IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL.

16, 2017 2943

Metamaterial Slabs Covered UWB Antipodal

Vivaldi Antenna
Xiangxiang Li, Student Member, IEEE, Hao Zhou, Zhiming Gao, Hailin Wang, and Guoqiang Lv

Abstract—An ultrawideband metamaterial slab (meta-slab) cov-

ered antipodal Vivaldi antenna (AVA) with high-gain and stable
radiation patterns is presented. The meta-slab exploiting the dis-
persive nature of metamaterial exhibits an effective permittivity
function higher than the one of the bare dielectric slabs. The high-
permittivity meta-slab sucks energy from the tapered slot and the
flare termination of the Vivaldi antenna and transmits the energy
to the endfire direction. Both the original antenna and proposed
antenna are simulated, fabricated, and tested. The simulated re-
sults show good agreement with the measured results. In the mea- Fig. 1. Photograph of the proposed antenna MSAVA consisting of a conven-
sured range of 3.68–43.5 GHz, S1 1 of both antennas are less than tional AVA and two meta-slabs.
–10 dB. Measured far-field results show the proposed antenna pro-
vides boresight gain >10 dBi over the 10–20 GHz, >15 dBi over the
20–32 GHz, and >17 dBi over the 32–40 GHz. It also exhibits stable (AZIM) obtains gain enhancement of about 2.2 and 3.8 dBi in
radiation patterns with narrower beamwidth and lower sidelobe the range of 9.5–10.5 GHz, respectively [6].
level in both the E- and H-plane compared to the AVA. Nevertheless, the frequency band of gain enhancement is rel-
atively narrow because of the resonant characteristic of AZIM.
Index Terms—Antipodal Vivaldi antenna (AVA), high gain,
metamaterial, tapered slot antenna, ultrawideband (UWB).
An array of the negative index metamaterial is inserted per-
pendicular to the antenna substrate providing up to 4 dBi gain
enhancement over 6.5–20 GHz [7]. However, the physical size
I. INTRODUCTION of overall antenna at 6.5 GHz is 2.08λ × 1.04λ × 0.69λ. To
achieve a broadband gain increase, the high index metamate-
ECENTLY, metamaterials have been applied to improve
R gains of the endfire antennas, such as bow-tie antenna,
log-periodic antenna, and horn antenna [1]–[3]. As one of the
rial, and anisotropic inhomogeneous artificial material are pre-
sented in [8]–[10]. With this technique, the gain enhancement of
1.3–3.6 dBi is achieved over 6–19 GHz [8]. Additionally, some
most popular endfire antennas, antipodal Vivaldi antenna (AVA) other techniques without metamaterials loading have been inves-
has the characteristics of broad band, moderate gain, low cost, tigated. The composite aperture structure, double slot structure,
and light weight, and, thus, is extensively used in wireless com- the lens, and the dielectric loading techniques are proposed to
munication, microwave imaging, and radar [4]. However, for increase Vivaldi antenna gain [11]–[15]. Recent publication ex-
the AVA with bandwidth up to multiple-octave (>10:1), several plains physical mechanism of broadband gain improvement for
challenging difficulties—i.e., gain decrease, main beam split shortened horn antenna from the intrinsic frequency dispersion
and tilt—restrict its application. The reasons for these problems of metamaterials [16]. The concept is applied in this letter to im-
have been concluded in [5]. prove the radiation properties of the ultrawideband (UWB) AVA.
Many kinds of metamaterials have been proposed to enhance Aiming at enhancing antenna performances in a broad band
Vivaldi antenna gain. The anisotropic zero-index metamaterial especially at high frequencies, metamaterial slabs (meta-slabs)
covered antipodal Vivaldi antenna (MSAVA) shown in Fig. 1 is
Manuscript received July 15, 2017; revised August 28, 2017; accepted proposed. The meta-slabs consisting of 10 × 25 array of high-
September 17, 2017. Date of publication September 20, 2017; date of cur- permittivity metamaterial unit cell can significantly increase
rent version October 30, 2017. This work was supported by Anhui Science gain and directivity, reduce sidelobe level, narrow beamwidth,
and Technology Major under Project 16030901001. (Corresponding Author:
Guoqiang Lv.) and correct main beam tilt of the conventional AVA without
X. Li, H. Zhou, and G. Lv are with the Key Laboratory of Special Display altering the length of the antenna. The measured boresight gain
Technology of the Ministry of Education, National Engineering Laboratory of is up to 17.7 dBi in the frequency band of 3.68–43.5 GHz.
Special Display Technology, National Key Laboratory of Advanced Display
Technology, Academy of Photoelectric Technology, Hefei University of Tech-
nology, Hefei 230009, China (e-mail: lxx2009@mail.hfut.edu.cn; zh_albert@ II. ANTENNA DESIGN
163.com; microwaveb@126.com).
Z. Gao is with the College of Electronic and Information Engineering, Nanjing We previously demonstrated in [17] that two unprinted di-
University of Information Science and Technology, Nanjing 210044, China electric slabs placed symmetrically in close proximity to the
(e-mail: gaozm0909@163.com). Vivaldi antenna can improve the radiation characteristics of
H. Wang is with the State Key Laboratory of Millimeter Waves, Southeast the antenna. The slabs form a guiding wave cavity that directs
University, Nanjing 210096, China (e-mail: 792243993@qq.com).
Color versions of one or more of the figures in this letter are available online
the electromagnetic (EM) waves to the endfire direction. On one
at http://ieeexplore.ieee.org. hand, the scattered fields distributed on both sides of the Vivaldi
Digital Object Identifier 10.1109/LAWP.2017.2754860 antenna are concentrated into the guiding structure. On the other

