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IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 60, NO. 6, JUNE 2012
A Compact Outer-Fed Leaky-Wave Antenna Using
Exponentially Tapered Slots for Broadside Circularly
Polarized Radiation
Shih-Kai Lin, Student Member, IEEE, and Yi-Cheng Lin, Senior Member, IEEE
Abstract—A novel leaky-wave slot antenna for broadside circularly polarized (CP) radiation is presented. The antenna simply
consists of a slotline for the feed, an exponentially tapered slot for
the radiator, and a shaped end for the terminator. Separating the
feed from the curved radiating slot makes the presented antenna
unique in design and compact in size. The resulting antenna yields
inherent broadband characteristics of leaky-wave antennas on the
input impedance, axial ratio (AR), and radiation gain. Particularly,
to solve the tilt-beam problem commonly seen in leak-wave antennas, we proposed an extended dual-slot design that successfully
achieved the pattern stability over a wide bandwidth. An antenna
module with a reflector for unidirectional pattern applications was
also developed. The measured performances of the final prototype
were found to exhibit 10-dB return loss bandwidth of 40%, 3-dB
AR bandwidth of 53%, and half-power gain bandwidth of 59%
with CP gain level about 8 dBic. The measured and simulated results were well consistent with each other.
Index Terms—Broadband, circular polarization, leaky-wave antennas, slot antennas, unidirectional patterns.
C
I. INTRODUCTION
IRCULARLY POLARIZED (CP) antennas have gained
increasing attention in many applications, such as satellite
communications, radio frequency identification (RFID) reader
[1] and the global positioning system (GPS) [2]. In order to increase the capacity or interoperate several systems in the same
module, CP antennas of broad bandwidth in axial ratio (AR)
and broadside radiation are desirable in various wireless systems. The most often used broadband CP antennas are traditional equiangular spiral antennas [3]. Spiral antennas inherently have broadband characteristics on the input impedance,
the AR, and gain patterns, mainly resulting from the mechanism
of leaky-wave radiation. However, the spiral antenna in general
is large, with several wavelengths required. Particularly, the antenna feed is complex, where the center-fed scheme, the vertical
joint of balun structure, and the strict magnitude/phase balance
are difficult to design and implement and expensive to fabricate.
Manuscript received December 04, 2010; revised November 27, 2011, acceptance December 02, 2011. Date of publication April 12, 2012; date of current
version May 29, 2012. This work was supported in part by the National Science
Council of Taiwan under Grant NSC-99-2219-E-002-006 and NSC-99-2622-E002-017-CC3, and in part by Excellent Research Projects of National Taiwan
University under Grant 99R80302.
S.-K. Lin is with the Graduate Institute of Communication Engineering, National Taiwan University, Taipei 10617, Taiwan.
Y.-C. Lin is with the Department of Electrical Engineering, National Taiwan
University, Taipei 10617, Taiwan (e-mail: yclin@cc.ee.ntu.edu.tw).
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.2012.2194639
Of the planar type of antenna structures, the printed patch
antennas [4]–[7] have been widely used with advantages of
simple feeding schemes and the low-cost manufacturing with
the printed circuit board (PCB) technology. The printed CP
patch antennas have been intensively investigated in recent
years with different emphases, including the antenna design
with size reduction [8], [9], the techniques of sequentially
rotated arrays [10]–[13] for enhanced bandwidth, and the
design of feeding schemes [14], [15] for effective coupling.
Nevertheless, the printed planar CP patch antennas in general
have limited usage because of their narrow bandwidth in the
input impedance and the axial ratio. To improve the bandwidth,
the leaky-wave concept has also been employed in microstrip
CP antenna design. For example, an open-loop antenna with
coaxial probe fed was investigated in [16] and a dual-fed antenna with two orthogonal feeding lines in 90 phase difference
was proposed in [17]. In general, these microstrip leaky-wave
antennas require a relatively large area.
Recently, printed aperture and slot CP antennas have been
intensively studied because of their advantages of wide bandwidths, low costs, and simple feeding schemes [18]–[21]. These
antennas perform bidirectional patterns and have limited usage.
