Submitted to IEEE TAP Miniaturized Wideband Metasurface...
Transcript of Submitted to IEEE TAP Miniaturized Wideband Metasurface...
Submitted to IEEE TAP
1
Abstract—A single-layer tightly-coupled metasurface with
narrow gaps in between and a dual-layer metasurface with
feasible wide gaps are proposed to realize the miniaturization of a
low-profile wideband antenna, respectively. The single-layer
metasurface consists of one square patch array while the
dual-layer metasurface is composed of two square patch arrays..
Both the single-layer and dual-layer metasurfaces supported by
grounded dielectric substrate are considered as waveguided
metamaterials to retrieve the effective refractive index along the
propagation direction. The effective propagation constant is
subsequently derived to initially estimate the resonant frequencies
of the dual-mode antenna. Both the single-layer metasurface with
narrow gap and the dual-layer metasurface exhibit increased
effective propagation constant and therefore achieve the antenna
miniaturization. Both types of antennas are able to produce the
realized gain greater than 6.5 dBi over the wide impedance
bandwidth of 27% with a reduced radiating aperture size of
0.460 0.460 and a thickness of 0.060 (0 is the free-space
wavelength at the center operating frequency of 5.5 GHz).
Index Terms—Wideband antenna, low-profile antenna,
miniaturization, waveguided metamaterial, metasurface, effective
medium theory.
I. INTRODUCTION
ONVENTIONAL broadband microstrip patch antenna
techniques usually require electrically-thick and
low-permittivity dielectric substrate (typically 0.10-thick
air-substrate for about 30% fractional bandwidth), such as the
utilization of capacitive probe feed, L-probe feed, aperture
coupling, U/E-slotted patch, and stacked patches [1]–[6].
Meanwhile, the investigation and demonstration of exotic
electromagnetic properties of metamaterials artificially
constructed of periodic sub-wavelength cells have profoundly
influenced physical and engineering research since the year of
2000 [7]–[10], in particular, have opened a new window for
Manuscript received May xx, 2017. This work was supported by the
Economic Development Board (EDB), Singapore, under the office for space
technology and industry (OSTIn) space industry alignment grant ("SIAG") S14-1139-IAF OSTIn-SIAG.
W. E. I. Liu is with Department of Electrical & Computer Engineering,
National University of Singapore, Singapore 117583 (e-mail: [email protected]).
Z. N. Chen is with Department of Electrical & Computer Engineering,
National University of Singapore, Singapore 117583 (e-mail: [email protected]).
X. Qing is with the Institute for Infocomm Research (I2R), Agency for
Science, Technology and Research (A*STAR), Singapore 138632 (e-mail: [email protected]).
J. Shi is with School of Electronics and Information, Nantong University, 9
Seyuan Road, Nantong 226019, China (e-mail: [email protected]). F. H. Lin is with Department of Electrical & Computer Engineering,
National University of Singapore, Singapore 117583 (e-mail:
designing innovative antennas with improved performance. For
instance, the composite right/left-handed (CRLH)
metamaterials have been widely applied in antenna designs
because of their unique dispersion characteristics [10]. With
deep analysis and understanding of the rich dispersion
characteristics and the operating modes of the metamaterial
structures, we have successfully proposed and developed a new
class of metamaterial-based low-profile broadband antennas
with consistent directional boresight radiation by exciting two
adjacent resonant modes over the desired frequency ranges
[11]–[13]. The aforementioned limitations of the existing
broadband microstrip patch antenna techniques have been
alleviated using developed metamaterial-based antenna
techniques, wherein the wide operating bandwidth with high
efficiency has been achieved with a thin substrate of relatively
high dielectric constant, typically 0.060-thick substrate with a
dielectric constant around 3.55 for achieving 30% fractional
bandwidth. The wideband low-profile metasurface antenna
proposed in [13] has been further developed by other groups to
realize low-profile broadband antennas with filtering function
[14] and circular polarization [15].
Besides the achieved performance, it is noted that the
radiating aperture of the previous metasurface antenna is as
large as 0.70 0.70 with a substrate having a dielectric
constant around 3.55 [13], [15]. The antenna aperture size will
be further enlarged if a substrate with a lower dielectric
constant is used. The higher-permittivity substrate can be used
to reduce the antenna aperture size at a price of reduced
bandwidth and radiation efficiency. However, a small
inter-element spacing is usually required in array configuration
for low sidelobe levels (SLLs) and suppressed grating lobes.
