chapter-8 polarization reconfigurable...
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Chapter-8
POLARIZATION RECONFIGURABLE ANTENNA
8.1 INTRODUCTION
With the remarkable growth of wireless communication systems, there has been an
increasing need for multifunctional Antennas that can cover various
communication environments [133]. In recent research, significant attention has
been paid to reconfigurable antennas, that is, antennas that can alter their radiating
topology within the same physical dimension, due to their selectivity of frequency,
radiation or polarization, and compact size [134]. Reconfigurable antennas can be
applied to a variety of radio frequency (RF) communication systems; for example,
frequency reconfigurable antennas for multiband mobile devices and polarization
reconfigurable antennas for Satellite communication systems and adaptive multi-
input multi-output systems. In order to obtain polarization reconfigurability,
several patch antenna configurations have been investigated [135-137]. To achieve
reconfigurability, the antenna requires the use of some RF switching devices, such
as PIN diodes, photo-conductive switches, MEMS switches and FETs. Basically
the selection of switch type depends on the switching speed demanded by the
application and the power handling capability [138].
In this chapter a simple patch antenna to achieve the triple polarization is
discussed. The antenna design is analyzed and optimized using Agilent ADS
software. Elliptical/Circular Polarization antenna is more sensitive than linearly
polarized. Many critical parameters affecting the axial ratio have to be considered
during design of circularly polarized antenna than linearly polarized. In this
design, 4 PIN diodes and necessary biasing networks are used to get the
polarization diversity. Linear polarization is achieved by edge feeding and
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Elliptical/CP is achieved by diagonal feeding. Feeding mechanism is having SP3T
switch to select a path of current excitation.
8.2 ANTENNA DESIGN & ANALYSIS
The antenna has been designed using RT5880 substrate (εr=2.2, Tanδ=0.0009)
having thickness of 20mil. Fig.8.1 shows the geometry of the design.
Figure 8.1 Antenna Geometry
The Geometry consists of a SP3T switch, biasing network, microstrip patches and
4 pin diodes (SMP1320-004LF) from Skyworks Solutions Inc.. The main
advantage of this geometry is that the structure can be reconfigured to achieve
LHCP and RHCP by switching ON or OFF the pin diodes. λ/4 lines are used as
the impedance transformer between input feed and antenna. The biasing Network
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Figure 8.2 PIN Diode Biasing Network
State ON
State OFF
Figure 8.3 Equivalent circuit of PIN diodes
as shown in the Fig.8.2 consisting of two Inductors L1 & L2 are realized using
high Impedance (120Ω) transmission line to block the analog signal entering the
DC supply and two by pass capacitors C1=10 pF (C0805C100J5GACTU) and
C2=4.7pF (C0805C479D5GACTU) are used to ground the analog ripples in the
DC supply voltage. When +5V is applied, diode is forward biased and make the
DC path closed through L1&L2.The RF-IN signal forwarded to the antenna
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through pin diode via C2. When the diode is reverse biased, RF signal path to
Antenna is blocked. Antenna is diagonally excited to achieve the elliptical
polarization and small patches separated by quarter wave length are incorporated
for impedance matching. All the 4 PIN diodes are excited simultaneously to
achieve the polarization diversity. When all the diodes are ON the antenna acting
as a regular square patch at the same time the middle path has been selected in the
SP3T switch. Therefore the square patch is excited diagonally thus achieving 450
slant elliptical polarization. If any other port is selected in SP3T, linear
polarization is realized with all diodes ON and LHCP/RHCP when all diodes are
OFF. If all the diodes are OFF the square patch become truncated on the corners as
shown in Fig.8.1, thus achieving LHCP/RHCP polarization. The equivalent circuit
of the PIN diode during OFF and ON state as shown in Fig.8.3. Due to the
parasitic effect, the centre frequency is changing a little while changing over to
other polarization. This can be optimized by implementing proper isolated biasing
network.
8.3 SIMULTED RESULTS
The Polarization Reconfigurable Antenna is simulated in Agilent ADS. Simulated
Return Loss, Gain Pattern and Axial ratio Patterns are given in Figures 8.4 to 8.8.
Figure 8.4 Antenna return Loss with SP3T Port-2 path
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Figure 8.5 Antenna return Loss with SP3T Port-1, 2, 3 paths
Fig.8.4 Shows the Simulated Return loss when the SP3T is at port-2 path and
Fig.8.5 Shows the variation in the resonating Frequency when SP3T switch at
three different paths thus achieving frequency re-configurability also. But, this
work has been carried out to achieve polarization re-configurability for single
frequency. The frequency range is 4.14GHz to 4.21GHz, this band still can be
increased by designing proper broad band matching network at the input
excitation.
The Fig.8.6 shows the simulated gain pattern of the antenna with gain of
4.3dBi.The Fig.8.7 and Fig.8.8 Show the Axial ratio when the all diodes are ON
and OFF when the SP3T switch is in path-3 and it is observed that when all the
diodes are ON in the path-3 the antenna is giving linear polarization with axial
ratio of 45dB and when all the diodes are OFF in the path-3 the antenna is giving
circular polarization with axial ratio of 2.8dB.
