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1872 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 62, NO. 4, APRIL 2014
A Wideband Circularly Polarized Antenna for
Microwave and Millimeter-Wave Applications
Mingjian Li and Kwai-Man Luk, Fellow, IEEE
Abstract—A wideband circularly polarized antenna with a single
feed is achieved by adjusting the shapes and dimensions of a lin-
early polarized magneto-electric (ME) dipole antenna. Experimen-
tally, an antenna prototype operating at around 2 GHz exhibits a
wide impedance bandwidth of 73.3% for and 46.6%
for , a 3-dB axial ratio bandwidth of 47.7% and
an antenna gain of 6.8 1.8 dBic. Based on the design operating
at lower microwave frequencies, a millimeter-wave antenna is de-
signed on a dielectric substrate. The antenna has a wide impedance
bandwidth of 56.7% covering 38.5 to 69 GHz and a
3-dB axial ratio bandwidth of 41% covering 45.8 to 69.4 GHz, over
which the antenna boresight gain varies from 5 to 9.9 dBic. This
validates that the design can be realized on a single-layer printed
circuit board.
Index Terms—Circularly polarized antenna, magneto-electric
dipole, wideband antenna, 60-GHz radio.
I. INTRODUCTION
CIRCULARLY POLARIZED (CP) radiation is useful
for multi-path interference suppression and polarization
mismatch reduction [1]. Owing to these features, circular
polarization is highly desirable for many applications such as
radar, satellite and millimeter-wave systems [2]. The basic op-
erating principle of a circularly polarized antenna is to radiate
two orthogonal field components with equal amplitude and in
phase quadrature. During the last years, many omni-directional
CP antennas with good performances have been investigated in
[3]–[5]. However, for some wireless communication systems,
unidirectional CP antennas are required because security and
efficiency issues should be considered by system designers.
The slot antenna is a competitive solution for its simple
structure. It can be divided into two categories: wide and ring
slot types. Different techniques such as employing parasitic
patch, L-shaped slot or artificial magnetic conductor (AMC)
reflector have been reported to enhance the AR bandwidth of
wide slot antenna, but these designs suffer from a large back
lobe [6]–[8]. Normally, the ring slot type has low back radiation
but a narrower AR bandwidth less than 20% [9]. By using a
pair of grounded hat-shaped patches, the AR bandwidth can
be enhanced to 36% [10]. Another approach for generating CP
Manuscript received June 29, 2013; revised November 20, 2013; accepted
December 19, 2013. Date of publication January 09, 2014; date of current ver-
sion April 03, 2014. This work was supported in part by the Research Grants
Council of the Hong Kong SAR and CityU Strategic Research Grant. [Project
No. CityU 9041677 and SRG7008113].
The authors are with the State Key Laboratory of Millimeter Waves, City
University of Hong Kong, Hong Kong (e-mail: mingjiali2-c@my.cityu.edu.hk).
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.2014.2298246
radiation is microstrip patch antenna which can be classified into
three categories: single feed, dual feed and sequentially rotated
patches. The single-fed type does not need an external circuitry
to excite CP wave, and therefore it has the simplest structure
among the three types. Unfortunately, its AR bandwidth is usu-
ally very narrow, basically less than 10% [11], [12]. A wider AR
bandwidth of over 30% can be achieved by employing a dual-fed
structure or sequentially-rotated patch array [13]–[15], but these
methods require feeding networks and occupy larger space. The
cross-dipole antenna is another way for producing CP radiation
and can be either dual-fed or single-fed. In [16], by adding para-
sitic loop resonators beside the dipole arms, the AR bandwidth
has been enhanced considerably to over 30%. In 1961, M. F.
Boister developed a new type of single-fed crossed dipole [17].
The basic concept is that the real parts of input admittances are
equal and the phase angles of the input admittances differ by 90 ,
which is realized by adjusting the lengths of the dipole arms.
Applying this concept, a cross-bowtie antenna with a complex
cavity has enlarged the AR bandwidth to 39% [18].
