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This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/LAWP.2018.2844293, IEEEAntennas and Wireless Propagation Letters
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Abstract— A broadband symmetrical dual-loop antenna with
enhanced gain is proposed in this letter. It is comprised of a
symmetrical dual-loop radiator, a pair of magneto-electric feeding
structures, two metal shorting pins, and a metal ground plane. To
simultaneously broaden its impedance bandwidth and improve its
radiation patterns, a symmetrical dual-loop radiator excited by a
pair of magneto-electric feeding structures is introduced. By
loading shorting pins between the radiator and the ground plane,
the peak gain can be significantly enhanced over the band of
operation. Measured results demonstrated that the proposed
antenna has a wide impedance bandwidth of 48.2% ranging from
1.68 to 2.76 GHz for VSWR<1.5 and a stable gain of 10.0±0.85 dBi.
Moreover, stable radiation patterns and low cross-polarization
are obtained in the desired frequency band. Due to these
performances above, the proposed antenna is very suitable for
base station applications.
Index Terms— Wideband, gain enhancement, shorting pin, low
cross-polarization, directional antenna.
I. INTRODUCTION
ITH the rapid development of wireless communication
systems, such as GSM1800/GSM1900, the third
generation (3G) system and long term evolution (LTE) system,
antennas with good features of broad impedance bandwidth,
low profile, high gain and stable radiation patterns are more
attractive. In recent years, a lot of open literatures about
broadband antennas with unidirectional radiation patterns have
been published [1]-[4]. In [1], by combining of shorted bowtie
patch antenna and an electric dipole, the proposed antenna
achieves a wide impedance bandwidth from 2.16 to 4.13 GHz
for VSWR< 2. A wideband MIMO antenna is proposed for
WLAN/WiMAX applications [2]. By adopting an integrated
balun, the antenna has a wide bandwidth of 60.6% covering the
band of 2.30-4.30 GHz for return loss >10 dB. Recently, a
parasitic element has been added to each tilted dipole for
bandwidth enhancement [3]. It is found that an impedance
bandwidth of 45% for return loss >10 dB from 1.62 to 2.55
GHz can be achieved by using a parasitic element close to the
dipole. In [4], a stepped transformer and shifted microstrip are
used to feed the parasitic patches stacked above the driven
patch; an enhanced operating bandwidth of 35% from 4.9 to
The authors are with the National Key Laboratory of Antennas and
Microwave Technology, Xidian University, Xi’an, Shaanxi, 710071, China.
(e-mail:[email protected];[email protected]; [email protected];
[email protected];[email protected])
7.05 GHz is achieved.
As mentioned above, many works have been carried out to
broaden its impedance bandwidth. However, in the case of
communication systems, high gain antennas are highly
demanded for lengthening the communication distance or
compensating the large propagation losses. Therefore, much
attention is paid to the antenna element with gain enhancement.
In recent years, numerous methods for enhancing the gain of
the antennas [5]-[10] have been proposed. In [5], several
directors and a truncated ground plane acting as a reflector have
been added to the antenna so as to maximize the antenna gain.
It is pointed out in [6] that the gain of the antenna could be
enhanced by the virtue of four parasitic rectangular patches. In
[7], the gain of the bow-tie antenna is significantly enhanced by
loading a 2×5 array of end-coupled split-ring cells. A
remarkable increase in the gain of the primary radiator such as
dielectric resonator antenna [8] has been achieved by
introducing the uniaxial anisotropic material. In [9], the gain of
the antenna is greatly improved with a peak enhancement of 2.5
dBi by centrally positioning the antenna above a double layered
EBG structures. In previous work, they have made the desired
results in increasing the antenna’s gain. However, gain
enhancement of these antennas is achieved at the cost of
enlarged equivalent caliber. In [10], a rectangular patch antenna
with slot-loaded technique is developed to reduce the sidelobe
level and enhance the gains. Additionally, another technique
for improving the antenna’s directivity is to load shorting pins
without increasing the occupied volume in [11]. However,
antennas of this type generally have high Q value, resulting in a
narrow band of operation.