1536-1225 © 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications standards/publications/rights/index.html for more information.
2944 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 16, 2017

Fig. 2. Configuration of the conventional AVA.

Fig. 4. (a) Configuration of the PL unit-cell. (b) S-parameters of the PL unit-

cell (uw = 0.2, ud = 0.2, dx = 2, dy = 2, dz = 2). (c) Retrieved permittivity
of the PL unit cell with different length.

Fig. 3. Simulated (a) S 1 1 , and (b) gain of the AVA and MSAVA. far-field performances at high frequencies stand for the limits
of bandwidth.
hand, the waves propagating along the slabs owning same phase
velocity result in smooth wavefront at the antenna aperture. The B. Metamaterial Unit Cell
advantage of this technique is to maintain the overall length of In order to design a proper unit cell (UC), which has a high
the antenna unchanged, while effectively enhancing the antenna permittivity property, a modified parallel-line (PL) structure in
gain. As concluded in [17], the high-permittivity slab with thin [8] is employed. The PL UC is printed on the Rogers RO4350B
thickness could achieve good performance. Metamaterials can substrate with a relative permittivity of 3.48 and a thickness of
be equivalent to homogeneous materials. Thus, the alternative 0.101 mm. The equivalent intrinsic parameter of the UC, i.e.,
to realize the concept of dielectric slabs covered Vivaldi antenna permittivity, was calculated using the inversion algorithm based
is to use the metamaterial. The metamaterial serves to reduce on Kramer–Krong relationship [18]. As shown in Fig. 4(a), the
the thickness of the slabs, cost, and weight. S-parameters of the PL are obtained on HFSS by assigning
the perfect electric conductor and perfect magnetic conductor
A. AVA Design boundary conditions in the yoz and xoz planes, respectively, and
two wave ports are located in z-direction. Fig. 4(b) illustrates
The geometry of a conventional AVA is demonstrated in the simulated S-parameters of the PL UC with different length.
Fig. 2. The AVA is printed on a Rogers RO4003C 0.508 mm The effective permittivity of the PL is illustrated in Fig. 4(c).
thick substrate (dielectric constant of 3.38 and loss tangent In fact, the effective permittivity of the PL UC is larger than
of 0.0027) with total size (l × w) of 60 × 40 mm2 . The AVA 1.7 over the nonresonant frequency range, which is larger than
consists of three major parts: an elliptically tapered ground; a effective permittivity of the unprinted dielectric slab (i.e., 1.13).
wm = 1.19 mm microstrip feedline matching the 50 Ω coaxial It is important to mention that the working bandwidth of the UC
line; as well as two dual exponentially tapered radiators printed should be far from its resonant frequency for reducing the loss.
on the opposite sides of the substrate. The end of the microstrip
line is linearly tapered in order to facilitate assembly with the
2.4 mm connector. The other geometrical parameters were op- C. AVA With Metamaterial Slab (MSAVA)
timized by genetic algorithm in order to achieve good reflection The proposed MSAVA is illustrated in Fig. 1. Two pieces
coefficients S11 at low frequencies. The values are given as fol- of meta-slabs made of M × N array of PL unit cells are sym-
lows: ws = 22 mm, lo = 30 mm, ls = 15 mm, r2 = 11.64 mm, metrically placed at a height of mh from the AVA. According
r1 = 19.4 mm, and lg = 20 mm. to the mechanism of AVA, the EM fields mainly radiate in the
The simulated S11 and gain of the AVA are shown in Fig. 3. It tapered slot region. The results in Fig. 5(a) confirm that the
can be observed that the AVA has a broad impedance bandwidth length ml = N × dz of the meta-slab should not be smaller
from 3.74 to more than 44 GHz, and a slight mismatch occurs than that of the tapered slot length for adequate sucking energy
between 5 and 6.25 GHz with S11 <–9.5 dB. However, the re- from the tapered slot and the flare termination. Fig. 5(b) shows
sults in Fig. 3(b) indicate that the main beam of the AVA does the width mw = M × dx of the meta-slab is close to the width
not point towards the axial direction of the tapered slot (z-axis) of tapered slot to ensure effective energy focusing. Parametric
above 34 GHz. It also can be found that the maximum gain of simulations have been carried out to reveal the impacts of height
the AVA is less than 11.4 dBi. Additionally, the boresight gain mh on boresight gain of the MSAVA. We can see in Fig. 6(a)
decreases dramatically at high frequencies. Note that the poor that larger height contributes to higher gain, yet only in a narrow
LI et al.: METAMATERIAL SLABS COVERED UWB ANTIPODAL VIVALDI ANTENNA 2945

