Aperture Coupled Microstrip Patch Antenna with an Asymmetric '8'
Shaped Slot for Dual Band Circular Polarization
Ajay Kumar Sharma', Ashok Mittal^ and B.V.R Reddy^
'•^ 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
DESCRIPTION
Dual band circular polarization (CP) with a small frequency ratio and wide impedance bandwidth
are the main characteristics of an antenna suitable for Satellite Communication, Global Positioning
System (GPS) and Radio Frequency Identification (RFID) applications. This poses a challenge in
case of a single feed and single patch microstrip antenna structure. A lot of research is underway
nowadays to achieve this.
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 /dual band circularly polarized radiation by cutting different shape slot
geometries like arc shaped slots, S shaped slot, ring shaped slot, annular slot etc into the radiating
patch have been attempted and reported in past.
For the first time an asymmetric '8' shaped slot is used to obtain dual band circularly polarized
radiation along with wide impedance bandwidth characteristics. An asymmetric '8'(eight) 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 in two close frequency bands, thereby radiating circularly polarized waves.
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
suppress the unwanted backside antenna radiation. The shield is positioned at a distance of
approximately X/4 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 '8'
shaped slot generates circularly polarized radiation in a dual band with a small frequency ratio.
Page 1 of 9
Measured results show that the proposed structure exhibits a 2:1 impedance bandwidth of 43.08 %
with respect to design frequency of 2.6 GHz. The 3 dB axial ratio bandwidth for lower and upper
bands are 2.1% and 3.3 % with respect to 2.32 GHz and 2.75 GHz respectively. A small frequency
ratio of 1.18 was achieved with this geometry which makes this antenna suitable for applications like
GPS and RFID with increase in range of detection of targets.
DESIGN DETAILS AND RESULTS
Circularly Polarized Aperture Coupled Patch Antenna with Asymmetric '8' Slot
A nearly square radiating patch is designed at the design frequency of 2.6 GHz. The patch
dimensions are calculated to be 39.0 mm x 37.0 mm and aspect ratio 1.05. 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 Cr =
2.2, tan 5 = 0.0009 and thickness, t = 31 mil. A foam layer of thickness 10.0 mm with 8r = 1.07, tan 8
= 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 La = 33 mm and Wa = 2.9 mm
respectively. An oval shaped stub with length Ls = 7.8 mm. Width Ws - 6.4 mm is used to match
the coupling aperture to the radiating patch. An asymmetric '8' 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-II.
Table I
Parameter Description Value
Patch Radiator (Square)
Lp
Wp
Lp/Wp
Patch length
Patch width
Aspect ratio
39 mm
37 mm
1.05
Coupling Aperture (Rectangular)
La
Wa
Aperture length
Aperture Width
33 mm
2.9 mm
Matching Stub (Oval Shaped)
Ls
Ws
Stub length
Stub Width
7.8 mm
6.4 mm
Parameter Description Value
Asymmetric 8 shaped slot
SLI
SL2
SL3
Su
SL5
SL6
SL7
Sw
As per Fig 1
As per Fig 1
As per Fig 1
As per Fig 1
As per Fig 1
As per Fig 1
As per Fig 1
Slot Width
14.3 mm
8.4 mm
18.0 mm
13.0 mm
17.3 mm
11.4 mm
27.0 mrii
2.0 mm
Page 2 of 9
ORIGINAL
0 H
16 m M^*
Table II Specifications of the proposed Aperture Coupled Microstrip Patch
Antenna with Asymmetric '8' Shaped Slot
(Based on Experimental results)
Antenna Parameter
Frequency Range
2:1 Impedance
Bandwidth
Gain
3 dB Axial Ratio
Bandwidtli
Specification
2.16-3.28 GHz
1.12 GHz (43.08% @ 2.6
GHz)
8.5±1 dBi
Band-I (2.30-2.35 GHz)
2.1% @ 2.32 GHz
Band-II (2.70-2.79 GHz)
3.3% @ 2.75 GHz
Simulated using IE3D from
Zealand, Inc
2.15-3.04 GHz
0.89 GHz (34.2 % @ 2.6
GHz)
8.0±0.3 dBi
Band-I (2.325-2.368 GHz)
1.83% @ 2.35 GHz
Band-II (2.755-2.843 GHz)
3.15% @ 2.79 GHz
Measured Results
2.16-3.28 GHz
1.12 GHz (43.08%)
8.5±1 dBi
Band-I (2.3-2.35 GHz)
2.1% @ 2.32 GHz
Band-II (2.70-2.79 GHz)
3.3% @ 2.75 GHz
Page 8 of 9
Aperture Coupled Microstrip Patch Antenna with an Asymmetric '8'
Shaped Slot for Dual Band Circular Polarization
Ajay Kumar Sharma', Ashok Mittal^ and B.V.R Reddy^
''^ 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
Specifications of the proposed Aperture Coupled Microstrip Patch Antenna with
Asymmetric '8' Shaped Slot
(Based on Experimental results)
Antenna Parameter
Frequency Range
2:1 Impedance
Bandwidth
Gain
3 dB Axial Ratio
Bandwidth
Specification
2.16-3.28 GHz
1.12 GHz (43.08% @ 2.6
GHz)
8.5±1 dBi
Band-I (2.30-2.35 GHz)
2.1% @ 2.32 GHz
Band-II (2.70-2,79 GHz)
3.3% @ 2.75 GHz
Simulated Results using
IE3D from Zealand, Inc
2.15-3.04 GHz
0.89 GHz (34.2 % @ 2.6
GHz)
8.0±0.3 dBi
Band-I (2.325-2.368 GHz)
1.83% @ 2.35 GHz
Band-II (2.755-2.843 GHz)
3.15% @ 2.79 GHz
Measured Results
2.16-3.28 GHz
1.12 GHz (43.08%)
8.5±1 dBi
Band-I (2.3-2.35 GHz)
2.1% @ 2.32 GHz
Band-II (2.70-2.79 GHz)
3.3% @ 2.75 GHz
C&r^R (^^^^^J

CLAIM
A novel geometry with an asymmetric slot I "8"(eight) shaped) cut into a square or nearly square
radiating patch is designed to achieve dual band circular polarization and wide impedance
characteristic using single feed 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 "8"
shaped slot cut into the patch area of a square or nearly square geometry generates circularly
polarized radiation in a dual band.
We Claim that:
1. Circularly Polarized (CP) radiation in a Dual band can be achieved by cutting an asymmetric
"8" shaped slot into a radiating patch of a square or nearly square geometry.
2. The asymmetric "8" 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 two loops of the "8" shaped slot, the frequency ratio of the
two operating bands can be controlled.
4. The aperture can be matched to the radiating patch over a wide bandwidth incorporating an
oval shaped stub in the feed line.