To achieve unidirectional patterns with enhanced peak gain, the
aperture and slot antennas are usually integrated with a backed
cavity or reflector [22]–[24]. Of these aperture antennas, a new
CP aperture antenna was recently proposed [25] and comprehensively investigated [26]. The curved leaky-wave currents
along the aperture edge were found and the CP radiation was
achieved through the design of the curved feed. However, the
antenna had a drawback in structure that varying the tapered
feed may also affect the input impedance and CP radiation simultaneously, making the optimization difficult.
In this paper, we present a novel leaky-wave CP antenna
using a curved tapered slot, instead of an aperture [26], to generate broadband CP radiation at broadside. The simple outer-fed
design makes the antenna fully planar and easy to integrate with
RF circuits or expand to array design on the same PCB substrate, leading to a low-cost manufacturing. This paper comprehensively covers the design concept, the parametric study,
and experimental results, including an extended design that resolves the practical beam-tilt problems and unidirectional pattern issues.
II. ANTENNA STRUCTURE AND DESIGN CONCEPT
Fig. 1 shows the geometry and coordinates of the presented
antenna. The antenna comprises three portions: a uniform slot-
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LIN AND LIN: COMPACT OUTER-FED LEAKY-WAVE ANTENNA USING EXPONENTIALLY TAPERED SLOTS FOR BROADSIDE CP RADIATION
2655
Fig. 2. Simulated surface current distribution on the ground plane in four dif,
). (Parameters:
,
ferent time steps. (
,
,
,
,
).
Fig. 1. (a) Geometry of the presented antenna, where the exponentially curved
slot is composed of three curves: , , and . (b) Detailed geometry of the
microstrip-to-slotline transition section.
line for the feed, a curved tapered slot for the CP radiation, and
a closing end for the termination. The radiating tapered slot is
formed by two exponential curves
(A to B) and
(C to D),
while the termination is determined by a circular arc
(B to
D). The exponential curves and of the tapered slot are expressed by the following equations in polar coordinates:
(1)
(2)
is the initial radius of the -th exponential curve with
where
respect to the origin denoted by O;
is the growing rate; and
is the stop angle of the curve end. The curve
is simply a
circular arc connecting the ends of
and .
The slotline feed can be directly connected to a slotline transmission line or, alternatively, converted to a microstrip transmission line which is commonly used in RF system. In this
paper, we incorporated a slotline-to-microstrip transition and
specified the antenna port with a microstrip feed for testing purpose. A nominal quarter-wavelength slotline-to-microstrip transition was employed, where the detailed geometry is shown
in Fig. 1(b). Note that a loaded T-stub instead of a straight
open-end was applied to the microstrip line end for avoiding
the extrusion to the edge of curved slot.
The feeding scheme of the presented antenna has several
advantages. Compared to the traditional spiral antennas [3]
fed at the center and terminated at outer end, the proposed
antenna is fed from outside and terminated near the center. This
outer-fed design facilitates the connection to the RF circuits
or array feeding networks all in a fully planar PCB substrate,
leading to low manufacturing cost. Additionally, separating the
antenna feed apart from the radiating area (the tapered slot) may
effectively consolidate the design process. For example, when
we tuned the taper slot parameters for optimal radiation performances, the impedance matching was almost not affected.
The presented antenna’s operation rests on the curved leakywaves radiating from the exponentially tapered slot. The excited
surface currents on the ground plane mainly propagate along the
tapered slot edges, thereby radiating bidirectional CP patterns
in broadside directions (
z-axis). Fig. 2 shows the surface
current distribution in different time steps. The dashed-line arrows show the standing waves resonance near the shorting end
of slotline feed, while the solid-line arrows reveal the null (constant phase) of the traveling surface currents propagating along
the curved slot. In this illustration, the surface currents propagate counter clockwise in the x-y plane; hence, the radiated
waves are of right-handed CP (RHCP) at the
-axis and of
left-handed CP (LHCP) at the
-axis. At the end of the slot,
the traveling waves were terminated by a curve with a simple
semi-circle in this study.