In the paper, a broadband symmetrical dual-loop antenna
loaded with shorting pins will be discussed. By introducing a
pair of magneto-electric feeding structures, a wide impedance
bandwidth is realized. Meanwhile, a symmetrical dual-loop
radiator is designed to suppress the cross-polarized field
component. Furthermore, a set of shorting pins is loaded
beneath the radiator patch to strength the peak gain.
II. ANTENNA DESIGN AND ANALYSIS
A. Symmetrical Dual-Loop Antenna Design
The evolution of the broadband symmetrical dual-loop
antenna is shown in Fig.1 (a). First, an ideal dipole antenna
referred to as antenna I with narrow bandwidth is presented. To
improve an impedance bandwidth, a magnetic dipole combined
Gain Enhancement of Broadband Symmetrical
Dual-Loop Antenna Using Shorting pins
Jinhai Liu, Student Member, IEEE, Zhaoyang Tang, Student Member, IEEE, Ziyang Wang, Student
Member, IEEE, Hui Li, and Yingzeng Yin, Member, IEEE
W
1536-1225 (c) 2018 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.
This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/LAWP.2018.2844293, IEEEAntennas and Wireless Propagation Letters
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Fig.1 (a) Evolution of the proposed antenna;
(b) Comparison of simulated VSWR.
with the electric dipole is introduced which is marked as
antenna II. However, the radiation patterns are degraded due to
the asymmetrical structure, especially the cross-polarization.
As shown in Fig.1 (a), to alleviate this problem, a symmetrical
dual-loop antenna III is designed to improve radiation patterns.
Herein, the resultant antenna has two symmetrical feeding
structures which should be commonly fed. Further, a set of
shorting pins is introduced in the centerline of the antenna IV to
strengthen the peak gain. The corresponding VSWR in
evolution is shown in Fig.1 (b). Note that, antenna II has more
resonances with better impedance matching by combing the
resonance of the magneto-electric feeding structure and the
ordinary loop antenna. Furthermore, antenna IV has an
impedance matching with bandwidth of 46.8% for VSWR < 1.5,
corresponding to the antenna with magneto-electric feeding
structures and symmetrical dual-loop radiator, whereas an
impedance bandwidth of 14.0% (VSWR < 2.5) is presented by
the ordinary loop antenna (antenna I). As desired, the operating
bandwidth performance of antenna IV is greatly improved.
The broadband symmetrical dual-loop antenna loaded with
shorting pins is optimized using the high-frequency structure
simulator software. A Wilkinson power divider is located under
the ground plane to excite the proposed antenna, and it is
fabricated on an F4B substrate with the thickness of 1.5mm and
Fig. 2 Vector surface current at 2.2GHz.
Fig.3 Comparison of simulated radiation patterns between
antenna II and antenna III at 2.2GHz.
relative permittivity of 2.2. Meanwhile, the patch radiator and
magneto-electric feeding structures are fabricated using a
copper with the thickness of 0.7mm. In addition, the optimized
parameters of the antenna shown in Fig.1 (a) are listed as
follows (units: mm):W=55, wf1=4, wf2=14, L=95, H=34,
L=95, D=17, wa=19, wf=19.5.
As shown in Fig.1 (a), when the ports are excited, the signal
is injected to the hook-shaped probes and then coupled to the
radiator. The surface current distribution of the antenna at 2.2
GHz is illustrated in Fig. 2. It can be seen that the current
intensely flows along ±x axis on the radiator, especially near the
coupling slot. In other words, the yoz-plane can be regarded as
the E-plane while the xoz-plane denotes the H-plane.
Additionally, there is a small amount of instantaneous current
on the horizontal patch and the currents almost in the ±x- axes
directions, which however contribute to the antenna’s
cross-polarization. To effectively suppress the spurious
radiation caused by the current flowing in ±x-axes direction, a
symmetrical configure with respect to antenna II is designed.
