Wideband Circularly Polarized Cavity-Backed Aperture Antenna With a Parasitic Square Patch

1. IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 13, 2014 197
Wideband Circularly Polarized Cavity-Backed
Aperture Antenna With a Parasitic Square Patch
Wenwen Yang, StudentMember, IEEE, and Jianyi Zhou, Member, IEEE
Abstract—A technique to design a wideband circularly polar-ized
(CP) cavity-backed aperture antenna is presented in this
letter. The proposed antenna consists of an aperture antenna, a
low-profile backed cavity, and a parasitic patch. Bidirectional
radiation of the aperture antenna is changed to unidirectional
radiation using the low-profile backed cavity. The parasitic patch
is adopted to provide a favorable axial-ratio (AR) bandwidth. The
proposed antenna combines the attractive features such as wide
impedance and AR bandwidths, compact size, high aperture effi-ciency,
as well as easiness of design, manufacture, and integration.
The antenna operates at 6-GHz band with the overall volume of
.Measured results show that the antenna
achieves a 10-dB impedance bandwidth of more than 70% and
a 3-dB AR bandwidth of 43.3% with a peak gain of 8.6 dBi.
Index Terms—Aperture antenna, cavity-backed, circularly po-larized,
wideband.
I. INTRODUCTION
CIRCULARLY polarized (CP) antennas have been widely
used in wireless communications such as satellite,
radar,and radio frequency identification (RFID) systems due
to its inherent advantage of insensitivity to depolarization.
However, they usually suffer from narrow impedance and
axial-ratio (AR) bandwidths. Many design methods have been
investigated to achieve wideband CP antennas [1]–[8]. The
printed aperture antennas are commonly used as they can
provide wideband bidirectional CP radiation [1], [2], but they
exhibit low radiation gain due to the bidirectional radiation
properties. The CP aperture antennas backed by ametal reflector
or cavity are proposed to offer unidirectional radiation [3]–[5].
As discussed in [4] and [5], the low-profile reflector or backed
cavity with height of less than deteriorated the CP
bandwidth of the antenna. In order to solve the problem, a
wideband CP aperture antenna backed by an artificial magnetic
conductor (AMC) reflector in place of the conducting metal
cavity that demonstrates a 33.2% AR bandwidth and a 36.2%
impedance bandwidth is proposed by Agarwal et al. [6]. How-ever,
the introduction of the AMC structure makes the design
procedure complicated, and the gain decreases rapidly in high
Manuscript received November 07, 2013; revised December 26, 2013; ac-cepted
January 02, 2014. Date of publication January 09, 2014; date of current
version February 05, 2014. This work was supported by the National Natural
Science Foundation of China under Grant 60702163, and in part by the National
Science and Technology Major Project of China under Grants 2010ZX03007-
002-01 and 2011ZX03004-003.
The authors are with the State Key Laboratory of Millimeter Waves,
Southeast University, Nanjing 210096, China (e-mail: wwyang@emfield.org;
jyzhou@seu.edu.cn).
Color versions of one or more of the figures in this letter are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LAWP.2014.2298252
frequency band. Furthermore, a broadband CP cavity-backed
slot antenna array with four linearly polarized disks located
in a single circular slot is presented in [7]. This antenna array
can achieve an AR bandwidth of 54.5% and an impedance
bandwidth of 92.1%. Nevertheless, the fabrication of the an-tenna
is complicated, and the size is much larger because of the
probe feeding structure and power division network. Recently,
several kinds of ring-patch CP antennas that use a parasitic ring
or patch suspended on the feeding structure to enhance the AR
bandwidth and gain are investigated in [8]–[10].
In this letter, a novel cavity-backed aperture antenna with a
parasitic patch is proposed. The circular-shaped aperture an-tenna
is optimized for achieving a wider AR bandwidth of the
overall antenna than the octagonal-shaped aperture antenna that
is described in [6]. The low-profile backed cavity is used to
offer favorable unidirectional radiation, and the parasitic patch
is adopted to enhance the CP performance that is deteriorated
by the low-profile backed cavity. The proposed antenna com-bines
the advantages of wide impedance and AR bandwidths,
high aperture efficiency, and easiness of design, manufacture,
and integration with RF circuits.