Path-2
Path-1
Path-3
Figure 8.6 G
Figure 8.7 Axial ratio when diodes ON, Path 3
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Figure 8.6 Gain and directivity pattern in Path-2
Figure 8.7 Axial ratio when diodes ON, Path 3
Figure 8.7 Axial ratio when diodes ON, Path 3
Figure 8.8 Axial ratio when diodes OFF, Path 3
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Figure 8.8 Axial ratio when diodes OFF, Path 3
Figure 8.8 Axial ratio when diodes OFF, Path 3
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8.4 MESURED RESULTS
The designed Antenna has been fabricated using photolithography process and
the photograph is shown in the below Fig.8.9 and Fig.8.10 shows the photograph
of the fabricated antenna before and after integrating the biasing network
respectively.
Figure 8.9 Photograph of Fabricated Antenna
The Polarization Reconfigurable Antenna is tested by measuring the radiation
parameters. The parameters of interest for measurements
1. Return Loss
2. Radiation Pattern Measurement
3. Measurement of Gain
4. Measurement of Axial Ratio
The Return loss Measurement Setup with Vector Network Analyzer is shown in
Fig.8.11. The Figures 8.12 to 8.14 shows the measured return loss of an antenna
Group-A
Group-B
Path-1
Path-2
Path-3
Short
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when SP3T switch in path-1, path-2 and path-3 respectively Fig.8.15 shows the
variation in the measured resonating frequency when SP3T switch at three
different paths
Figure 8.10 Fabricated antenna with biasing network
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Figure 8.11 Photograph of antenna under return loss measurement
Figure 8.12 Measured return loss in Path-1
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Figure 8.13 Measured return loss in Path-2
Figure 8.14 Measured return loss in Path-3
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Figure 8.15 Measured return loss with SP3T Port-1, 2, 3 paths
Figure 8.16 Antenna mounted on the top of the positioner
Figure 8.17 Antenna mounted on the top of the positioner (Zoomed)
Figure 8.18 Measured radiation pattern in Path
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Figure 8.17 Antenna mounted on the top of the positioner (Zoomed)
Figure 8.18 Measured radiation pattern in Path-2
Figure 8.17 Antenna mounted on the top of the positioner (Zoomed)
Figure 8.19 Measured gain pattern in Path
The Figures 8.16 and 8.17 show the Radiation pattern measurement setup in
outdoor environment. Fig.8.18 shows the radiation pattern and Fig.8.19 shows the
Gain pattern of an antenna at 4.2GHz when the SP3T switch is at path
all the diodes are in ON and it is observed that the measured Gain is 2.6dBi.
Figures 20 and 21 show the measured axial ratio when all the diodes are in ON
and SP3T switch position is at path
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Figure 8.19 Measured gain pattern in Path-2
The Figures 8.16 and 8.17 show the Radiation pattern measurement setup in
outdoor environment. Fig.8.18 shows the radiation pattern and Fig.8.19 shows the
Gain pattern of an antenna at 4.2GHz when the SP3T switch is at path
all the diodes are in ON and it is observed that the measured Gain is 2.6dBi.
show the measured axial ratio when all the diodes are in ON
and SP3T switch position is at path-3 and path-2 respectively.
The Figures 8.16 and 8.17 show the Radiation pattern measurement setup in
outdoor environment. Fig.8.18 shows the radiation pattern and Fig.8.19 shows the
Gain pattern of an antenna at 4.2GHz when the SP3T switch is at path-2 and when
all the diodes are in ON and it is observed that the measured Gain is 2.6dBi.
show the measured axial ratio when all the diodes are in ON
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Figure 8.20 Measured axial ratio – Linear Polarization
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Figure 8.21 Measured axial ratio – Circular Polarization
8.5 DISCUSSION ON MESURED RESULTS
There is a shift in frequency due to effect of biasing circuit, assembly and
fabrication error ,this can be avoided by a better matching circuit and isolated
power supply for the pin diodes. In the axial ratio measurement Fig.8.17 shows the
variation in the received power is more than 30dB which indicates that the antenna
is linearly polarized and Fig.8.18 shows the variation in the received power is less
than 4dB this indicates that the antenna is circularly polarized the Table 8.1 give
the polarization states of the developed antenna
All Diodes
Group-A & B
Path Polarization
1 2 3
ON ON OFF OFF Linear-H
ON OFF ON OFF Elliptical (450 slant)
ON OFF OFF ON Linear-V
OFF ON OFF OFF LHCP
OFF OFF ON OFF Out of band resonance
OFF OFF OFF ON RHCP
TABLE 8.1 Different Polarization states of the Reconfigurable Antenna
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8.6 CONCLUSION
A new microstrip antenna with triple-polarization diversity has been designed
and fabricated for C-band application. To achieve the polarization
reconfigurability, 4 PIN diodes has been used to connect the truncated patches to
the main patch. The type of achieved polarization are linear, circular and
elliptical. The purity of polarization has been estimated by measuring the axial
ratio which is less than 4dB for CP and more than 30dB for linear. The gain of the
antenna for all states in better than 2.5dBi. The controlling of PIN diodes in real
application can be implemented using FPGA to achieve the fast switching speed.
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