As shown in Fig.2 (antenna III), two symmetrical ports can be
excited to produce out-of-phase currents at other horizontal
portions due to the introduction of the symmetrical structure
and commonly feeding technique. Thereby, A and C can be
considered as the joint points connecting radiator with y-axis
component of surface current flowing in the same direction,
thus A-C plane functioning as a perfect magnetic wall. B and D
are joint points connecting radiator with x-axis component of
surface current flowing out of phase, thus B-D plane work as an
electric wall. Theoretically, the spurious radiations can be
cancelled in far field due to the currents’ component along
x-axis with equal amplitude but out of phase. This operating
mechanism introduces the perfect magnetic wall and electric
wall model by physical connection of the surface currents,
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Fig.4 Current distributions on the radiator at 2.7GHz
(I1 & I3 represent the source currents).
Fig.5 Pattern comparisons of the proposed antenna at 2.7GHz.
TABLE I
RADIATION PARAMETERS OF ANTENNA III AND ANTENNA IV
Antennas Antenna IV Antenna III
E-plane HBPW (º) 57 70.1
H-plane HBPW (º) 46.5 51.0
Gain (dBi) 10.95 9.70
which achieves an improved phase canceling effect and
suppresses the cross-polarized radiation.
To further illustrate the improvement of the
cross-polarization, a comparison between the simulated
radiation patterns of antenna II and antenna III is presented in
Fig.3. It is observed that the polarization purity of the antenna
III can be greatly improved compared to that of the antenna II,
especially in the E-plane.
B. Gain Enhanced with Shorting pins
In order to illustrate the operating principle of gain
enhancement of the proposed antenna, surface current
distributions of the antennas with and without shorting pins are
compared and shown in Fig. 4. As shown, the surface current
intensity on the radiator (antenna IV) is more uniform in the
middle area and a much larger area is strengthened because of
the presence of the shorting pins. Meanwhile, it is obvious (as
shown in Fig.2 (antenna III)) that the vector current on the
central line of the radiator flows with the same direction as that
of the source currents on both sides of the radiator.
Consequently, currents on the vertical portions of the
symmetrical radiator flow in same directions and their radiation
can be superposed in the far field. As such, the electrical size of
Fig.6 Comparison of simulated gains between antenna III and
antenna IV.
Fig .7 VSWR and gain against frequency.
the proposed antenna is effectively enlarged and the radiation
gain is hence enhanced.
Furthermore, the simulated radiation patterns of the
proposed antenna with and without shorting pins are plotted in
Fig.5. Herein, it can be seen that the half-power beamwidth
(HPBW) of the antenna with shorting pins is narrowed in both
E- and H-planes. Meanwhile, the HPBWs and the gains are
numerically investigated, as tabulated in Table I. As shown in
Fig.5, the pin-loaded antenna has notably narrowed HPBW
compared with those of the antenna without pins. To be specific,
the HPBWs of the antenna with shorting pins are, respectively,
57 º and 46.1 º at E- and H-plane, while they are 70.1 º and 51 º
for the antenna without shorting pins loaded. The gain of the
proposed antenna has been enhanced to as high as 10.95 dBi,
with an increase of 1.25 dB.
To further illustrate the function of the shorting pins, the
comparison of simulated gains between antenna III and antenna
IV is shown in Fig.6. Compared with the antenna without
shorting pins, the peak gain of antenna IV is greatly increased
within the operating frequency band, and its gain without a
significant decline at high frequencies scope.
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This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/LAWP.2018.2844293, IEEEAntennas and Wireless Propagation Letters
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Fig.8 Simulated and measured radiation patterns of the
proposed antenna in the E- and H-plane.