In addition, circularly-polarized antennas have been consid-
ered for millimeter-wave applications. Since the unlicensed
spectrum of 57–64 GHz has been assigned for short-range
wireless communications, several CP arrays with remarkable
performances have been proposed for this frequency band. In
[19], an AR bandwidth of more than 7 GHz is achieved by
positioning a slot-coupled rotated strip above a slot etched in
the broadside wall of a substrate integrated waveguide. The use
of two open tape rings realizing CP radiation at 60 GHz has
been presented in [20]. By employing the sequentially rotation
technique, a grid array, U-slot patch array and slot array exhibit
wide AR bandwidths of 12.9%, 20.5% and 34.6% in [21]–[23],
respectively.
Recently, several magneto-electric (ME) dipole antennas
with linearly polarization or dual polarization were presented
in [24]. The basic design, consisting of a magnetic dipole and
an electric dipole, exhibits a wide impedance bandwidth and
stable gain. In this paper, based on a modified linearly polarized
magneto-electric dipole antenna and the single-fed cross-dipole
concept, new wideband circularly polarized antennas are de-
signed for both 2 GHz and 60 GHz bands. Our designs are
single-fed antennas which exhibit very wide axial ratio and
impedance bandwidths.
II. MICROWAVE CIRCULARLY POLARIZED ANTENNA DESIGN
A. Linearly Polarized Antenna
Fig. 1 depicts a new linearly polarized magneto-electric
dipole antenna which consists of four metallic plates, four
0018-926X © 2014 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.
LI AND LUK: A WIDEBAND CIRCULARLY POLARIZED ANTENNA FOR MICROWAVE AND MILLIMETER-WAVE APPLICATIONS 1873
Fig. 1. Perspective view of the linearly polarized antenna.
TAB L E I
DIMENSIONS FOR THE LINEARLY POLARIZED ANTENNA
metallic posts, an L-shaped probe and a planar reflector.
For realizing an ME dipole antenna, the electric dipole and
magnetic dipole should be excited simultaneously [24]. Two
planar half-wave electric dipole antennas are formed by the
metallic plates . And two vertically
quarter-wave shorted microstrip patch antennas ,
radiating as a magnetic dipole, are formed by the inner surfaces
of two adjacent metallic posts together with the planar reflector
between them. For exciting this antenna, the L-shaped probe
is placed between the metallic posts which act as the electrical
ground of the probe. This combination forms a stripline like
transmission line. The distance between the probe edge and
the post is . Detailed dimensions of the linearly
polarized antenna are summarized in Table I.
According to simulations, this antenna exhibits a wide
impedance bandwidth of93%from1.62to4.42
GHz as shown in Fig. 2. The antenna boresight gain is about
9 dBi at lower frequencies and decreases significantly after 3
GHz, which is because the radiation main beam is divided into
two beams due to the higher order mode operation at higher
frequencies. Basically, there are four resonances according to
the SWR curve in Fig. 2. The 1st and 3rd resonances at 1.75
and 2.6 GHz respectively are mainly influenced by the length
of the electric dipole, . And the 2nd and 4th resonances at
3.4 and 4.25 GHz respectively are mainly controlled by the
height of the antenna, H, which is the length of the shorted
microstrip patch antennas. To further investigate the antenna
operating principle, the currents on the antenna at different
times and T/4 are depicted as shown in Fig. 3, where
T is the period of oscillation at 2.4 GHz. At time ,the
currents on the planar electric dipoles attain maximum strength
whereas the currents on the radiating slot edges of the shorted
microstrip patches attain minimum strength. Thus, the currents
on the electric dipole are dominated. At time ,the
currents on the planar electric dipoles attain minimum strength
whereas the currents on the radiating slot edges of the shorted
microstrip patches attain maximum strength. Thus, the currents
Fig. 2. Simulated SWR and boresight gain of the linearly polarized antenna.
