Dual-Band Circularly Polarized S-Shaped Slotted Patch Antenna With a Small Frequency-Ratio

Abstract

A dual-band single-feed circularly polarized, S-shaped slotted patch antenna with a small frequency-ratio is proposed for GPS applications. An S-shaped slot is cut at the centre of a square patch radiator for dual-band operation. A single microstrip feed-line is underneath the center of the coupling aperture ground-plane. The frequency-ratio of the antenna can be controlled by adjusting the S-shaped slot arm lengths. The measured 10-dB return loss bandwidths for the lower and upper-bands are 16% (1.103-1.297 GHz) and 12.5% (1.444-1.636 GHz), respectively. The measured 3-dB axial-ratio (AR) bandwidth is 6.9% (1.195-1.128 GHz) for the lower-band and 0.6% (1.568-1.577 GHz) for the upper-band. The measured gain is more than 5.0 dBic over both the bands. The measured frequency-ratio is 1.28. The overall antenna size is 0.46 ??_o ?? 0.46??_o ?? 0.086??_o at 1.2 GHz.

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2112 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 58, NO. 6, JUNE 2010
much weaker. The signal levels decreased due to the propagation loss
of the human body and the dielectric interface between free space
and the body [2], [6]. With the actual antenna design, we were able
to reconstruct the myocardial events of a volunteer, as shown in the
lower panel of Fig. 9. The bandpass filtered radar signal provides the
opportunity to distinguish between different heart cycles, in terms
of the different contributions of the left and right ventricles to the
systolic (contraction) and diastolic (relaxation) phases [9]. Eventually,
the heart rate was evaluated as 66 min . Mapping the voltage axes
of Fig. 9 to calibrated mechanical amplitudes and further combined
UWB and MRI measurement campaigns are parts of current work.
VII. CONCLUSION
We have described the design, realization, and test of MR-compat-
ible double-ridged horn antennas for use in a bistatic radar arrange-
ment in strong static and dynamic magnetic and electromagnetic fields.
The scattering parameters and radiation properties could be improved
significantly compared to previous antenna designs. This progress en-
abled us to perform first valuable measurements of the respiration and
heart beat of test persons in a 3-T MR scanner under full operation,
underlining the great potential for medical diagnostics. Based on these
introductory results, advanced studies are in progress, including tests
in a 7-T MR scanner. The fusion of MRI diagnosis with UWB-radar
navigation as well as the MR-compatible double-ridged horn antenna
design are patented.
ACKNOWLEDGMENT
The authors are grateful to E. Hamatschek, M. Fritz, and U. Schmidt
at TU Ilmenau for valuable technological support.
REFERENCES
[1] U. Schwarz, F. Thiel, F. Seifert, R. Stephan, and M. Hein, “Ultra-wide-
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Dual-Band Circularly Polarized -Shaped Slotted
Patch Antenna With a Small Frequency-Ratio
Nasimuddin, Zhi Ning Chen, and Xianming Qing
Abstract—A dual-band single-feed circularly polarized, -shaped
slotted patch antenna with a small frequency-ratio is proposed for GPS
applications. An -shaped slot is cut at the centre of a square patch radi-
ator for dual-band operation. A single microstrip feed-line is underneath
the center of the coupling aperture ground-plane. The frequency-ratio
of the antenna can be controlled by adjusting the -shaped slot arm
lengths. The measured 10-dB return loss bandwidths for the lower and
upper-bands are 16% (1.103–1.297 GHz) and 12.5% (1.444–1.636 GHz),
respectively. The measured 3-dB axial-ratio (AR) bandwidth is 6.9%
(1.195–1.128 GHz) for the lower-band and 0.6% (1.568–1.577 GHz) for
the upper-band. The measured gain is more than 5.0 dBic over both the
bands. The measured frequency-ratio is 1.28. The overall antenna size is
0.46 at 1.2 GHz.
Index Terms—Circular polarization, circularly polarized antenna, dual-
band antenna, GPS antenna, microstrip antenna, slotted patch, slot.
