chapter-8 polarization reconfigurable...

157

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

158

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

159

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

160

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

161

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

162

Figure 8.6 Gain and directivity pattern in Path-2



Figure 8.8 Axial ratio when diodes OFF, Path 3

163



164

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

165

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

166

Figure 8.11 Photograph of antenna under return loss measurement

Figure 8.12 Measured return loss in Path-1

167



168

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

169


Figure 8.18 Measured radiation pattern in Path-2


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

170

Figure 8.19 Measured gain pattern in Path-2



Gain pattern of an antenna at 4.2GHz when the SP3T switch is at path


show the measured axial ratio when all the diodes are in ON

and SP3T switch position is at path-3 and path-2 respectively.



Gain pattern of an antenna at 4.2GHz when the SP3T switch is at path-2 and when


show the measured axial ratio when all the diodes are in ON

171

Figure 8.20 Measured axial ratio – Linear Polarization

172

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

173

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.

chapter-8 polarization reconfigurable...

Documents