Fig. 3. Current distribution of the linearly polarized antenna at 2.4 GHz.
on the magnetic dipole are dominated. At time ,the
currents on electric dipoles are dominated again with opposite
direction to the currents at time .Attime ,
the currents on the magnetic dipole are dominated again with
opposite direction to the currents at time . Hence, two
degenerated modes with nearly the same amplitudes and 90
phase differences are excited by the planar electric dipoles and
shorted microstrip patch antennas separately.
B. Circularly Polarized Antenna
Fig. 4(a) shows the proposed circularly polarized magneto-
electric dipole antenna. Standing from the linearly polarized
ME dipole antenna described above, one pair of the diagonally
opposite metallic plates is truncated at its corners in order to
increase its resonant frequency. The other pair is added with
hook-shaped strips in order to reduce its resonant frequency.
And two of the four gaps are filled with small metallic blocks in
order to increase the resonant frequency of the equivalent ver-
tically quarter-wave shorted patch antenna. Thus, the proposed
CP antenna consists of a pair of metallic plates with extended
strips, a pair of metallic plates with corner truncated, a pair of
U-shaped posts, an L-shaped probe feed and a planar reflector.
Each U-shaped post is formed by two posts with a small metallic
block between them. Fig. 4(b) depicts the top and side views
of the proposed antenna. The metallic plates are direct-current
grounded through the U-shaped posts. The proposed antenna
with the L-shaped probe is mounted on a planar reflector of size,
. A SMA connector is placed under
1874 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 62, NO. 4, APRIL 2014
Fig. 4. Microwave circularly polarized antenna. (a) Perspective view, (b) top
and side views.
TAB L E I I
DIMENSIONS FOR THE CIRCULARLY POLARIZED ANTENNA
the reflector for transmitting signal to the L-shaped probe. In
making the prototype, the plates, posts and reflector were made
of aluminum due to its advantages of low-cost and corrosion
prevention. Copper strip (thickness ) was used to
make the probes for ease of bonding. Detailed dimensions of
the antenna are summarized in Table II.
Fig. 5. Effect of the length of the hook-shaped strip (keeping
and ).
Fig. 6. Effect of the length of the truncated corner (keeping
and ).
C. Antenna Analysis
The crossed electric dipoles have dipole arms in different
lengths. The vertically quarter-wave shorted patch antennas
have different depths by adding the metallic blocks between the
metallic posts. Thus, the magneto-electric dipole can generate
a wideband CP radiation. Fig. 5 shows the simulated axial ratio
(AR) and standing wave ratio (SWR) versus the hook-shaped
strip length . It can be seen that
theARandSWRareinfluenced by this parameter significantly.
Larger deteriorates the impedance matching and causes a
larger AR at higher frequencies, whereas smaller enlarges
the in-band AR value. Hence, was set to be 25 mm. Fig. 6
shows the simulated AR and SWR versus
.As is increased, the 3-dB AR band is shifted
to higher frequencies and both AR and SWR become larger.
Although the minimum AR is generated when ,
was selected in order to achieve a wide over-
lapping AR and SWR bandwidth.
To further understand the antenna operating principle, the
currents on the antenna at different times ,T/4,T/2and
3T/4 are depicted as shown in Fig. 7, where T is the period of
LI AND LUK: A WIDEBAND CIRCULARLY POLARIZED ANTENNA FOR MICROWAVE AND MILLIMETER-WAVE APPLICATIONS 1875
Fig. 7. Current distribution at 1.8 GHz. (a) ,(b) ,(c) ,
(d) .
oscillation at 1.8 GHz. It can be seen that electric currents on the
antenna surface rotate in the counter-clockwise direction. Like
the linearly polarized antenna, the CP antenna also can be ex-
cited with two degenerated modes which come from the electric
dipole and the microstrip patch antenna separately. At and
T/2, the currents on the apertures of the shorted microstrip patch
antennas attains maximum and are in opposite directions for the
two times. Thus, the equivalent magnetic current from the patch
apertures is along x-axis and has 90 phase difference with the
electric surface currents on the patch apertures. At and
3T/4, the currents on the edges of the cross planar electric dipole
antennas attains maximum and are in opposite directions. It can
be seen that the equivalent electric current on the electric dipoles
is also along x-axis. Hence, the equivalent magnetic and electric
currents are along the same direction and in phase, which con-
firms that the proposed antenna can generate a CP radiation [25].