I. INTRODUCTION
Recently, dual-band circularly polarized (CP) microstrip antennas
(CPMAs) have received much attention in the field of wireless commu-
nications. In many dual-band applications such as global positioning
system (GPS), a small frequency-ratio is required. This poses a chal-
lenge for a single-feed, single-patch, microstrip antenna structure. A
single-band broadband CP can be generated from a patch antenna with
an aperture-coupled feed with cross-slots using two parallel feed-lines
[1]. Various types of antenna structures with different feeding network
systems for dual-band CPMAs have been reported [2]–[8]. Tanaka et
al. have proposed a dual-feed CPMA which combines slots and patch
for dual-band operation [2]. Yang and Wong have investigated a single-
layer slit-loaded square microstrip patch antenna for dual-band CP ra-
diation with a frequency-ratio of 1.76 [3]. A single-feed dual-band
CPMA has been proposed in [4]. They have realized dual-band CP op-
eration by cutting two arc-shaped slots close to the boundary of a cir-
cular patch radiator and protruding one of the arc-shaped slots with a
narrow slit. The frequency-ratio of the dual-band antenna is 1.48. In
[5], Cai et al. have proposed a ring type slot, aperture-coupled, an-
gular-ring patch antenna with L-shaped feed-line for dual-band CP
operation. The dual-band frequency-ratio of the antenna is 1.32. Bao
and Ammann have been proposed a probe-feed single-layer dual-band,
CPMA with a small frequency-ratio of 1.21 [6]. The antenna consists of
a small circular patch surrounded by two concentric annular-rings. An
unequal lateral cross-slot is loaded on the ground-plane for dual-band
CP operation. The gain of the antenna at 1.224 GHz and 1.480 GHz
is around 1.35 dBic and 3.5 dBic, respectively. Su and Wong have
studied a dual-band CP stacked microstrip antenna using a coaxial-feed
for GPS applications. The antenna comprises two stacked patches with
a combination of air and dielectric layers. The gain of the antenna is
less than 2.0 dBic for the lower-band and more than 4.0 dBic for the
Manuscript received May 25, 2009; revised October 19, 2009; accepted De-
cember 01, 2009. Date of publication March 29, 2010; date of current version
June 03, 2010.
The authors are with the Institute for Infocomm Research, Singapore 138632,
Singapore (e-mail: nasimuddin@i2r.a-star.edu.sg).
Color versions of one or more of the figures in this communication are avail-
able online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TAP.2010.2046851
0018-926X/$26.00 © 2010 IEEE
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IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 58, NO. 6, JUNE 2010 2113
Fig. 1. Proposed dual-band CPMA: (a) cross-section view, (b) -shaped
slotted patch radiator, and (c) aperture-coupled feeding structure.
upper-band [7]. A dual-band circularly polarized stacked microstrip an-
tenna with cross-slot and aperture-coupled feed has been proposed for
GPS [8]. Three Wilkinson power combiners have been used to add the
signals from the four feed-lines at the slots with equal amplitudes and
90 phase-shifts.
In this communication, a dual-band single-feed single-patch CPMA
with a small frequency-ratio is proposed. The antenna consists of an
-shaped slotted square patch radiator and an aperture-coupled feeding
structure. Dual-band CP radiation is achieved by cutting an asymmet-
rical -shaped slot from the radiating patch, without increasing the
size and the thickness of the patch antenna. The antenna design and
optimization is conducted with the help of commercial EM software,
IE3D [9].
TABLE I
DIMENSIONS OF THE OPTIMIZED ANTENNA DESIGN
Fig. 2. Effect of the on antenna parameters: (a) return loss, (b) axial-ratio
at the boresight, and (c) gain at the boresight.
II. ANTENNA STRUCTURE AND DESIGN
The proposed antenna configuration is shown in Fig. 1. The patch is
fed through an aperture-coupled 50- microstrip feed-line under the
ground-plane. The overall size of the antenna is . The
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2114 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 58, NO. 6, JUNE 2010
Fig. 3. Current distributions on the -shaped slotted patch radiator at
(a) 1.227 GHz and (b) 1.575 GHz.
50- microstrip feed-line and the aperture are etched on the opposite
sides of an RO4003 substrate ( mm, and tan
). The open end of the microstrip feed-line extends
from center of the aperture. The aperture size is .
The asymmetrical -shaped slot acts as a perturbation of the patch
to excite the two orthogonal modes with a 90 phase-shift for CP op-
eration at the lower-band. The asymmetrical S-shaped slot itself res-
onates at the upper-band and generates CP radiation for the upper-band
[10]. By varying the length of one of the slot arms, the operating fre-
quency-ratio of the two operating bands can be controlled. From simu-
lation, it is found that the dimensions of the -shaped slot significantly
affect the performance of the antenna. Based on the simulation, a pro-
cedure of the antenna design is suggested as follows.