The following three sections constitute the main part of the
letter. The antenna geometry and design consideration are de-scribed
in Section II. Section III discusses the simulated results
and measured performances of the proposed antenna and shows
the comparison to the antenna from [6] and [7]. Section IV sum-marizes
the results obtained in the letter.
II. ANTENNA ELEMENT DESIGN
The configuration of the proposed cavity-backed aperture
antenna is shown in Fig. 1. The antenna consists of a parasitic
patch, a circular-shaped aperture antenna, and a low-profile
backed cavity. The proposed antenna is designed with the
center frequency of 6 GHz. The circular-shaped aperture an-tenna
with L-shaped stub feeding line is designed on an FR4
substrate for the convenience of comparison to [6]
and integration with other RF devices, whereas the parasitic
patch is fabricated on a substrate of Taconic TLX ( ,
loss tangent is 0.0019 at 10 GHz) to reduce the dielectric losses.
For measurement convenience, an impedance transformer is
introduced to transfer the input impedance of the antenna to
50 . The distance between the parasitic patch and aperture an-tenna
is defined as . A low-profile square cavity of width
and height is located beneath the aperture antenna.
Fig. 2 demonstrates the comparison of the octagonal-shaped
aperture and circular-shaped aperture based on numerous sim-ulations
by using Ansoft HFSS. It can be found that the cir-cular-
shaped aperture antenna has slightly wider AR bandwidth
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2. 198 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 13, 2014
Fig. 1. (a) Geometrical configuration of the proposed antenna. (b) Aperture
antenna layer. (c) Side view of the proposed antenna.
Fig. 2. Comparison of octagonal-shaped aperture and circular-shaped aperture.
in high frequency band. However, this phenomenon becomes
more obvious as the low-profile backed cavity and parasitic
patch are added. The octagonal-shaped cavity-backed aperture
antenna can achieve an AR bandwidth of 38%, while the cir-cular-
shaped cavity-backed aperture antenna can achieve an AR
bandwidth of 42%. Moreover, the return loss of the circular-shaped
cavity-backed aperture antenna seemsmuch better in the
higher frequency band. Hence, the circular-shaped aperture an-tenna
is adopted to obtain optimal performance.
The working principle of the circular-shaped aperture an-tenna
is studied first. As depicted in Fig. 3, the lower resonant
frequency is corresponding to the TE mode of the circular
slot, while the higher resonant frequency is caused by the TE
mode. However, the current distributions of the two resonant
modes indicate that the corresponding relations are just approx-imate
equivalent since they are not strictly symmetric along the
circular slot. From the above discussion, it can be concluded
Fig. 3. Current distributions at different times of
from left to right. (a) 5 GHz. (b) 6.7 GHz.
Fig. 4. Comparison of the proposed antenna in different cases.
that the resonant frequencies of the circular-shaped aperture are
mainly decided by the dimensions of and . The authors
would like to use
(1)
(where is the speed of light in free space) to calculate the res-onant
frequency of TE ; the second term in the equation is the
correction factor considering the presence of different dielectric
media on the two sides of the aperture antenna [11]. The center
frequency can be estimated once is obtained.
The effects of the low-profile backed cavity ( at
6 GHz) and parasitic patch are also studied as shown in Fig. 4.
It is seen that the AR bandwidth of the aperture antenna without
a backed cavity and parasitic patch is the widest, while the gain
is quite low due to its bidirectional radiation properties. How-ever,
the gain of the cavity-backed aperture antenna without
parasitic patch is about 1 dB less than the proposed antenna in
the whole band since the AR performance is deteriorated by
the low-profile backed cavity (more than 5 dB). In other words,
the parasitic patch can be used to complement the deteriorated
CP performance of the aperture antenna with a low-profile
backed cavity. According to [12], the parasitic patch should be
designed to resonant at the highest frequency in the AR band of
the aperture antenna to obtain the widest AR bandwidth since
the amplitudes difference of the patch’s two eigen-resonant

3. YANG AND ZHOU: WIDEBAND CP CAVITY-BACKED APERTURE ANTENNA WITH PARASITIC SQUARE PATCH 199
Fig. 5. Effects of varying on the return loss and axial ratio.
Fig. 6. Effects of varying on the return loss and axial ratio.