D. Antenna Performances
An antenna prototype was constructed and tested, as shown
in Fig. 8. Measurements on SWR, axial ratio, gain and radiation
patterns were accomplished by Agilent N5230A network ana-
lyzer and SATIMO complex antenna system.
Fig. 9 shows simulated and measured SWRs and gains. The
simulated impedance bandwidth is 52.3% from
1.58 to 2.7 GHz and 60.7% from 1.48 to 2.77 GHz.
The measurements indicate that the antenna prototype can
achieve a wide impedance bandwidth of 46.6%
from 1.68 to 2.7 GHz and 73.3% from 1.32 to
Fig. 8. Antenna prototype of the microwave CP antenna.
Fig. 9. Simulated and measured SWRs and gains of the microwave CP antenna.
Fig. 10. Simulated and measured axial ratios of the microwave CP antenna.
2.85 GHz. Within the 1.5:1 SWR band, the antenna boresight
gain varies in the range of 5 to 8.6 dBic. Fig. 10 plots the mea-
sured and simulated antenna axial ratios. It can be seen that
the measured 3-dB axial ratio band is 47.7% from 1.69 GHz
to 2.75 GHz, which is generally covered by the
operating band. The measured and simulated radiation patterns
at 1.8, 2.1 and 2.4 GHz are depicted in Fig. 11. It can be seen
that the measured right-hand circularly polarized (RHCP) com-
ponents in H- and V- plane patterns, i.e. the co-polarization,
exhibit unidirectional radiation characteristic. The left-hand
circularly polarized (LHCP) components in both planes are the
cross-polarization of the radiation pattern. The 3-dB axial ratio
1876 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 62, NO. 4, APRIL 2014
Fig. 11. Simulated and measured radiation patterns of the microwave CP an-
tenna at 1.8, 2.1 and 2.4 GHz.
TAB L E I II
KEY DATA O F WIDEBAND CIRCULARLY POLARIZED ANTENNAS
All designs for comparison are unidirectional CP antennas.
is the free space wavelength referring to the antenna center frequency.
beamwidth in H-plane is quite wide, generally larger than 100 .
But the 3-dB axial ratio beamwidth in V-plane is narrow and
decreases with frequency from 60 at 1.8 GHz to 40 at 2.4 GHz.
It is noted that as the ground plane size is decreased, the upper
edge frequency of 3-dB axial ratio is shifted upward and the
back lobe level is increased slightly. Furthermore, the key data
of our work and other designs is summarized in Table III. Our
work achieves wide impedance and 3-dB AR bandwidths but the
proposed antenna suffers from a relatively large antenna size.
Fig. 12. Millimeter-wave circularly polarized antenna. (a) Perspective view,
(b) antenna prototype, (c) top view.
III. MILLIMETER-WAV E CIRCULARLY POLARIZED ANTENNA
DESIGN
A. Millimeter-Wave Antenna Geometry
A millimeter-wave circularly polarized magneto-electric
dipole antenna is proposed as shown in Fig. 12. This antenna
was printed on a conventional dielectric substrate, which vali-
dates that the original design in Fig. 4 can be realized by using
single-layer printed circuit board technique. Due to the dielec-
tric substrate, the antenna volume and weight can be reduced
significantly. More importantly, the impedance and 3-dB axial
ratio bandwidths still remain wide. In the experiment, we chose
the Duroid 5880 (thickness ,and
at 60 GHz) as our substrate. It can been seen
from Fig. 12(a) that the metallic posts and the vertical portion
of the L-shaped probe are realized by using plated through hole
technology. All via holes have a diameter of .
