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: eleliwe@nus.edu.sg).

Z. N. Chen is with Department of Electrical & Computer Engineering,

National University of Singapore, Singapore 117583 (e-mail: eleczn@nus.edu.sg).

X. Qing is with the Institute for Infocomm Research (I2R), Agency for

Science, Technology and Research (A*STAR), Singapore 138632 (e-mail: qingxm@i2r.a-star.edu.sg).

J. Shi is with School of Electronics and Information, Nantong University, 9

Seyuan Road, Nantong 226019, China (e-mail: jinshi0601@hotmail.com). F. H. Lin is with Department of Electrical & Computer Engineering,

National University of Singapore, Singapore 117583 (e-mail:

fhlinphd@gmail.com).

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

3

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

4

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

6

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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