Aperture Coupled Microstrip Patch Antenna with an Asymmetric 'n'
Shaped Slot for Circular Polarization
Ajay Kumar ~harma',A shok ~ittala'n d B.V.R ~ e d d ~ ~
University School of Information & Communication Technology GGSIPU, Sector 16-C Dwarka Delhi - 110 078
Ambedkar Institute of Advanced Communication Technologies and Research, Geeta Colony, Delhi - 1 10 031
DESCRIPTION
Circular polarized microstrip patch antennas find application in navigation, mobile, radar, wireless,
and satellite communication systems. In comparison to linearly polarized antennas, the circularly
polarized antennas provides greater flexibility in terms of orientation between the transmitter and
receiver. The basic principle of operation in such antennas is to radiate two orthogonal field
components in equal magnitude and phase quadrature. Singly fed microstrip patch antennas are most
popular configurations for CP operation because of their several merits like reduced weight, less
complexity and freedom from use of external hybrids and power dividers or couplers. However,
singly fed microstrip antennas suffers from low 3 dB axial ratio bandwidth. A lot of research is
underway nowadays to achieve wide axial ratio bandwidth using single feed .
To improve upon the impedance bandwidth of microstrip patch antennas the aperture coupled
configuration is a good choice. The aperture coupled microstrip antennas has numerous advantages
over conventional probe fed microstrip antennas. In aperture coupled antennas, separate substrate
can be used for the feed circuit and the antenna element to isolate spurious feed radiation from the
antenna element by use of a common ground plane. Further, the input impedance can be controlled
by the size and position of the aperture. Any excess reactance generated by the coupling aperture can
be tuned out by the use of tuning stubs in the feed line. The shape of the aperture can be chosen
appropriately to improve coupling with the feed line and the patch. Further, aperture coupled
configuration exhibits low cross polarization levels and wide bandwidth making it suitable for
circularly polarized antennas for wideband applications.
FIELD OF INVENTION
The field of invention is "Circularly Polarized Microstrip Patch Antennas using single feed". The
concept of generating single band circularly polarized radiation by cutting different asymmetric
shape slot geometries like U-shaped slot, C-shaped slot, F-shaped slot etc into the radiating patch
have been attempted and reported in past.
For the first time an asymmetric 'n' shaped slot is used to obtain wideband circularly polarized
radiation along with wide impedance bandwidth characteristics. An asymmetric 'n'(Pi) shaped slot
has been cut into a nearly square radiating patch singly feed in aperture coupled configuration. This
resulted in perturbation of patch currents in a manner so as to excite orthogonal modes with 90
degree phase difference thereby radiating circularly polarized waves over a wide frequency band.
The coupling aperture is rectangular in shape and is matched to the radiating patch with a unique
oval shaped stub in feed line. A foam layer with a conducting plane is used as a back shield to
Page 1 of 9
suppress the unwanted backside antenna radiation. The shield is positioned at a distance of
approximately 0.16 h from the aperture plane. This has resulted in gain enhancement of the antenna.
It has been investigated through simulation and proved experimentally that an asymmetric 'z'
shaped slot generates circularly polarized radiation in a wide band. Measured results show that the
proposed structure exhibits a 2: 1 impedance bandwidth of 40.38 % with respect to design frequency
of 2.6 GHz. The 3 dB axial ratio bandwidth is 3.5 % with respect to measured minimum axial ratio
frequency of 2.57 GHz. This makes this antenna suitable for Wireless and Satellite Communication
applications.