Eight of them are located at the inner edge of the rectangular
metallic patches and connected to the ground for forming verti-
cally quarter-wave shorted patch antennas. One of them (Probe
via hole) under the horizontal patch of the L-shaped probe
accomplishes a transmission line together with the adjacent via
holes as electrical ground of the probe. According to the HFSS
simulations, this transmission line has characteristic impedance
of about 42 ohm and introduces small inductance which can be
easily compensated. A W-type connector (Anritsu: W1-103F)
is launched underneath the antenna fixture for transmitting
signal to the L-shaped probe. Four metallic screws were used
to fix the antenna single-layer substrate on the antenna fixture
LI AND LUK: A WIDEBAND CIRCULARLY POLARIZED ANTENNA FOR MICROWAVE AND MILLIMETER-WAVE APPLICATIONS 1877
TAB L E I V
DIMENSIONS FOR THE MILLIMETER-WAV E CIRCULARLY POLARIZED ANTENNA
Fig. 13. Simulated and measured SWRs and gains of the millimeter-wave CP
antenna.
which has the dimension of 10 mm 10 mm. Note that since
in practice the antenna will be connected to a millimeter wave
system directly, the antenna fixture is not a compulsory part
and the ground plane can be reduced so as to achieve a smaller
antenna size. Detailed dimensions of the millimeter-wave
antenna shown in Fig. 12(c) are summarized in Table IV.
B. Millimeter-Wave Antenna Performances
As shown in Fig. 12(b), an antenna prototype was fabricated
and tested to verify the millimeter-wave antenna design. Mea-
surements on impedance bandwidth, gain and radiation patterns
were accomplished by a millimeter wave band Agilent E8361A
Network Analyzer with N5260-60003 waveguide T/R module
and an NSI spherical near-field millimeter-wave measurement
system. The simulations implemented by the EM simulation
software Ansoft HFSS.
Fig. 13 shows simulated and measured SWRs and bore-
sight gains. The simulated and measured impedance band-
widths are 58.1% (38.2–69.5 GHz) and 56.7%
(38.5–69 GHz). In the operating frequency range, the simulated
boresight gain varies in the range of 5 to 9.9 dBic. The measured
gain is consistent with simulations over a frequency band of 50
to 65 GHz. The reason that the radiation measurement was done
only within this frequency band instead of the whole operating
band is that our millimeter-wave measurement system provides
the operation from 50 to 65 GHz at the moment. Fig. 14 plots
simulated and measured antenna axial ratios. It can be seen that
the simulated 3-dB axial ratio band is 41% (45.8–69.4 GHz).
Fig. 14. Simulated and measured axial ratios of the millimeter-wave CP
antenna.
Fig. 15. Simulated and measured radiation patterns at 50, 60 and 65 GHz.
The measured 3-dB axial ratio from 50 to 65 GHz is lower
than 3 dB and higher than the simulated value generally.
The measured and simulated radiation patterns at 50, 60 and
65 GHz are depicted in Fig. 15, and measurements basically
1878 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 62, NO. 4, APRIL 2014
agree with simulations. The co-polarization component of the
radiation pattern is LHCP, which is not the same as the design at
microwave band. The reason is that both designs at 2 GHz and
60 GHz have mirrored geometries of each other. In addition, it
can be observed that the cross-polarization component (RHCP)
is higher than the simulated values mainly due to substrate de-
formation caused by the uneven surface of the antenna fixture.
Like the design at 2 GHz, the 3-dB axial ratio beamwidth in
H-plane is much wider than the beamwidth in V-plane which
increases with frequency.