The theoretical modeling of the presented antenna in terms
of leaky-waves may consider a curved and tapered line-source
propagating a leaky-mode, which complex propagation constant
is modulated along the width-tapered curved slot in order to
obtain the desired circularly-polarized radiation pattern pointing
at broadside. By the equivalent principle, it can be considered as
magnetic currents confined in the slot with complex propagation
constants. For this purpose, a leaky-mode analysis and design
theory for curved tapered leaky-modes could be applied, such as
the one presented in [27]. However, the theoretical leaky-mode
analysis and design is out of the scope of this paper work.
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IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 60, NO. 6, JUNE 2012
Fig. 3. Axial ratio at the zenith versus frequency with different
.
Fig. 4. RHCP gain at the zenith versus frequency with different
.
III. DESIGN GUIDELINES AND PARAMETRIC STUDY
The exponential curves and are the main parameters to
determine the CP radiation performances of the presented antenna. The initial value for the radius
of
is approximated
as
, where
is the free space wavelength at the
center frequency. The initial radius
of the inner curve
is selected slightly less than
according to the width of the
slotline feed, which is directly related to the input impedance.
In this study, we designed the characteristic impedance of the
slotline about 100 ohms. Besides
the input impedance of the
presented antenna can also be affected by the dielectric constant
and thickness of the PCB substrate.
The important design parameters of the presented antenna
also include the growing rates (
and
) of the exponential
curves
and
and the ground plane dimension (L). Effects
of these parameters on the CP radiation and impedance performances are described as follow.
A. Effects of the Growing Rate
of
The smaller the growing rate
the shorter the total length
of . In general, the length of curve
(controlled by
)
determined the first AR resonance at the lower frequency in the
operating band. Fig. 3 shows the axial ratio (AR) at the zenith
( -axis) of the proposed antenna versus the frequency with
different
in the range from 0.01 to 0.03. From Fig. 3,
one can observe that
apparently affects the AR level at the
lower frequencies in the band. For the experimental prototype
antenna,
was chosen by 0.01 from the optimization. The
input impedance of the antenna is not influenced much by .
To fairly compare the proposed antenna with traditional uniform-width spiral slot antennas, we simulated a reference design
using small spiral slot in a similar ground plane, as shown in
Fig. 3. The reference design was modified from a curl antenna
[28] that employed a horizontal metal spiral arm with a small
number of turns to radiate CP patterns at broadside. Note that the
presented antenna contains a tapered slot with two different increasing rates, while the reference design uses a uniform-width
spiral slot with a single increasing rate. The main parameters
for the reference design are:
,
,
,
, with a same ground plane
Fig. 5. Axial ratio at the zenith versus frequency with different
.
as the presented antenna. It was observed that the reference antenna has a 3-dB axial ratio bandwidth of only 4.5% (with a
co-pol gain level about 1.6 dBic). By contrast, the proposed
antenna uses two different increasing rates to design the two
surface currents along the edges of the tapered slot leading to
dispersive AR resonances, which broaden the AR bandwidth
to nearly 40%. Moreover, the presented antenna also avoids
the out-of-phase cancellation of the surface currents on the slot
edges, thus the CP gain at zenith may reach a good level about
5.3dBic, as shown in Fig. 4.
B. Effects of the Growing Rate
of
Figs. 4 and 5 show the radiation gain and AR, respectively, of
the proposed antenna while
varies in the range from 0.15 to
0.35. The total length of the exponentially curved decreases
as
varies from 0.15 to 0.35. As expected, Figs. 4 and 5
show that the resonant frequency of RHCP gain and AR shifts to
a higher frequency as
decreases. The antenna peak gain was
found about 5dBic. When
increases further to the value of
, the exponentially curved slot approaches a uniform-width
slot transmission line that contains more guided-mode waves
and reduces the leaky-wave radiation. This explains why the
mentioned reference design has a negative gain ( 1.6 dBic).
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LIN AND LIN: COMPACT OUTER-FED LEAKY-WAVE ANTENNA USING EXPONENTIALLY TAPERED SLOTS FOR BROADSIDE CP RADIATION
2657
Fig. 6. Measured and simulated reflection coefficients of the prototype antenna,
,
,
,
,
,
where
,
,
,
, and
,
,
, and
.