III. EXPERIMENTAL RESULTS
To validate the antenna design, a broadband symmetrical
dual-loop antenna with a set of shorting pins is designed and
fabricated. The fabricated antenna is shown in Fig.7. Herein,
the VSWR versus frequency is also illustrated in Fig.7. There is
a reasonable agreement between the simulated and measured
results and a wide measured impedance bandwidth of 48.2% is
obtained, ranging from 1.68 to 2.76 GHz. Fig.8 depicts the
simulated and measured radiation patterns at frequencies of 1.7,
2.2 and 2.7 GHz, respectively. It is observed that the proposed
antenna achieves unidirectional radiation patterns with stable
radiation directivity and low cross-polarization within the
operating band. The measured cross-polarization radiation
level is less than -21 dB in the operating frequency band. It
should be pointed out that the simulated cross-polarization at
some frequencies is lower than −40 dB, which cannot be
appeared in the figures. Finally, the gain of the proposed
antenna was measured in an anechoic chamber with the Satimo
system, as shown in Fig.7. It can be seen that the measured gain
varying within 10.0 ±0.85 dBi is obtained across the whole
operating frequency band.
IV. CONCLUSION
A wideband and high gain symmetrical dual-loop antenna is
presented. By employing the magneto-electric feeding
structures, an impedance bandwidth of 48.2% ranging from
1.68 to 2.76GHz is obtained for VSWR<1.5. Meanwhile, the
spurious radiations can be effectively suppressed by a
symmetrical dual-loop radiator. Furthermore, since a pair of
shorting pins is symmetrically loaded beneath the radiator,
surface current across the centerline of the antenna can be
remarkably strengthened so as to enhance its radiation
directivity and gain. The proposed antenna exhibits advantages
of wide impedance bandwidth, stable radiation patterns and
high gain. Owing to these properties, the proposed antenna will
be an appropriate replacement for conventional Yagi–Uda
antenna in some point-to-point communications, or as a part of
a broadband high-gain antenna array for base-station
applications.
V. REFERENCE
[1]. Hang Wong, Ka-Ming Mak, and Kwai-Man Luk, “Wideband Shorted Bowtie Patch Antenna With Electric Dipole,” IEEE Trans. Antennas
Propag., vol. 56, No. 7, pp. 2098-2101, 2008.
[2]. HanWang, Longsheng Liu, Zhijun Zhang, Yue Li, and Zhenghe Feng, “A Wideband Compact WLAN/WiMAX MIMO Antenna Based on Dipole
With V-shaped Ground Branch,” IEEE Trans. Antennas Propag., vol. 63,
No. 5, pp. 2290-2295, 2015. [3]. Yi Fan, XuLin Quan, Yan Pan, YueHui Cui, and RongLin Li, “Wideband
Omnidirectional Circularly Polarized Antenna Based on Tilted Dipoles,”
IEEE Trans. Antennas Propag., vol. 63, No. 12, pp. 5961-5966, 2015. [4]. Ankita Katyal, and Ananjan Basu, “Compact and Broadband Stacked
Microstrip Patch Antenna for Target Scanning Applications,” IEEE
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Antennas Propag., vol. 57, No.11, pp. 3672-3676, 2009. [6]. Wenwen Yang, Jianyi Zhou, Zhiqiang Yu, and Linsheng Li, “Bandwidth-
and Gain-Enhanced Circularly Polarized Antenna Array Using
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[7]. Z Abdolmehdi Dadgarpour, Behnam Zarghooni, Bal S. Virdee, Tayeb A.
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[8]. Saeed Fakhte, Homayoon Oraizi, Ladislau Matekovits, and Gianluca Dassano, “Cylindrical Anisotropic Dielectric Resonator Antenna With
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1404-1409, 2017. [9]. Pui-Yi LAU, Kenneth Kin-On YUNG, Zhi-Ning CHEN, “A Wideband
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[10]. Xiao Zhang, Lei Zhu and Qiong-Sen Wu, “Side-lobe-reduced and
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Loading of Shorting Pins,” IEEE Trans. Antennas Propag., vol. 64, No. 8,
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