IV. CONCLUSION
Two single L-probe fed wideband circularly polarized mag-
neto-electric dipole antennas, operating at microwave and mil-
limeter-wave bands, have been investigated respectively. The
microwave antenna was designed by modifying the shapes and
dimensions of a linearly polarized magneto-electric dipole an-
tenna. The millimeter-wave antenna was developed based on the
microwave design. Experimentally, an antenna prototype was
constructed for 2 GHz band. It achieves an impedance band-
width of 73.3%, a 3-dB axial ratio bandwidth of
47.7% and a gain of 6.8 1.8 dBic. The oper-
ating band almost covers the whole3-dBARband.Themil-
limeter-wave CP antenna was fabricated for 60 GHz band by
using printed circuit board technique. This antenna exhibits an
impedance bandwidth of 56.7%, a 3-dB axial ratio bandwidth
of 41% and a gain of 7.45 2.45 dBic. This attempt has proved
this circularly polarized antenna design can be realized by using
the conventional PCB technique which is low-cost and fabri-
cated easily.
ACKNOWLEDGMENT
The authors would like to thank K. B. Ng for helping with the
antenna measurements.
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Mingjian Li (S’10) received the B.Sc. (Eng.) degrees
in electronic and communication engineering from
City University of Hong Kong in 2010, where he is
currently pursuing Ph.D. degree.
His recent research interests include wideband
antennas, millimeter-wave antennas and arrays, base
station antennas, circularly-polarized antennas and
small antennas.
Mr. Li received the Honorable Mention at the stu-
dent contest of 2011 IEEE APS-URSI Conference
and Exhibition held in Spokane, US. He was awarded
the Best Student Paper Award (Second Prize) in the 2012 IEEE International
Workshop on Electromagnetics (IEEE iWEM2012) held in Chengdu, China.
LI AND LUK: A WIDEBAND CIRCULARLY POLARIZED ANTENNA FOR MICROWAVE AND MILLIMETER-WAVE APPLICATIONS 1879
Kwai-Man Luk (M’79–SM’94–F’03) was born
and educated in Hong Kong. He received the B.Sc.
(Eng.) and Ph.D. degrees in electrical engineering
from The University of Hong Kong in 1981 and
1985, respectively.
He joined the Department of Electronic Engi-
neering, City University of Hong Kong, in 1985
as a Lecturer. Two years later, he moved to the
Department of Electronic Engineering, Chinese
University of Hong Kong, where he spent four years.
He returned to the City University of Hong Kong in
1992, and is currently Chair Professor of Electronic Engineering and Director
of State Key Laboratory in Millimeter waves (Hong Kong). He is the author
of three books, nine research book chapters, over 290 journal papers and 220
conference papers. He has received five US and more than 10 PRC patents. His
recent research interests include design of patch, planar and dielectric resonator
antennas, and microwave measurements.
Prof. Luk is a Fellow of the Chinese Institute of Electronics, PRC, a
Fellow of the Institution of Engineering and Technology, UK, a Fellow of the
Institute of Electrical and Electronics Engineers, USA and a Fellow of the
Electromagnetics Academy, USA. He is Deputy Editor-in-Chief of PIERS
journals. He was a Chief Guest Editor for a special issue on “Antennas in
Wireless Communications” published in the PROCEEDINGS OF THE IEEE
in July 2012. He was Technical Program Chairperson of the 1997 Progress in
Electromagnetics Research Symposium (PIERS), General Vice-Chairperson
of the 1997 and 2008 Asia-Pacific Microwave Conference (APMC), General
Chairman of the 2006 IEEE Region Ten Conference (TENCON), Technical
Program Co-chairperson of 2008 International Symposium on Antennas and
Propagation (ISAP), and General Co-chairperson of 2011 IEEE International
Workshop on Antenna Technology (IWAT). He received the Japan Microwave
Prize at the 1994 Asia Pacific Microwave Conference held in Chiba in De-
cember 1994, and the Best Paper Award at the 2008 International Symposium
on Antennas and Propagation held in Taipei in October 2008. He was awarded
the very competitive 2000 Croucher Foundation Senior Research Fellow in
Hong Kong and the 2011 State Technological Invention Award (2nd Honor)
of China.