In the case of
, Fig. 5 reveals that the resonant frequency of AR even shifts out of the operating band of 5–6 GHz.
Considering both AR level and the flatness of CP gain bandwidth, the
was chosen as 0.25 in the prototype antenna.
C. Effects of the Ground Plane Size
In a simulation of the presented antenna with the ground plane
size L varying from 55 mm to 65 mm, we found that the antenna
gain and input impedance were slightly affected; however, the
axial ratio was relatively influenced. The co-polarized gain becomes greater at lower frequency when a larger ground plane
is used. The ground plane size influences the AR performance
at the higher frequency in the operating band, thereby determining the performance of AR bandwidth. It appears that the
surface current distribution may be affected by the outer conformal boundary of the ground plane. The reason for the conformal ground plane shape in this study is mainly for maximizing the AR bandwidth at the zenith. For illustrating the optimal process based on a given finite ground size, the parameter
L of the prototype was chosen as 60 mm in the results of later
section.
Fig. 7. Measured and simulated radiation patterns of the proposed antenna at
different frequencies: (a) 4.7 GHz, (b) 5.5 GHz, and (c) 6.5 GHz.
IV. EXPERIMENTAL VERIFICATION
To verify the proposed CP leak-wave slot antenna, we fabricated a prototype with an FR4 PCB having a dielectric constant
of 4.4, a loss tangent of 0.02, and a thickness of 0.6 mm. The
prototype antenna design was optimized at a center frequency of
5.5 GHz with the feed port implemented by a tapered microstrip
line varied from 0.5 mm to 1.27 mm for the 50-ohm SMA connector. Fig. 6 shows the measured and simulated reflection coefficients of the experimental prototype. The measured 10-dB
return loss bandwidth is very wide, about 70% from 3.9 GHz to
8.0 GHz, which agrees well with simulation results.
Fig. 7(a)–(c) shows the measured and simulated patterns of
the prototype antenna in two elevation cuts (the XZ plane and
the YZ plane) at three different frequencies. The CP measurements here were conducted by measuring two orthogonal complex fields (V-pol and H-pol) of the antenna in a far-field anechoic chamber. Fig. 7 shows that the peak of the CP gain pat-
Fig. 8. Measured and simulated gain and AR at the zenith of the presented
antenna.
terns tilts off the zenith as the frequency increases. As a result,
the CP gain at the zenith ( -axis) drops rapidly at higher frequencies, although the peak gain maintains the same level. To
this point, we summarized the frequency response of the CP radiation performances in Fig. 8. It shows the RHCP gain and the
axial ratio at the zenith as a function of frequencies. Also in-
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Fig. 9. Geometry and dimensions of the dual-slot design of the presented antenna.
Fig. 10. Measured and simulated gain and AR at the zenith of the dual-slot
design of the presented antenna.
cluded in Fig. 8 is the peak RHCP gain defined by the maximum of the main lobe which may tilt with frequencies. It appears the main lobe is pointed to the zenith at low frequencies
and tilted to the horizon at high frequencies. The measured 3-dB
AR bandwidth is about 38% (4.5–6.6 GHz) and the half-power
gain bandwidth is around 48% (3.6–5.9 GHz) with a maximum
gain of 5.3 dBic. Note that the 3-dB AR bandwidth and the
half-power gain bandwidth are not matched well in the frequency band, while the peak gain remains stable over the entire
3-dB AR bandwidth.
V. ACHIEVING PATTERN STABILITY
UNIDIRECTIONAL BEAM
AND
A. Dual-Slot Design for Pattern Stability
As mentioned in the previous section, the peak of the radiation patterns tilts when the frequency increases. This beam-tilt
problem limits the antenna usage in many practical applications,
particularly for the point-to-point wireless communications. To
overcome the beam-tilt problem, we propose a dual-slot design
based on the presented antenna.
Fig. 9 shows the geometry of the dual-slot design, where the
exponentially curved slot in the single-slot design is duplicated
in relation to a rotationally symmetric structure. The parameters
are the same as those of the singleof the curves , , and
slot design. The only change is the feed setting. The original
quarter-wavelength shorting end of the slotline is removed and
replaced by a symmetric slot of the courter part. Note that, by
using such a compact feeding scheme, the equivalent magnetic
currents of the two slots near the feed are 180 out of phase.