Fig. 8. Photograph of the MSAVA in the anechoic chamber to measure the

radiation patterns and gain.
Fig. 5. Effects of varying (a) length ml and (b) width mw of meta-slab on
boresight gain (ul = 1.4 mm, mh = 3.5 mm).

Fig. 9. Measured and simulated (a) reflection coefficient S 1 1 , and (b) gain.

Fig. 6. (a) Effect of varying height mh on boresight gain of the MSAVA

(10 × 25 array, ul = 1.4 mm). (b) Effect of varying length of PL unit cell on
the gain bandwidth of the MSAVA (10 × 25 array, mh = 3.5 mm). III. RESULTS AND DISCUSSION
Based on the above analysis, the example is designed. The
dimensions of the meta-slab in millimeter are as follows: dh =
3.5, dt = 0.1, dw = 22, and dl = 50. The software HFSS and
CST MWS are utilized to design and optimize the antennas.
The developed antenna is fabricated and measured to verify its
simulated performances. The antenna is fed using a 2.4 mm
end-launcher-connector as shown in Fig. 1.
The simulated S11 in Fig. 3(a) shows that the loading of
meta-slabs has subtle influence on the frequency bandwidth.
The S11 of both fabricated antennas are measured with the
help of Agilent PNA-X Network Analyzer N5244A. As seen
in Fig. 9(a), simulated results are in good agreement with
the tested data in the measured bandwidth (3–43.5 GHz). The
Fig. 7. E-field distributions on the (a) xoz plane (antenna surface) and
(b) yoz plane at 35 GHz (10 × 25 array, mh = 3.5 mm, ul = 1.4 mm). measured S11 of both antennas are lower than –10 dB over
3.68–43.5 GHz. Compared to the simulated results, the im-
provement of the measured S11 in the band of 4.7–6.3 GHz is
operational bandwidth. To achieve higher gain enhancement and due to the fabrication effect of the 2.4 mm connector.
wider gain bandwidth, the value of mh should be selected prop- As shown in Fig. 3(b), with the meta-slabs, the value of the
erly. It should be mentioned that when the electrical length of the peak gain is exactly identical with that of the boresight gain, indi-
height is too small, the ability of confinement of the EM energy cating the MSAVA produces a more directed beam. Additionally,
within the guiding cavity formed by the meta-slabs is limited the gain is also significantly enhanced. The far-field radiation of
due to that the E-field is loosely controlled and can extend far the MSAVA was tested in a compact range anechoic chamber,
beyond the guiding cavity, resulting in faint gain increase at low as shown in Fig. 8. Fig. 9(b) shows the measured boresight gain
frequencies. Fig. 6(b) shows the boresight gain of the MSAVA of the MSAVA. The result is in agreement with the simulation
in the band of 3–50 GHz with ul varying from 1.3 to 1.6 mm. over most of the frequency range. The meta-slabs dramatically
The results indicate that the length of the PL UC needs to be improve the AVA high frequency performance. The MSAVA can
tuned according to the operational band. realize gain of more than 10 dBi at the frequency above 10 GHz,
To better understand the effect of the meta-slabs, Fig. 7 plots and the measured maximum gain is up to 17.7 dBi at 38 GHz.
the E-field distributions on the xoz plane, as well as yoz plane Figs. 10 and 11 depict the E-plane and H-plane radiation
at 35 GHz of the AVA and MSAVA. It is evident that E-field patterns at some frequencies, respectively. As observed, the
intensity in the tapered slot and flare termination is reduced, MSAVA exhibits good directional radiation with narrow beam
while in the meta-slabs is enhanced. On the other hand, the especially at high frequencies. The beam-squinting of the AVA
guided waves transmitting along the meta-slabs superpose at at 40 GHz is eliminated. In addition, lower sidelobe levels are
the endfire direction, producing smoother wavefront compared achieved at high frequencies with the meta-slabs compared to
to the AVA. the AVA.
2946 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 16, 2017

made of 10 × 25 array of parallel-line unit cells are spaced by an

air layer with the antenna board. The measured results confirm
that the proposed antenna achieves gain >10 dBi in the range of
10–20 GHz, >15 dBi in the range of 20–32 GHz, and >17 dBi
in the range of 32–40 GHz. The proposed method does not con-
flict with previous techniques such as ripple edge and thus can
be combined to achieve a high-gain compact-size UWB antenna
over entire band for microwave and millimeter-wave imaging.

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