1. Determine the initial dimensions of the square patch with an
-shaped slot according to the lower-band frequency and the
antenna size constraint;
2. Optimize the aperture-coupled feeding structure to achieve good
impedance matching over the operating bands;
3. Select the length of one arm of the -shaped slot and adjust
the length of the other arm to generate dual-band CP oper-
ation. Make sure that the 3-dB AR bandwidth falls totally within
the 10-dB return loss bandwidth; and
4. Further optimize the antenna by changing the foam thickness and
-shaped slot parameters ( , and .
If the desired performance over the required frequency is not
achieved at the end of Step 4, change the initial parameters in Step
1 and iterate the steps. The optimal dimensions of the antenna are
tabulated in Table I.
A parametric study is conducted to understand the effect of the
-shaped slot on the dual-band CP operation. The procedure adopted
for study is that only one parameter is changed at a time while all other
parameters are kept unchanged.
Fig. 4. Measured and simulated return loss.
Fig. 5. Measured and simulated axial-ratio at the boresight.
Fig. 6. Measured and simulated gain at the boresight.
Fig. 2 shows the effect of on the performance of the antenna,
where varies from 9.5 mm to 23.0 mm. From Fig. 2(a), it is found
that as increases, the 10-dB return loss bandwidth and impedance
matching improve at the upper-band. Fig. 2(b) illustrates the axial-ratio
(AR) at the boresight with different . When mm,
namely the -shaped slot is symmetrical, the antenna generates CP
radiation only at the lower-band. As decreases, the antenna gen-
erates CP radiation at the upper-band with an increase in the oper-
ating frequency, whereas the operating frequency at the lower-band
changes slightly. As a result, the frequency-ratio of the two operating
frequency bands can be controlled by adjusting . It is also found
that the bandwidth at the lower-band is slightly affected by , which
offers more flexibility to achieve a desired frequency-ratio. The bore-
sight gain with varying is shown in Fig. 2(c). The boresight gain
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IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 58, NO. 6, JUNE 2010 2115
Fig. 7. Measured radiation patterns at (a) 1.227 GHz, and (b) 1.575 GHz.
variation with frequency does not significantly depend on the for
the lower-band. However, upper-band gain dip decreases with an in-
crease in . Increasing shifts the operating frequency down. Note
that when mm, there is no gain dip at the upper-band.
However, such a symmetrical -shaped slotted microstrip antenna can
produce CP radiation only at the lower-band.
The current distributions of the antenna at 1.227 GHz and 1.575 GHz
are shown in Figs. 3(a) and (b), respectively. It is found that at 1.227
GHz, the current is much stronger around the edges of the -shaped
slotted patch. This implies the lower-band operation is dependent on
the patch size. From Fig. 3(b), it is observed that the majority of the
current distribution is around the -shaped slot at 1.575 GHz. This
suggests that the upper-band radiation is mainly from the asymmetrical
-shaped slot.
III. MEASURED RESULTS AND DISCUSSIONS
The optimized antenna was fabricated and measured. Fig. 4 com-
pares the measured and simulated return loss. The measured 10-dB
return loss bandwidth is 16% (1.103–1.297 GHz) for the lower-band
and 12.5% (1.444–1.636 GHz) for the upper-band. Fig. 5 shows the
measured and simulated AR at the boresight. The measured 3-dB AR
bandwidths at the lower- and upper-bands are 6.9% (1.195–1.280 GHz)
and 0.6% (1.568–1.577 GHz), respectively. Both the GPS bands are
covered with less than 3-dB AR. The measured AR at 1.227 GHz and
1.575 GHz are 2.0 dB and 1.34 dB, respectively. The frequency-ratio
of the measured minimum AR for dual-band is 1.28. Fig. 6 shows the
measured and simulated gain at the boresight. The gain is more than 5.0
dBic with a variation of less than 0.5 dB across the 3-dB AR bandwidth
for both the bands. The gain at the upper-band is around 2.0 dB below
that of the lower-band because of a gain dip. However, the upper-band
gain is still greater than 5.0 dBic which is suitable for GPS application
[11]. Fig. 7 shows the measured radiation patterns at 1.227 GHz and
1.575 GHz in the and planes, respectively. The 3-dB AR
beamwidth is more than 90 for the lower-band and more than 60 for
the upper-band.
IV. CONCLUSION
A single-feed single-patch dual-band circularly polarized microstrip
antenna with a small frequency-ratio has been investigated. The pro-
posed antenna with an -shaped slot has achieved good impedance
matching, high gain and wide CP beamwidth at the GPS lower and
upper-bands. The antenna has been realized for a small dual-band fre-
quency-ratio of 1.28. The proposed single-feed single-patch -shaped
slotted patch antenna is useful for small frequency-ratio dual-band CP
antenna and array designs.
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