This phasing property satisfies the feed requirement for a CP
antenna configured with two identical CP radiators fed at the
same point. In view of this, the presented dual-slot design should
be considered as an antenna element, not a sub-array, for its
single feed point and the phase center are collocated at the same
position. This notion is important when applied to the design of
array antennas and feeding networks.
Fig. 10 shows the RHCP gain and the AR at the zenith
( -axis) versus frequency. Compared to the single-slot design,
the measured 3-dB AR bandwidth improved from 38% to 57%
Fig. 11. Measured and simulated radiation gain patterns of the dual-slot design
at different frequencies: (a) 3.9 GHz, (b) 5.5 GHz, and (c) 6.5 GHz.
(4.0–7.2 GHz) and the half-power gain bandwidth improved
from 48% to above 75% (from below 3 GHz to 6.6 GHz) with a
maximum gain of 7.2 dBic at 4.5 GHz. Note that the difference
between the peak gain and the zenith gain, meaning the extent of
the beam-tilt problem, is significantly reduced. The main reason
for the pattern stability and the improved bandwidth of gain and
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LIN AND LIN: COMPACT OUTER-FED LEAKY-WAVE ANTENNA USING EXPONENTIALLY TAPERED SLOTS FOR BROADSIDE CP RADIATION
2659
Fig. 13. Measured and simulated reflection coefficients of the dual-slot design
of the presented antenna with a reflector.
Fig. 12. Polar plot of the peak gain direction angle
as a function of frequency varying from 3 GHz to 6.5 GHz with a comparison of the single slot and
the dual-slot design of the presented antenna.
AR at the zenith are the rotational symmetry of the dual-slot
design of the presented antenna, as shown in Fig. 9.
Fig. 11 shows the measured and simulated patterns of the
dual-slot design of the presented antenna in two elevation cuts
(the XZ plane and the YZ plane) at three different frequencies.
Note that the peak gain remains at the zenith over the entire
band. To quantitatively describe the improvement of beam-tilt
problem, Fig. 12 shows the contour of the peak gain direction
of the radiation patterns in a polar plot format as a function of frequency. For the single-slot design, the beam tilts from
about the zenith (0 ) to an elevation angle of 25 when the frequency varies from 3.0 GHz to 6.5 GHz. However, with the
dual-slot design, the peak gain direction exhibits considerable
stability with the elevation angle within 10 from the zenith over
the entire band of observation. The above results ensure that the
dual-slot design exhibits enhanced performances in terms of the
impedance bandwidth, the axial ratio and gain bandwidth, the
gain level, and the pattern stability.
B. Backed Reflector for Unidirectional Patterns
For practical applications, we extend the design of the presented antenna for radiating the unidirectional patterns. Unidirectional patterns are preferable in the practical usage of most
point-to-point communication systems. In general, the slot or
aperture antennas radiate bidirectional patterns with different
polarization sense in the opposite directions. To this end, the
presented antenna design was extended and configured with a
backed reflector.
The prototype of the dual-slot design with a reflector 15 mm
below was made. When the reflector was introduced, all the design parameters remained the same as those in Fig. 9 except for
the open stub length of the feed line that was changed from 4 mm
to 8 mm for tuning the impedance matching. Fig. 13 shows the
measured and simulated reflection coefficients of the dual-slot
design of the presented antenna with a reflector. The measured
10-dB return loss bandwidth is about 40% (4.5–6.7 GHz), which
comply well with the simulated results. Compared to the bandwidth in Fig. 14, the reduced bandwidth in Fig. 13 is mainly due
Fig. 14. Measured and simulated gain and AR at the zenith of the dual-slot
design of the presented antenna.
to the proximity effects between the dual-slot in the upper PCB
and the backed reflector with quarter-wavelength distance.
Fig. 14 shows the spectrum of RHCP gain and AR at
the zenith ( -axis) of the dual-slot design with a reflector.
