1124 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 66, NO. 3, MARCH 2018
Low-Profile Planar Filtering Dipole Antenna
With Omnidirectional Radiation Pattern
Yao Zhang, Student Member, IEEE, Xiu Yin Zhang , Senior Member, IEEE, and Yong-Mei Pan , Member, IEEE
Abstract—This paper presents a low-profile planar dipole
antenna with omnidirectional radiation pattern and filtering
response. The proposed antenna consists of a microstrip-to-
slotline transition structure as the feeding network and a planar
dipole as the radiator. Filtering response is obtained by adding
nonradiative elements, including a coupled U-shaped microstrip
line and two I-shaped slots, to the feeding network. Within the
operating passband, the added nonradiative elements do not
work, and thus the in-band radiation performance of the dipole
antenna is nearly not affected. However, at the side stopbands,
the added elements resonate and prevent the signal passing
through the feeding network to the dipole antenna, suppress-
ing the out-of-band radiation significantly. As a result, both
satisfactory filtering and radiation performances are obtained.
For demonstration, an omnidirectional filtering dipole antenna
is implemented. Single-band bandpass filtering responses in both
the reflection coefficient and realized gain are obtained. The
measured in-band gain is ∼2.5 dBi, whereas the out-of-band
radiation suppression is more than 15 dB.
Index Terms—Dipole antenna, filtering antenna, omnidirec-
tional radiation pattern.
I. INTRODUCTION
D
UE to large-signal coverage, omnidirectional antennas
that having uniform radiation patterns are desirable
for certain applications in mobile communications. Various
dipole antennas [1]–[3], loop antennas [4]–[7], as well as slot
antennas [8] have been developed to generate omnidirectional
radiation pattern. On the other hand, filtering antennas are
expected to be implemented in modern wireless communi-
cation systems due to the advantages of compact size and
low insertion loss [9]–[18]. As a result, various omnidirec-
tional antennas with filtering response were developed in
recent years [19]–[33]. In [19], a fourth-order bandpass filter
was directly connected with a circularly polarized mono-
pole antenna to obtain filtering performance. In [20]–[22],
three wideband balun filters were designed independently
and then cascaded with dipole antennas. Other filtering net-
works, such as loop-resonator filter [23], multimode resonator
Manuscript received September 13, 2017; revised December 15, 2017;
accepted December 30, 2017. Date of publication January 5, 2018; date of
current version March 1, 2018. This work was supported in part by the
National Science Foundation of China under Grant 61725102 and Grant
61671210 and in part by the Science and Technology Planning Project of
Guangdong Province, China, under Grant 2017B090901053. (Corresponding
author: Xiu Yin Zhang.)
The authors are with the School of Electronic and Information Engineering,
South China University of Technology, Guangzhou 510641, China (e-mail:
zhangxiuyin@scut.edu.cn).
Color versions of one or more of the figures in this paper are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TAP.2018.2790169
filter [24], step-impedance resonator filters [25], [26], half-
wavelength resonator filter [27], quarter-wavelength resonator
filter [28], coupled line resonator filter [29], and band notch
filters [30]–[33], were also employed to feed omnidirectional
antennas. In these designs, satisfying filtering performances
were achieved, but the filtering networks introduce extra inser-
tion loss which degrades the antenna in-band gain undesirably.
Very recently, simple parasitic elements were used instead
of the complex filtering circuits to design omnidirectional
filtering patch antennas or monopole antennas [34], [35]. For
example, 18 shorting pins were inserted into a triangular
patch to provide a radiation null at the upper band edge [34],
whereas four shorting pins were added to a wideband mono-
pole antenna to improve its frequency selectivity [35]. These
shorting pins are very useful to realize the filtering function.
However, the antennas in [34] and [35] are of considerable
size (1.1 × 1.1λ
2
g
and 0.97 × 0.95λ
2
g
) due to the large ground
plane and the tapered-shaped radiators, respectively.
In this paper, a novel omnidirectional filtering dipole
antenna is proposed. It consists of an out-of-phase power
divider as the feeding network and a planar dipole as the
radiator. Parasitic nonradiative elements, including a coupled
U-shaped microstrip line and two I-shaped slots, are intro-
duced to the feeding network to realize filtering response while
keeping the in-band radiation performance unaffected. Since
no specific filter is involved, the design therefore becomes very
compact. Also, due to the elimination of the insertion loss of
filter, the in-band antenna performance is little affected. For
verification, an omnidirectional planar filtering dipole antenna
is implemented. The working mechanism of the proposed
antenna is analyzed in detail, and the design guidelines as
well as measured results are also presented.
II. A
NTENNA CONFIGURATION AND
WORKING MECHANISM
A. Antenna Configuration
Fig. 1 depicts the layout of the omnidirectional filtering
dipole antenna. The antenna is fabricated on a substrate with
relative permittivity of 3.38 and thickness of 0.508 mm.
It consists of a pair of rectangular patch dipoles which
are connected by a section of microstrip line, and a feed-
ing network which is based on a microstrip-to-slotline-to-
microstrip transition structure. The patch dipole, together
with a coupled U-shaped microstrip line and a feeding line
with circle end is printed on the top side of the substrate,
whereas a small rectangular ground plane with dimension of
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