The inter-element spacing is also a key consideration for wide
scanning phased array antennas. A smaller inter-element
spacing results in a smaller inter-element phase difference for
scanning the beam to a wide angle, such that low SLLs and
good axial ratios could be maintained and the occurrence of the
grating lobes would be avoided within the wide-angle beam
steering. In this case, antenna miniaturization is even more
critical to achieve low mutual coupling within a small
inter-element spacing [16], [17]. Meanwhile, miniaturization of
wideband antenna is increasingly demanded in compact
wideband multiple-input multiple-output (MIMO) antenna
systems for modern communications [18]–[20]. As a result, the
wideband antenna element with further miniaturized size (e.g.
0.50 0.50) is highly desired in high-performance wideband
fixed/phased array antennas and compact wideband multiple
antenna systems. More recently, a metasurface inspired printed
Miniaturized Wideband Metasurface Antennas Wei E. I. Liu, Member, IEEE, Zhi Ning Chen, Fellow, IEEE, Xianming Qing, Senior Member, IEEE, Jin Shi,
Member, IEEE, and Feng Han Lin, Student Member, IEEE
C
Submitted to IEEE TAP
2
antenna embedded in a half-wavelength square cavity was
proposed for reaching a large fractional −10-dB bandwidth
around 30% but with a large thickness up to 0.150 [21].
In this paper, we propose two ways to miniaturize the
low-profile wideband metasurface antennas for potential
applications in high-performance wideband fixed/phased array
antennas and compact wideband MIMO antenna systems. First,
a low-profile single-layer metasurface antenna (SIMA) with
narrowed gaps in between the metasurface patch array elements
is investigated for potential antenna miniaturization while
maintaining wideband performance. Then, a wideband
miniaturized low-profile dual-layer metasurface antenna
(DUMA) is proposed and studied with wider gaps between the
metasurface patch array elements to facilitate the fabrication.
Both the single-layer and dual-layer metasurfaces working
together with the supported grounded substrate are considered
as the waveguided metamaterials to extract the effective
refractive index [22], [23]. The effective propagation constant
is then derived for estimating the resonant frequencies of the
TM10 and antiphase TM20 modes of the antenna [13]. The
wideband performance of the proposed miniaturized
low-profile dual-layer metasurface antenna is verified by the
measurement results, which are in good agreement with the
simulation results by using CST Microwave Studio [24].
II. MINIATURIZED SINGLE-LAYER TIGHTLY-COUPLED
METASURFACE ANTENNA
The low-profile single-layer tightly-coupled metasurface
antenna is revisited to study the potential for size
miniaturization. As shown in Fig. 1, the single-layer
metasurface is composed of a 44 square patch array with a
periodicity of p and a narrow gap width of g, printed on the top
surface of a grounded substrate with a thickness of h1. The
antenna is fed by a 50-Ohm microstrip line implemented on the
bottom substrate of thickness h0 through a rectangular aperture
cut on the ground plane. The Rogers RO4003C substrate (ɛr =
3.55, tan δ = 0.0027) is utilized for all the antennas throughout
the paper.
The single-layer metasurface working together with the
grounded substrate is regarded as a waveguided metamaterial.
Fig. 2(a) depicts the simulation model with the applied
boundary conditions and TEM wave excitations, wherein two
metasurface unit cells are loaded in the upper metal plate of the
planar waveguide. Since the period of the metasurface unit cell
is much smaller than the operating wavelength within the
frequency band of interest, the volume occupied by the
metasurface in the planar waveguide can be taken as an
effective medium. The effective medium parameters are
obtained from the simulated reflection and transmission
coefficients and the subsequent standard retrieval process [25].
The retrieved y-components of the refractive indexes of the
metasurface waveguided metamaterial with different
metasurface gap widths are compared in Fig. 2(b). With the
Fig. 1. Configuration of the single-layer tightly-coupled metasurface antenna.