Fig. 7. Effects of varying and on the return loss and axial ratio.
currents becomes larger quickly in the band that is higher than
its resonant frequency.
To evaluate the effects of the antenna dimensions on the re-turn
loss and AR, a parametric study is performed. The simu-lated
return loss and AR of the proposed antenna by varying
are shown in Fig. 5. It is seen that the higher resonant fre-quency
is mainly determined by the parasitic patch, and the CP
performance in the AR band is affected by the coupling be-tween
the cavity-backed aperture antenna and parasitic patch.
The optimized return-loss and AR bandwidths can be achieved
when mm ( at 6 GHz), which is similar as de-scribed
in [8]. Fig. 6 shows the simulated return loss and AR
with different values of , where we can observe that the peak
Fig. 8. Photograph of the fabricated antenna.
Fig. 9. Simulated and measured return loss of the proposed antenna.
appearing in AR curve moves to the lower frequency as in-creases.
This is the effect due to the internal resonance of the slot
antenna loaded cavity. Therefore, the width of the cavity should
be adjusted to keep away from the internal resonance for an op-timal
performance The variations of return loss andARwith the
depth of the cavity and width of the parasitic patch are
depicted in Fig. 7. It can be seen that good CP performance in the
central zone of theAR band can be stillmaintained as long as the
parasitic patch grows properlywith decreasing. The achieved
AR bandwidth is about 42% for mm % for
mm % for mm , and 16%
for mm . However, the return loss of the an-tenna
changes slightly as the parameters mentioned above are
not dramatically varied.
From the discussion above, it can be learned that a favorable
CP performance can be achieved by suspending a patch above
the low-profile cavity-backed aperture antenna that has a dete-riorative
CP characteristic in itself as known in [4] and [5].
III. EXPERIMENTAL VERIFICATION
In order to validate the simulated performance of the pro-posed
antenna, prototypes of the antenna are fabricated by using
printed circuit board (PCB) process as shown in Fig. 8. Detailed
geometrical parameters of the proposed antenna are listed as fol-lows
(all in millimeters):
.
The simulated and measured of the proposed antenna
are plotted in Fig. 9. It can be found that the return loss is below
10 dB from 4.73 to more than 9 GHz. The overall tendencies

4. 200 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 13, 2014
Fig. 10. Simulated and measured radiation pattern of the proposed antenna.
(a) -plane at 5 GHz. (b) -plane at 5 GHz. (c) -plane at 7 GHz.
(d) -plane at 7 GHz.
Fig. 11. Simulated and measured axial ratio and gain of the proposed antenna.
TABLE I
PERFORMANCE COMPARISON AT 6 GHZ
of the simulation and measurement agree well with each other.
Fig. 10 depicts the simulated and measured radiation patterns
in the - and -plane, respectively, for 5 and 7 GHz. The CP
performance was measured using a rotating linearly polarized
transmitting horn antenna. The ripples in the pattern represent
the quality of the CP radiation. As illustrated in Fig. 11, the
measured 3-dB AR bandwidth is around 43.3% (4.8–7.4 GHz),
which is completely within the 2-VSWR impedance bandwidth.
The measured peak gain is 8.6 dBi at 7.4 GHz, while a gain of
more than 6 dBi in the whole AR bandwidth is achieved.
The comparison between the measured performances of
the proposed antenna and antennas reported in [6] and [7] is
listed in Table I. Compared to the antenna presented in [6],
the proposed antenna shows larger impedance and 3-dB AR
bandwidths while having slightly larger volume. On the con-trary,
the proposed antenna has smaller AR bandwidth than the
antenna presented in [8] while having much smaller volume.
Furthermore, the proposed antenna is much easier for design,
fabrication, and integration than both mentioned above.
IV. CONCLUSION
In this letter, a technique to design a wideband circularly po-larized
cavity-backed aperture antenna is presented.Aprototype
operating at 6 GHz has been designed and measured to validate
the concept. The prototype achieves an impedance bandwidth
of more than 70%, an AR bandwidth of 43.3%, and a peak gain
of 8.6 dBi. The proposed antenna exhibits promising character-istics
of wide impedance and AR band, high aperture efficiency
(more than 65% except 7–7.4 GHz), and easiness of design, fab-rication,
and integration.
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