig. 7, the effect of the different part of the hybrid
absorber on working frequency band can be found. In lower
frequency range (1.43 GHz-6 GHz), the energy is absorbed by
the resonance-based structure. The SSPP structure takes charge
of the absorption in higher frequency range (10 GHz -13.5
GHz). Moreover, in the middle frequency range (6 GHz - 10
GHz), both resonance-based and SSPP structures take effect in
absorption. This phenomenon demonstrates that the
combination of absorbers with different absorption
mechanisms is the key to realize ultra-wideband absorption.
C.
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|S11| (dB)
0
-10
Orginal hybrid absorber
Optimized hybrid absorber
-20
1
(a)
3
5
7
9
11
Frequency (GHz)
13
15
(b)
Fig. 6. (a)Schematic of the hybrid absorber. The parameter values are (unit:
mm): 𝑤1 = 4.3 , 𝑤9 = 7.9 (linearly increase), 𝑤10 = 8 , ℎ𝑠 = 6 . (b)
Comparison of the simulated reflection coefficient of the original hybrid
absorber and the optimized one.
0
-10
-10
|S11| dB
|S11| (dB)
（a）
0
-20
TE polarization,q=0°
TE polarization,q=30°
-30
1
Fig. 7. Surface power loss density at different frequencies.
3
5
7
9
11
-20
TM polarization,q=0°
TM polarization,q=30°
-30
13
15
1
3
Frequency (GHz)
(b)
15
(c)
0
0
-10
-10
|S11| (dB)
It is obvious that the above hybrid absorber is polarization
sensitive, leading to the limitations in applications. Thus, a
dual-polarized absorber is proposed based on the previous
absorber. The 3-D view of the unit cell of dual-polarized
absorber is shown in Fig. 8(a) and the parameters of the present
structure are slightly adjusted to ensure the absorption
performance. The simulated reflection coefficient of the
absorber is given in Fig. 8(b) and (c). Under normal incident
wave, the absorber can achieve -10 dB reflectivity from 1.45
GHz to 13.33 GHz with an FBW of 160.7%. However, as the
incident angle increases, the absorption deteriorates in some
frequency ranges, especially in the frequency range of 8.5 GHz9.6 GHz under TE polarization, the worst reflection coefficient
is about -6.7 dB. To improve the stability under oblique
incidence, the number of strip lines of SSPP structure is
increased to 15, and their length is also adjusted, which linearly
changes from 4.3 mm to 8.8 mm. As is shown in Fig.9, the
optimized structure shows better incident angle stability, which
keeps -10 dB reflection coefficient or lower in most frequency
range and the worst reflection coefficient under 30°incident
wave is about -8 dB.
13
Fig. 8. (a) Unit cell structure of the dual-polarized absorber. The parameter
values are (unit: mm): 𝑤1 = 4, 𝑤10 = 9 (linearly increase), ℎ𝑠 = 8, b = 17.6, c
= 8. Simulated reflection coefficient of the optimized absorber under (b) TE
polarization incident waves. (c) TM polarization incident waves.
|S11| (dB)
III. DUAL-POLARIZED ULTRA-WIDEBAND ABSORBER
5
7
9
11
Frequency (GHz)
-20
TE polarization,q=0°
TE polarization,q=30°
-30
1
3
5
7
9
11
Frequency (GHz)
-20
TM polarization,q=0°
TM polarization,q=30°
-30
13
15
1
3
(a)
5
7
9
11
Frequency (GHz)
13
15
(b)
Fig. 9. Simulated reflection coefficient of the optimized absorber under (a) TE
polarization incident waves. (b) TM polarization incident waves.
and the amended reflection coefficient can be obtained by
subtracting the former two reflectivity. The measured results
are plotted and compared in Fig. 10(b), which basically consists
with the simulation. The agreement between the simulated
results and measured results validates our design. Finally, Table
I compares the performance of our presented absorber and that
of recently reported absorbers.
IV. EXPERIMENTAL VERIFICATION AND DISCUSSION
V. CONCLUSION
To verify the performance of the proposed absorber, a
prototype has been fabricated. The sample includes 10*10 unit
cells with a dimension of 20 mm*20 mm*20.6 mm and the
absorption performance is measured by free space method. The
prototype and measurement environment are depicted in Fig.
10(a). Before measuring the reflectivity of the absorber, the
reflectivity of a metal plate placing at the same place is firstly
recorded. Then the performance of the absorber is also tested
In conclusion, we propose a 3-D absorber via combining
resonance-based absorber and SSPP structure without
introducing extra thickness. The combination of different
absorption mechanisms is the key to achieve the ultra-wideband
absorption, which provides a novel method to design ultra-wide
-band absorbers. The absorber achieves absorption over 90%
from 1.45 GHz to 13.33 GHz under both TE and TM
polarization waves with the thickness of 20.6 mm (0.1 𝝀𝑳 ) and
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TABLE I
COMPARISONS WITH OTHER ABSORBERS
Ref.
-10dB Bandwidth (GHz)
/ FBW (%)
Polarization
Thickness (𝝀𝑳 )
Absorption Mechanisms
[6]
1.03-13.27/171.2%
Single
0.101
Resonance-based
[7]
1.50-12.31/156.6%
Dual
0.113
Resonance-based
[14]
9-35/118.2%
Dual
0.15
SSPP
[19]
3.9-10.6/92.4%
Dual
0.091
Resonance-based + SSPP
This
work
1.45-13.33/160.7%
Dual
0.1
Resonance-based + SSPP
𝜆𝐿 is the wavelength at the lowest frequency in free space of the absorption band
[5]
[6]
[7]
(a）
0
[8]
|S11| (dB)
-10
[9]
-20
[10]
-30
Simulated |S11|
Measured |S11|
[11]
-40
1
3
5
7
9
11
Frequency (GHz)
13
15
(b)
[12]
Fig. 10. (a) Photograph of the fabricated absorber and the measurement
environment. (b) Comparison of simulated and measured results.
the performance is verified experimentally. The structure also
possesses the advantages of lightweight and good mechanical
stability, which ensures it to possess promising application in
both military and civil areas such as radar cross section
reducing and EMC.
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