(a)
4.5 5.0 5.5 6.0 6.5 7.0 7.50.0
0.4
0.8
1.2
1.6
Ref
ract
ive
Index
Frequency (GHz)
Re(ny ), g = 0.25 mm
Re(ny ), g = 0.5 mm
Im(ny ), g = 0.25 mm
Im(ny ), g = 0.5 mm
(b)
4.5 5.0 5.5 6.0 6.5 7.0 7.50.0
0.1
0.2
0.3
0.4
0.5
0.0
0.1
0.2
0.3
0.4
0.5
Att
enu
atio
n (
Np
/)
e p , g=0.25 mm
e p , g=0.5 mmPh
ase
shif
t (r
ad/
)
Frequency (GHz)
e p , g=0.25 mm
e p , g=0.5 mm
( fr L)
( fr L)
(c)
Fig. 2. (a) Simulation model for extracting the effective medium parameters,
(b) retrieved effective refractive index, and (c) derived propagation
characteristics of the miniaturized single-layer metasurface. (SIMA I: p = 7.05,
g = 0.25, h1 = 3.25, h0 = 0.813, Wm = 1.8, Ls = 20, Ws = 0.4, s = 10.5, Lg = 40;
SIMA II: p = 7.05, g = 0.5, h1 = 3.25, h0 = 0.813, Wm = 1.8, Ls = 18.5, Ws = 0.6,
s = 10, Lg = 40. unit: mm)
Submitted to IEEE TAP
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unchanged dimensions of p = 7.05 mm and h1 = 3.25 mm, the
metasurface with a narrower gap width of g = 0.25 mm exhibits
the higher effective refractive index and a lower effective
extinction coefficient.
The effective propagation constant is derived from the
refractive index to estimate the dual-mode frequencies of the
antenna using the same empirical equations in [13], which are
reproduced as follows:
𝛽𝑒𝑝 𝜋⁄ = (1 − 2𝛽𝑓𝑟∆𝐿 𝜋⁄ )/4, TM10 (1)
𝛽𝑒𝑝 𝜋⁄ = (1 − 2𝛽𝑓𝑟∆𝐿 𝜋⁄ )/2, antiphase TM20 (2)
𝛽𝑓𝑟 = 2𝜋𝑓√𝜀𝑓𝑟 𝑐⁄ (3)
∆𝐿
ℎ1= 0.412
(𝜀𝑓𝑟 + 0.3)(4𝑝 ℎ1⁄ + 0.262)
(𝜀𝑓𝑟 − 0.258)(4𝑝 ℎ1⁄ + 0.813) (4)
𝜀𝑓𝑟 =𝜀𝑟 + 1
2+𝜀𝑟 − 1
2(1 + 3ℎ1/𝑝)
−1/2 (5)
where is the operating frequency, c is the speed of light in
vacuum, e is the effective phase constant of the metasurface
waveguided metamaterial, and fr is the phase constant in the
effective extended region with a length of L at each end of the
antenna along the resonant direction (i.e. along y-axis) due to
the effect of the fringing fields.
The derived propagation characteristics of the single-layer
metasurface waveguided metamaterial are presented in Fig.
2(c). The dual-mode resonant frequencies of the metasurface
with g = 0.25 mm are initially calculated to be 5.10 GHz and
6.35 GHz, lower than 5.36 GHz and 6.63 GHz of the
metasurface with g = 0.5 mm, respectively. The simulated
reflection coefficient and gain of the single-layer metasurface
antennas are plotted in Fig. 3. The matching frequencies of the
antenna are found to be 5.08 GHz and 6.14 GHz for SIMA I,
and 5.58 GHz and 6.60 GHz for SIMA II, close to the estimated
resonant frequencies from the analysis of the propagation
characteristics. With the decrease of the gap width from g = 0.5
mm to g = 0.25 mm, the −10-dB reflection coefficient
frequency band of the single-layer metasurface antenna has
been downward shifted from 5.16–6.86 GHz to 4.71–6.38 GHz.
The single-layer tightly-coupled metasurface antenna SIMA I
with a narrow gap width of 0.25 mm is able to produce 30%
fractional bandwidth with a low profile of 0.060 and a
miniaturized aperture area of 0.510 0.510. The simulated
realized gain varies from 6.5 dBi to 8.0 dBi and the radiation
efficiency is higher than 90% over the wide operating
bandwidth. The antenna miniaturization is achieved due to the
increased effective refractive index caused by the reduced
metasurface gap width.