DESIGN DETAILS AND RESULTS
Circularlv Polarized A~ertureC ou~ledP atch Antenna with Asymmetric 'n' Slot
A nearly square radiating patch is designed at the design frequency of 2.6 GHz. The patch
dimensions are calculated to be 40.0 mm x 37.5 mm and aspect ratio 1.06. Top view of the patch
geometry is shown in Fig. 2 while view of the aperture side and feed side of the substrate is shown in
Fig 3. The patch and the feed substrate used is Roger Corporation's RT Duroid (5880) having E, =
2.2, tan 6 = 0.0009 and thickness, t = 3 1 mil. A foam layer of thickness 10.0 mm with E, = 1.07, tan 6
= 0.0003 is used to increase height between patch and ground plane. Length and width of the
rectangular coupling aperture cut in the feed ground plane are L, - 29.7 mm and W, = 3.5 mm
respectively. An oval shaped stub with length Ls = 7.4 mm, Width Ws = 3.2 mm is used to match
the coupling aperture to the radiating patch. An asymmetric In' shaped slot is cut into the patch area
with dimensions as per Table I, with details shown in Fig 1. Antenna specifications are tabulated in
Table-11.
Table I
-
2.5 mm
29.0 mm
Aspect ratio As per Fig 1 9.0 mm
--- -- - - - A- ---A -- A A A - -- -- - - - I - - -- -- -- -- I La
Aperture length 29.7 rnm 1 S5 I As per Fig 1 1.5 mm
Parameter
- A - -- - -- - - -
Aperture Width I l5mm 1 A Sh I As per Fig 1 12.2 mm
Description
Page 2 of 9
Patch Radiator (Square)
Value
-
Description
Asymmetric sc shaped slot
Parameter Value
Design parameters of Circularly Polarized Patch Antenna with n shaped slot
L"
Fig. 1 Dimensions of Asymmetric n shaped slot in radiating patch
Fig. 2 Top view of the Asymmetric n shaped slotted aperture coupled patch antenna
s7
sw
As per Fig 1
Slot Width
Matching Stub (Oval Shaped)
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14.0 mm
Ls 2.0 mm
Ws
Stub length
Stub Width
7.4 mm
3.2 mm
Rectangular
Aperture
Oval
Shaped
Matching
Stub
- Feed line
Fig. 3 (a) View of Coupling Aperture Fig. 3 (b) View of Feed line with Oval Stub
(a) Return Loss
The antenna return loss is measured using Agilent Technologies PNA-X Network Analyzer
model N5244A. The 2:l impedance bandwidth exhibited by the antenna is 1.05 GHz which is
40.38% with respect to the design frequency of 2.6 GHz. The return loss plot is shown in Fig 4.0
below.
r - ----- - --
Cm. CHI. S11 C 1-Port LCL
Fig. 4 Return Loss of Asymmetric IC shaped slotted aperture coupled patch antenna
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(b) Axial Ratio
The axial ratio of the fabricated antenna is measured using an Orbit MiDAS antenna test system and
plotted in the bore sight direction. The axial ratio bandwidth is defined in the range where the axial
ratio is below 3 dB. The plot of axial ratio vs. frequency for measured and simulated case is shown
in Fig 5. The measured 3 dB axial ratio bandwidth is 3.5 % with respect to measured minimum axial
ratio frequency of 2.57 GHz.
(c) Gain
The simulated and measured Gain plot of the circularly polarized z shaped slotted patch antenna is
shown in Fig. 6. A nominal gain of 8.0 dBi is obtained with the proposed geometry.
Measured
2.1 2.3 2.5 2.7 2.9 3.1
Frequency, GHz
Fig. 5 Axial Ratio of the antenna vs. frequency measured in the boresight direction
2.2 2.4 2.6 2.8 3 3.2
Frequency, GHz
Fig. 6 Gain of the antenna vs. frequency measured in the boresight direction
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a
(d) Power Pattern
The Azimuth and Elevation power pattern measured using an Orbit MiDAS antenna test system is
shown in Fig. 7 and Fig 8 at circularly polarized resonating frequency of 2.57 GHz. The measured
3 dB Azimuth & Elevation beamwidth at 2.57 GHz are 5 lo & 42' respectively.