We found that the measured 3-dB AR bandwidth is 53%
(4.3–7.4GHz) and the half-power gain bandwidth is 59%
(3.9–6.6GHz), which are still wideband numbers, even though
the reflector may reduce the bandwidth. The overlapped bandwidth (from satisfying the 10-dB return loss, the 3-dB axial
ratio, and the half-power gain performances at the same time) of
the presented dual-slot antenna module is 38% (4.5–6.6GHz).
Fig. 15 shows the measured and simulated patterns in two
elevation cuts (the XZ plane and the YZ plane) at three different frequencies. The RHCP gain level is improved by the
backed reflector and found to be about 8 dBic with stable radiation patterns achieved at the three frequencies. Moreover, the
front-to-back ratio is about 20dB over the entire band, which is
very good in view of the small backed reflector.
Table I compares the performances of the presented dual-slot
antenna of backed reflector with the cavity-backed aperture antenna in [26] and the curl antenna in [28]. Note that the antenna
in [26] is aperture-type that the main leaky-waves are propagating along the aperture edge, while the presented antenna is
slot-type with two different rates for the tapered slot. In general,
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IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 60, NO. 6, JUNE 2012
demonstrated that curved leaky waves may generate broadband
CP radiation using a compact tapered slot. Compared to traditional spiral antennas, the presented antenna has many advantages including 1) the compact size—only a single-arm and
single-turn slot involved, 2) the outer-fed design—easy to integrate with RF circuits or array feeding networks on the same
substrate, and 3) the fully planar PCB design—leading to a
low manufacturing cost. In this paper, a comprehensive study is
given, covering the single-slot design, the dual-slot design, and
the reflector-embedded design, all verified with experimental
prototypes. Particularly, the dual-slot design of the presented
antenna has overcome the beam-tilt problem. By its rotational
symmetry, the dual-slot design not only achieves the pattern stability but also improves the bandwidth of impedance, AR, and
gain at the zenith. Additionally, we accomplished an antenna
module with a reflector for practical unidirectional pattern applications. The measured results of the final antenna prototype
exhibit a 10-dB return loss bandwidth of 40%, a 3-dB AR bandwidth of 53%, and a half-power (3-dB) gain bandwidth of 59%
with a gain level about 8 dBic.
REFERENCES
Fig. 15. Measured and simulated radiation patterns of the dual-slot design of
the presented antenna with a reflector at different frequencies: (a) 4.4 GHz, (b)
5.5 GHz, and (c) 6.9 GHz.
TABLE I
COMPARISON OF THE PRESENTED ANTENNA
WITH THOSE IN REFERENCE [26], [28]
the presented antenna has better bandwidth covering the input
impedance, AR, and gain with pattern stability.
VI. CONCLUSION
This paper has presented a novel leaky-wave circularly polarized (CP) slot antenna. For the first time, we have successfully
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2661
Shih-Kai Lin (S’10) was born in Taichung County,
Taiwan, 1982. He received the B.S. degree in
electrical engineering from the Department of
Electrical Engineering, National Taiwan University,
Taipei, Taiwan, in 2006 and the M.S. degree in
electrical engineering from the Graduate Institute
of Communication Engineering, National Taiwan
University, Taipei, Taiwan, in 2008, where he is
currently working toward the Ph. D. degree
His research interests include the design, implementation, and measurement of PCB antennas, circularly polarized antennas, and millimeter-wave antennas.
Yi-Cheng Lin (S’92–M’98) received the B.S. degree
in nuclear engineering from National Tsing-Hua University, Hsingchu, Taiwan, in 1987, the M.S. degree
in electrical engineering from National Taiwan University, Taipei, Taiwan, in 1989, and the Ph.D. degree in electrical engineering from the University of
Michigan, Ann Arbor, in 1997.
From 1997 to 2003, he was with Qualcomm Inc.,
San Diego, CA, responsible for the design and development of various antennas for satellites and terrestrial communication systems. Since 2003, is with
the faculty of the Department of Electrical Engineering and the Graduate Institute of Communication Engineering, National Taiwan University, where he is
currently an Associate Professor. His research interests include antenna miniaturization, ultra-wideband and multiband antennas, millimeter-wave antennas,
and diversity antennas for MIMO systems.
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