III. MINIATURIZED DUAL-LAYER METASURFACE ANTENNA
A. Design
The configuration of the proposed wideband miniaturized
dual-layer metasurface antenna and the corresponding
Cartesian coordinate system are shown in Fig. 4. A 44 square
patch array with a periodicity of p and gap width of g, and a 66
square patch array with a periodicity of pr and gap width of gr,
are separately printed onto the top and bottom surface of a piece
of thin substrate with a thickness of h2 to form the dual-layer
metasurface. The antenna substrate has the same overall
thickness of h1.
The dual-layer metasurface supported by the grounded
substrate is considered as a waveguided metamaterial. Its
effective medium parameters are extracted using the simulation
model shown in Fig. 5(a), wherein the 22 upper patch cells
and 33 lower patch cells of the dual-layer metasurface are
incorporated in the planar waveguide as the relation between
the two periodicities is set as 2p = 3pr. The dimensions of the
dual-layer metasurface of the antenna (DUMA I) are as follows:
p = 7.05 mm, g = 0.6 mm, pr = 4.7 mm, gr = 1.6 mm, h2 = 0.508
mm, h1 = 3.25 mm. The retrieved y-component of the refractive
index of the dual-layer metasurface waveguided metamaterial
in DUMA I is presented in Fig. 5(b) in comparison with that of
the single-layer tightly-coupled metasurface in SIMA I. In
4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5-40
-30
-20
-10
0
|S11|, SIMA I
|S11|, SIMA II
|S11| (
dB
)
Frequency (GHz)
-3
0
3
6
9
Gain, SIMA I Gain, SIMA II
Bo
resi
gh
t G
ain
(d
Bi)
Fig. 3. |S11| and realized boresight gain of the single-layer metasurface
antennas.
Fig. 4. Configuration of the dual-layer metasurface antenna.
Submitted to IEEE TAP
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order to achieve the same high effective refractive index for
antenna miniaturization, the gaps in the dual-layer metasurface
can be much wider than those in the single-layer metasurface.
From the derived effective propagation characteristics given in
Fig. 5(c), the two resonant mode frequencies of DUMA I are
initially predicted to be 5.13 GHz and 6.27 GHz using the
empirical equations (1)–(5).
Fig. 6 shows the simulated reflection coefficient and realized
boresight gain of the DUMA I. The matching frequencies of the
DUMA I are 5.09 GHz and 5.92 GHz, close to the prediction
from the mode analysis. The proposed DUMA I with a
radiating aperture area of 0.510 0.510 achieves an operating
bandwidth of 27% (4.70–6.17 GHz) with |S11| ≤ −10 dB,
realized boresight gain varying from 6.5 dBi to 7.8 dBi, and
radiation efficiency higher than 90%.
Fig. 7 depicts the simulated electric field distributions in the
cross-section of the proposed dual-layer metasurface antenna at
the frequencies of 5.09 GHz and 5.92 GHz. It indicates that the
conventional TM10 and antiphase TM20 modes remain in the
dual-layer metasurface antenna and contribute to the wideband
performance. The enhanced capacitive couplings in the
dual-layer metasurface give rise to the increase of the effective
refractive index, and consequently the antenna miniaturization.
B. Miniaturization
The miniaturization potential of the proposed wideband
dual-layer metasurface antenna is further discussed in this
subsection. To realize a wide bandwidth centered at ~5.5 GHz
with a smaller periodicity of p = 6.3 mm, the metasurface gaps
of DUMA I are optimized to be g = 0.3 mm and gr = 1.2 mm to
create the DUMA II as well as g = 0.14 mm SIMA III for the
required high effective refractive index.
The retrieved propagation characteristics are compared in
Fig. 8(a). Fig. 8(b) shows the simulated |S11| and gain of the
antennas DUMA II and SIMA III with a radiating aperture area
of 0.460 0.460. The dual-layer metasurface antenna DUMA
II achieves an operating bandwidth of 27% (4.76–6.29 GHz)
for |S11| ≤ −10 dB, realized boresight gain varying from 6.5 dBi
to 7.5 dBi, and radiation efficiency higher than 90%. The
single-layer SIMA III exhibits almost the same impedance
bandwidth and gain as the dual-layer DUMA II. However, the
minimum gap width in SIMA III is only 0.14 mm against 0.3
mm in DUMA II. The gaps of SIMA III, 0.14 mm in width, are
too small to fabricate using the standard PCB process while the
antenna performance is quite sensitive to fabrication tolerance
of the gaps.
