Circularly Polarized Microstrip Patch Antenna

Fed by Substrate Integrated Waveguide

Tomas Mikulasek, Jaroslav Lacik
Dept. of Radio Electronics
Brno University of Technology
Brno, Czech Republic
mikulasek.t@phd.feec.vutbr.cz


AbstractThis paper presents a circularly polarized microstrip
patch antenna fed by a substrate integrated waveguide. The
antenna was designed in Ansoft HFSS for the operating
frequency 10 GHz. The antenna works with right-handed
circular polarization. At the desired frequency, the antenna
reaches 4.4% impedance bandwidth (the measured value for s
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better than 10 dB), 7.1% Axial Ratio (AR) bandwidth (the
simulated value for AR < 3) and the simulated gain 9.3 dBi.
Keywords-circularly polarization; substrate integrated waveguide
I. INTRODUCTION
Substrate Integrated Waveguide (SIW) structures have
gained considerable attention in recent years [1], [2]. SIW is
electrically similar to a conventional metallic waveguide, and
can be easily fabricated by a low cost Printed Circuit Board
(PCB) process. The SIW structure consists of two rows of
metal vias created in a dielectric substrate. The top and bottom
side of the substrate is covered by metal sheets which are
connected by the vias.
Circularly polarized (CP) antennas are widely used in
satellite and radar systems. In this type of communication, low-
-cost low-profile antennas are required. Recently, CP antennas
based on the SIW technology were designed and published.
Cavity backed crossed-slot antennas for CP applications with
different feeding methods were presented in [3] and [4]. These
antennas were fabricated from a single substrate where the
backed SIW cavity fed by a single grounded coplanar
waveguide or a single coaxial probe was integrated. These
antennas suffered from a narrow Axial Ratio (AR) bandwidth
(typically about 1 %) and a low gain (typically about 5.5 dBi).
In [5], a circular ring slot embedded in a single-layered SIW
was applied to reach the circular polarization. This antenna
reached 2.3% AR bandwidth. A similar concept of a cavity-
-backed CP antenna was presented in [6]; the published
antenna had a high gain (about 7.8 dBi) and 3% AR bandwidth.
In our contribution, we propose a square patch fed by two
probes built from a SIW in order to excite the circular
polarization (the dual-feed method). Signals with an equal
magnitude and 90 phase difference are obtained due to
a proper position of the probes in the SIW. This paper
presents a designed geometry, a design procedure and results of
a simulation and a measurement of the CP antenna.
II. GEOMETRY OF PROPOSED ANTENNA
The geometry of a model of the proposed CP antenna is
depicted in Figure 1. The antenna of overall dimensions L and
W consists of two dielectric layers, a SIW layer and a patch
layer.
In the SIW layer of the dielectric constant
r1
and the
thickness h
sub1
, the substrate integrated waveguide is created.
To reduce computational costs of the design, the SIW is
substituted by an equivalent rectangular waveguide (the long
dashed line) of the equivalent width w
wg
and the length l
wg
[2].
The waveguide is fed by a port on the left side and operates in
the fundamental TE
10
mode. The right end of the waveguide is
shorted.
The patch layer of the dielectric constant
r2
and the
thickness h
sub2
is placed on the SIW layer. The metal sheet
The presented research was financially supported by the project
CZ.1.07/2.3.00/20.0007 WICOMT, financed from the operational program
Education for competitiveness, and by the grant 102/08/H027 Advanced
Methods, Structures and Components of Electronic Wireless Communication
provided by the Czech Science Foundation.

1
y
y
x
z
A
A
TE
10
p 2
wg
wg
1
x
3
4
port

(a) Top view

(b) Section A-A
Figure 1. Geometry of antenna model.
6th European Conference on Antennas and Propagation (EUCAP)
978-1-4577-0919-7/12/$26.00 2011 IEEE 2380
between these layers is a ground plane for the patch. In the
ground plane, two circular slots (the short dashed line) of the
diameter d
3
are etched. On the top side of this layer, a square
patch of the dimension L
p
is located. The patch is fed from the
waveguide through the slots by two probes of the diameter d
2
.
The distance between the probes and the center of the patch is
a
4
. Dimensions a
2
and a
3
denote a position of the patch and the
waveguide. A proper choice of these dimensions ensures an
equal magnitude and 90 phase difference of signals for the
circular polarization. In this case, the patch generates the Right-
-Handed Circular Polarization (RHCP).
In order to improve the impedance matching of the antenna,
two metal vias of the diameter d
1
and the position v
x
and v
y
are
placed in the waveguide.
III. PROCEDURE OF ANTENNA DESIGN
The described antenna was designed in Ansoft HFSS for
the frequency 10 GHz. The CuClad 217 (
r1
= 2.17) and the
FoamClad (
r2
= 1.25) were chosen as the dielectric substrate
for the SIW layer and the patch layer, respectively.
The procedure of the antenna design can be divided into
several steps. In the first step, the patch layer, including the
ground plane, the patch and the feeding probes, is designed
separately. The patch is fed by two 50 coaxial probes of the
diameters d
2
and d
3
. The signals of the probes are with 90
phase difference. The length of the patch L
p
and the parameter
a
4
are evaluated by optimizing the reflection coefficient of the
single patch for 10 GHz.
In the next step, the width w
wg
of the waveguide is
determined for the operating frequency 10 GHz. A cutoff
frequency of the fundamental mode TE
10
is 7.2 GHz. This
waveguide is used for the feeding of the patch from the
previous step. The dimensions a
2
and a
3
are optimized for the
widest AR bandwidth. An initial position of the probe of the
patch (see Figure 1b) is one half of the waveguide wavelength
from the short, and is slightly shifted along the waveguide axis.
Finally, two metal vias with the diameter d
1
are used for the
improvement of impedance matching of the antenna. The
positions v
x
and v
y
are optimized for the best impedance
matching. The resultant parameters of the optimized antenna
model are summarized in Table 1.
The simulated reflection coefficient and the axial ratio of
the optimized antenna model are depicted in Figures 2 and 3,
respectively. The antenna reaches 3.5% impedance bandwidth
for s
11
better than 10 dB and 7.3% axial ratio bandwidth for
AR < 3 dB. Figure 4 shows simulated normalized radiation
patterns at 10 GHz for RHCP. The simulated gain of the
antenna model is 9.4 dBi.
In order to verify the results obtained from Ansoft HFSS,
the antenna model was modeled by the transient solver of CST
Microwave Studio (CST MWS). CST results are compared
with Ansoft ones in Figures 2 to 4; a good correspondence of
results is obvious.
TABLE I. PARAMETERS OF ANTENNA MODEL IN FIGURE 1
Parameter Value (mm)