Fig. 7 Radiation pattern of antenna in Azimuth Plane at 2.57 GHz
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Fig. 8 Radiation pattern of antenna in Elevation Plane at 2.57 GHz
Table I1 Specifications of the Aperture Coupled Microstrip Patch Antenna with
Asymmetric 'x' Shaped Slot
(Based on Experimental results)
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I Antenna Parameter
Frequency Range
2:l Impedance
Bandwidth
Gain
3 dB Axial Ratio
Bandwidth
Specification
2.3075 - 3.3575 GHz
1.05 GHz (40.38% @ 2.6
GHz)
8.1*0.3 dE3i
2.52-2.61 GHz
3.5% @ 2.57 GHz
Simulated using IE3D from
Zealand, Inc
2.32-3.17 GHz
0.85 GHz (32.69 % @ 2.6
GHz)
7.8*0.1 dl3i
2.56-2.645 GHz
3.27% @ 2.6 GHz
Measured Results
2.3075 - 3.3575 GHz
1.05 GHz (40.38% @ 2.6
GHz)
8.1*0.3 dBi
2.52-2.61 GHz
3.5% @ 2.57 GHz
Aperture Coupled Microstrip Patch Antenna with an Asymmetric 'x'
Shaped Slot for Circular Polarization
Ajay Kumar ~harma',A shok ~ i t t aaln~d B.V.R ~ e d d ~ ~
University School of Information & Communication Technology GGSIPU, Sector 16-C Dwarka Delhi - 110 078
Ambedkar Institute of Advanced Communication Technologies and Research, Geeta Colony, Delhi - 1 10 03 1
Specifications of the Aperture Coupled Microstrip Patch Antenna with Asymmetric
'A' Shaped Slot
(Based on Experimental results)
Antenna Parameter
Frequency Range
2:l Impedance
Bandwidth
Gain
3 dB Axial Ratio
Bandwidth
Specification
2.3075 - 3.3575 GHz
1.05 GHz (40.38% @ 2.6
GHz)
8.1h0.3 dBi
2.52-2.61 GHz
3.5% @ 2.57 GHz
Simulated using IE3D from
Zealand, Inc
2.32-3.17 GHz
0.85 GHz (32.69 % @ 2.6
GHz)
7.8hO. 1 dBi
2.56-2.645 GHz
3.27% @ 2.6 GHz
Measured Results
2.3075 - 3.3575 GHz
1.05 GHz (40.38% @ 2.6
GHz)
8.150.3 dBi
2.52-2.61 GHz
3.5% @ 2.57 GHz

J U I-- Aperture Cou~ledM icrostrir, Patch Antenna with an AsymmetrI-i-c 'n'
Shaped Slot for Circular Polarization
Ajay Kumar ~harma',A shok ~ i t t a l 'a nd B.V.R ~ e d d ~ ~
'"University School of Information & Communication Technology GGSIPU, Sector 16-C Dwarka Delhi - 110 078
Ambedkar Institute of Advanced Communication Technologies and Research, Geeta Colony, Delhi - 110 031
CLAIM
A novel geometry with an asvmmetric slot ("zl'(Pi) shaped) cut into a nearly square radiating patch
is designed for wide band circular polarization and wide impedance characteristic using single feed
in aperture coupled configuration. The coupling aperture is matched to the patch with a unique oval
shaped stub incorporated in the feed line.
It has been investigated through simulation and proved experimentally that an asymmetric "n"
shaped slot cut into the patch area of a nearly square geometry generates circularly polarized
radiation in a wide frequency band.
We Claim that:
1. Circularly Polarized radiation in a Wide band can be achieved by cutting an asymmetric "n"
shaped slot into a radiating patch of a nearly square geometry.
2. The asymmetric "z" shaped slot perturbs the patch currents in a manner so as to excite the
two orthogonal modes with a 90 degree phase shift.
3. By varying the line lengths of the "z" shaped slot, the operating frequency of circularly
polarized band can be controlled.
4. The aperture can be matched to the radiating patch over a wide bandwidth by incorporating
an oval shaped stub in the feed line.