Furthermore, the proposed wideband dual-layer metasurface
antenna can be miniaturized by reducing the substrate thickness
h2 as well. When printed onto a thinner substrate, the dual-layer
(a)
4.5 5.0 5.5 6.0 6.5 7.00.0
0.4
0.8
1.2
1.6
Ref
ract
ive
Ind
ex
Frequency (GHz)
Re(ny ), DUMA I
Re(ny ), SIMA I
Im(ny ), DUMA I
Im(ny ), SIMA I
(b)
4.5 5.0 5.5 6.0 6.5 7.00.0
0.1
0.2
0.3
0.4
0.5
0.0
0.1
0.2
0.3
0.4
0.5
Att
enu
atio
n (
Np
/)
e p , DUMA I
e p , SIMA IPh
ase
Sh
ift
(rad
/)
Frequency (GHz)
e p , DUMA I
e p , SIMA I
( fr L)
( fr L)
(c)
Fig. 5. (a) Simulation model for extracting the effective medium parameters,
(b) retrieved effective refractive index, and (c) derived propagation
characteristics of the dual-layer metasurface. (DUMA I: p = 7.05, g = 0.6, pr =
4.7, gr = 1.6, h2 = 0.508, h1 = 3.25, h0 = 0.813, Wm = 1.8, Ls = 21, Ws = 0.4, s =
11, Lg = 40, unit: mm)
4.0 4.5 5.0 5.5 6.0 6.5 7.0-40
-30
-20
-10
0
|S11|
|S11| (
dB
)
Frequency (GHz)
-3
0
3
6
9
Gain
Bo
resi
gh
t G
ain (
dB
i)
Fig. 6. Simulated |S11| and realized boresight gain of the dual-layer metasurface
antenna (DUMA I).
Fig. 7. Simulated electric field distribution of the dual-layer metasurface
antenna at the resonant frequencies of 5.09 GHz and 5.92 GHz.
Submitted to IEEE TAP
5
metasurface waveguided metamaterial generates a higher
effective refractive index for the size reduction of the antenna.
Therefore, the proposed dual-layer metasurface is much
more promising for achieving a wideband miniaturized
low-profile antenna operating at higher frequency bands, by
considering the miniaturization ability and ease of fabrication.
IV. EXPERIMENTAL VALIDATION
To verify the proposed dual-layer metasurface antenna
design, DUMA I was fabricated with slight changes without
loss of validity. The variation of the lower layer of the
dual-layer metasurface is illustrated in Fig. 9(a). The 66
square patch array is separated in the middle with a larger
center gap width of gc = 2 mm. The other modified dimensions
are as follows: p = 7 mm, g = 1 mm, gr = 0.8 mm, Ls = 20 mm, s
= 11.5 mm. The occupied aperture area of the fabricated
dual-layer metasurface antenna is 27.4 mm 28.6 mm, or
0.500 0.520 at the center operating frequency of 5.5 GHz.
An additional transition from the grounded coplanar waveguide
(GCPW) to microstrip line (MSL) is introduced to the
prototype as shown in Fig. 9(b). The designed GCPW-to-MSL
transition has a wide bandwidth of 4.5–6.5 GHz at −35-dB
reflection coefficient with the insertion loss better than 0.15 dB.
Fig. 9(c) compares simulated and measured |S11| and realized
boresight gain of the dual-layer metasurface antenna. The
simulated bandwidth of the antenna with −10-dB reflection
coefficient is 24.4% (4.82–6.16 GHz) while the measured one
is 24.5%, from 4.86 to 6.22 GHz with slight frequency shift.