Parameter Value (mm)
a1 8.73

hsub2 1.88
a2 9.08

L 41.00
a3 2.20

Lp 11.84
a4 3.15

lwg 38.00
d1 0.80

vx 4.32
d2 0.80

vy 27.38
d3 1.60

W 36.00
hsub1 1.52

wwg 14.14

Figure 2. Simulated reflection coefficient of antenna model.

Figure 3. Simulated axial ratio of antenna model in main-lobe direction.

(a) xz plane
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(b) yz plane
Figure 4. Simulated RHCP radiation patterns of antenna model at 10 GHz.
IV. EXPERIMENTAL RESULTS
In order to verify the designed antenna experimentally, the
feeding port was replaced by a direct non-radiated SMA-to-
-SIW transition [7] and the waveguide was converted to the
SIW [1]. The antenna was modeled by Ansoft HFSS, verified
by CST MWS and fabricated. Figure 5 depicts the final
antenna configuration with the SMA-to-SIW transition. Due to
a small change of the reflection coefficient after the
conversion, the position of the matching vias is slightly
corrected. The supplementary parameters of the final antenna
are given in Table 2.
The antenna was fabricated by a low cost PCB process.
Both the layers were fixed using an acrylic resin. The
photographs of the fabricated antenna prototype are shown in
Figure 6. The results of the simulation and measurement are
depicted in Figures 7 to 9.
Figure 7 shows a good agreement between the simulated
and measured reflection coefficient of the fabricated antenna.
The antenna reaches up to 4.4% impedance bandwidth in
comparison with the simulated impedance bandwidth 3.9 %.
In Figure 8, simulated and measured AR of the fabricated
antenna are compared. Obviously, the measured AR behavior
is very different from the simulated one. Sensitivity of the
fabrication process and fabrication tolerances might be the
reason of the mismatch. Therefore, we are working on the
validation of an antenna fabricated with a higher accuracy. The
simulated AR bandwidth is 7.1 %.
The normalized radiations patterns in xz and yz plane are
depicted in Figure 9. Only the co-polarized RHCP radiation of
the antenna was measured. The agreement of the simulated and
measured values is satisfactory. The gain of the antenna was
not measured. The simulated one is 9.3 dBi.

(b) Top view

(b) Section A-A
Figure 5. Antenna configuration with SMA-to-SIW transition.

(a) Top view

(b) Bottom view
Figure 6. Fabricated antenna.
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TABLE II. SUPPLEMENTARY PARAMETERS OF ANTENNA IN FIGURE 5
Parameter Value (mm)

Parameter Value (mm)
a5 8.22

px 2.17
a6 2.49

py 2.39
d4 1.40

vx 4.39
fx 5.59

vy 27.72
fy 10.32

W 36.00
L 48.00

wSIW 15.17
lSIW 43.03



Figure 7. Simulated and measured reflection coefficient of antenna
with SMA-to-SIW transition.

Figure 8. Simulated and measured axial ratio of antenna with
SMA-to-SIW transition in main-lobe direction.

(a) xz plane

(b) yz plane
Figure 9. Simulated and measured RHCP radiation patterns of antenna
with SMA-to-SIW transition at 10 GHz.
V. CONCLUSION
In the paper, the low-cost low-profile CP patch antenna fed
by the SIW was introduced for centimeter wave applications.
The designed antenna was validated by the measurement.
Unfortunately, the axial ratio of the antenna was very sensitive
on fabrication tolerances. The proposed antenna reached the
wide axial ratio bandwidth 7.1 % (the simulated value in
Ansoft HFSS) and 4.4% impedance bandwidth (the measured
value) at the desired frequency 10 GHz. The simulated gain of
the antenna was 9.3 dBi.
REFERENCES
[1] L. Yan, W. Hong, K. Wu, T. J. Cui, Investigations on the propagation
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[2] F. Xu, K. Wu, Guided-wave and leakage characteristics of substrate
integrated waveguide, IEEE Transaction on Microwave Theory and
Techniques, 2005, vol. 53, no. 1, pp. 6673.
[3] G. Q. Luo, L. L. Sun, Circularly polarized antenna based on dual-mode
circular SIW cavity, in Proceedings of the International Conference on
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[4] G. Q. Luo, L. L. Sun, L. X. Dong, Single probe fed cavity backed
circularly polarized antenna, Microwave and Optical Technology
Letters, 2008, vol. 50, no. 11, pp. 29962998.
[5] D. Kim, J. W. Lee, C. S. Cho, T. K. Lee, X-band circular ring-slot
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[7] A. Morini, M. Farina, C. Cellini, T. Rozzi, G. Venanzoni, Design of
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