The simulated realized boresight gain varies from 6.5 dBi to 7.8
dBi with radiation efficiency higher than 88% over the
4.5 5.0 5.5 6.0 6.5 7.00.0
0.1
0.2
0.3
0.4
0.5
0.0
0.1
0.2
0.3
0.4
0.5
Att
enu
atio
n (
Np/
)
e p , DUMA II
e p , SIMA III
Ph
ase
Shif
t (r
ad/
)
Frequency (GHz)
e p , DUMA II
e p , SIMA III
( fr L)
( fr L)
(a)
4.0 4.5 5.0 5.5 6.0 6.5 7.0-30
-20
-10
0
|S11|, DUMA II
|S11|, SIMA III
|S11| (
dB
)
Frequency (GHz)
2
4
6
8
Gain, DUMA II Gain, SIMA III
Bo
resi
gh
t G
ain (
dB
i)
(b)
Fig. 8. (a) Propagation characteristics and (b) simulated |S11| and realized
boresight gain of the single-layer and dual-layer metasurface antennas. (DUMA
II: p = 6.3, g = 0.3, pr = 4.2, gr = 1.2, h2 = 0.508, h1 = 3.25, h0 = 0.813, Wm = 1.8,
Ls = 20.5, Ws = 0.3, s = 10.3, Lg = 40; SIMA III: p = 6.3, g = 0.14, h1 = 3.25, h0
= 0.813, Wm = 1.8, Ls = 20.5, Ws = 0.3, s = 10.2, Lg = 40. unit: mm)
(a) (b)
4.0 4.5 5.0 5.5 6.0 6.5 7.0-40
-30
-20
-10
0
Simu. |S11|
Meas. |S11|
|S11| (
dB
)
Frequency (GHz)
-3
0
3
6
9
Simu. Gain Meas. Gain
Bo
resi
gh
t G
ain (
dB
i)
(c)
Fig. 9. Dual-layer metasurface antenna: (a) variation of the lower layer of the
metasurface, (b) photos of the antenna prototype, (c) simulated and measured
|S11| and realized boresight gain. (p = 7, g = 1, pr = 4.7, gr = 0.8, gc = 2, Ls = 20,
s = 11.5, unit: mm)
Fig. 10. Simulated and measured normalized radiation patterns in E/H-planes
at 5.0, 5.4, and 6.0 GHz.
Submitted to IEEE TAP
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impedance bandwidth. The measured and simulated gains are
in good agreement with a maximum gain drop of 0.9 dB at 5.3
GHz. Fig. 10 plots the measured and simulated normalized
radiation patterns at the operating frequencies of 5.0, 5.4, and
6.0 GHz. Over the operating band, the simulated front-to-back
ratio (FBR) is greater than 11 dB and up to 18 dB at 5.4 GHz,
and the 3-dB beamwidth is 69±10 in E-planes and 69±7 in
H-planes, all in good coincidence with the measurements.
Within the 3-dB beamwidth, the cross polarization levels are
simulated to be less than −37 dB in H-planes and negligible in
E-planes, while the measured are less than −28 dB and −23 dB
in H- and E-planes, respectively.
V. CONCLUSION
The wideband low-profile single-layer and dual-layer
metasurface antennas have been proposed for miniaturization.
Both the single-layer and dual-layer metasurfaces working
together with the supported grounded dielectric substrate have
been considered as the waveguided metamaterials to extract the
effective refractive index for estimating the resonant
frequencies of the antennas. The antenna miniaturization has
been achieved with the increased effective refractive index,
resulted from the narrowed gaps of the single-layer metasurface
and the enhanced capacitive couplings of the dual-layer
metasurface, respectively. Both single-layer and dual-layer
metasurface antennas with the respective minimum gap width
of 0.14 mm and 0.3 mm are able to obtain the realized gain
greater than 6.5 dBi over the wide impedance bandwidth of 27%
with a reduced radiating aperture size of 0.460 0.460 and a
low profile of 0.060 (0 is the free-space wavelength at center
operating frequency of 5.5 GHz). The miniaturized single-layer
tightly-coupled wideband metasurface antenna is more suitable
for working at low frequency ranges due to the simpler
structure, while the miniaturized dual-layer wideband
metasurface antenna for the high frequency operation wherein
feasible wide gaps are much preferred. The proposed wideband
miniaturized low-profile antennas would be promising for
various communication systems, such as high-performance
wideband fixed array antennas, high-performance wideband
wide-scanning phased array antennas with either linear or
circular polarization, and compact wideband MIMO antenna
systems.
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