Chapter-4
RECONFIGURABLE WHEEL ANTENNA
4.1
INTRODUCTION
More number of antennas in communication system will increase the capacity and
diversity of the system. If we want to either transmit or receive multiple signals
then we have to choose among (a) multiple antennas, (b) multiband antenna, (c)
reconfigurable antenna. While multiple antennas provide serious isolation
problem, multiband antenna shows degraded performance with respect to SNR
when compared to reconfigurable antenna and [89-93] shows the benefit of
reconfigurable antennas with switching capability when they are used as elements
of a MIMO(Multiple Input and Multiple Output) system. The capacity of the
MIMO communication system depends not only on the eigen structure of the
channel matrix but also on the mean receive signal-to-noise ratio [91]. Therefore it
is desirable to use reconfigurable antennas in a MIMO system which will increase
receive SNR and multipath richness. In this chapter we proposed a novel
reconfigurable wheel antenna (RWA). In our antenna, reconfiguration is achieved
by switching in various parts of the antenna into the current path. In general to
increase the capacity, the pattern and beam width of the antenna element of the
MIMO array should be selected such that the receive SNR and the multipath
richness are increased.
4.2 ANTENNA DESIGN & ANALYSIS
Reconfigurable wheel antenna has been simulated in Agilent ADS. In the design
and development stage, the RWA has been fabricated using photo lithography
technique in RT duroid 5880 substrate (εr=2.2) and Tanδ=0.0009. The RWA
consists of 32 patch elements in the same substrate. The connection between one
element to the other is achieved by switching of PIN diodes. In this work, we use
40 nos. of PIN diodes as the controlling elements for making connections between
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the patches. As the size of the PIN Diode is small (1mm), copper ribbons are used
to connect the PIN diodes with the patch elements. The excitation of the RWA is
achieved using SMA connector. The PIN diodes are driven by a separate ‘driver’
circuit. By controlling on/off of the diodes, the RWA is made to resonate at multiband of frequencies. Fig 4.1 shows the geometry of the design with PIN diodes.
Location of
PIN Diode
Figure 4.1 ADS Layout of wheel antenna with PIN diodes
Specifications
Reconfigurable Frequency : 3.8 GHz (S-Band), 9.6 GHz (X-Band)
Gain
: 2dB to 6 dB (Reconfigurable)
VSWR
: 2: 1 (Max)
HPBW
: ±430 (Az) and ±420 (El) – For S-Band
: ±350 (Az) and ±330 (El) – For X-Band
4.3 SIMULATED RESULTS
The Reconfigurable wheel antenna is simulated in Agilent ADS. The simulated
return loss, radiation pattern, gain pattern and efficiency are given at S-Band and
X-Band in Figures 4.2 to 4.11. Fig. 4.2 shows the simulated return loss when the
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all the diodes located on both inner and outer wheel are in ON condition (i.e, statestate
I). In state-I,
I, the antenna is resonating at 3.8 GHz (S
(S-Band).
Band). Fig.3 shows the
simulated return loss when all the diodes located on inner wheel are ON and outer
wheell are OFF condition (i.e, state
state-II). In state-II,
II, the antenna is resonating at 9.63
GHz(X-Band).
Band). These bands can be changed by changing the dimensions of patch.
The return loss and bandwidth can be improved by designing proper broad band
matching network att the input excitation.
Figure 4.2 Antenna return loss in state
state-I (S-Band)
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Figure 4.3 Antenna return loss in state-II (X-Band)
Figure 4.4 Antenna radiation pattern at S-Band
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Figure 4.5 Antenna gain pattern at S-Band
Figure 4.6 3D radiation pattern of antenna at S-Band
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Figure 4.7 Antenna efficiency at S-band
Fig. 4.4 shows the simulated normalized radiation pattern of the antenna with
cross polarization level is 38 dB over ±900 angle, Fig.5 shows the gain and
directivity pattern of the antenna with gain of 5dBi, Fig.4.6 shows the 3D radiation
pattern of the antenna and Fig.4.7 shows the efficiency of 63% of an antenna in
state-I respectively.
Figure 4.8 Antenna radiation pattern at X-Band
Fig.4.8 shows the simulated normalized radiation pattern of the antenna with cross
polarization
on level is 11 dB over ±900, Fig.4.9 shows the gain and directivity
pattern of the antenna with gain of 6dBi, Fig.4.10shows the 3D rad
radiation
iation pattern of
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the antenna and Fig.4.11 shows the efficiency of 68% of an antenna in state-II
respectively.
Figure 4.9 Antenna gain pattern at X-Band
Figure 4.10 3D radiation pattern of antenna at X-Band
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Figure 4.11 Antenna efficiency at X
X-band
4.4 .MEASURED RESULTS
The designed antenna has been fabricated using photolithography process and the
photograph is shown in the below Fig.4.13.
Figure 4.12 Photograph of fabricated RWA unit with PIN diodes
The reconfigurable wheel antenna is tested by measuring the radiation parameters.
parameter
The parameters of interest for measurements are,
1. Return Loss
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2. Radiation Pattern Measurement
3. Measurement of Gain
The Figures 4.13 & 4.14 show the measured return loss of an antenna when the
antenna in state-I and state-II respectively.
Figure 4.13 Measured return loss of an antenna in State-I
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Figure 4.14 Measured return loss of an antenna in State-II
The Figures 4.15 and 4.16 show the radiation pattern measurement setup in
outdoor environment. Fig.4.17 and Fig.4.18 shows the radiation pattern at S-band
and X-band respectively. The measured HPBW is 840 in azimuth and 760 in
elevation and 660 in azimuth and 600 in elevation at S- band and X-band
respectively. The less ripple is observed in the radiation pattern.
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Figure 4.15 Antenna mounted on the top of the positioner
Figure 4.16 Antenna mounted on the top of the positioner (closed view)
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Figure 4.17 Measured radiation pattern of antenna at S-Band
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Figure 4.18 Measured radiation pattern of antenna at X-Band
4.5 DISCUSSION ON MESURED RESULTS
The simulated results are in good agreement with the measured results for single
element, fabrication accuracy can further improve the results of the designed
antenna. There is a little shift in the frequency due to biasing circuits of pin diodes.
This can be improved by maintaining proper isolation between DC supply and RF.
The below Table 4.1 gives the comparison between simulation and measured
results of the RWA in the two different bands.
S.No
1
Parameter
Freq( GHz)
Simulation Results
Measured Results
S-Band
X-Band
S-Band
X-Band
3.8
9.63
3.26
9.42
105
2
Gain ( dBi)
5
6
4.5
5.4
3
Beamwidth(Az)
860
700
840
660
Beamwidth(El)
840
660
760
600
RL (dB)
-17.18
-28.22
-18.95
-28.32
4
5
TABLE 4.1 Comparison of RWA simulation and measured results
4.6 RWA ARRAY ANALYSIS
After validating the single element, array analysis has been carried with the
following specifications.
Specifications
Reconfigurable Frequency : 3.8 GHz (S-Band), 9.6 GHz (X-Band)
Gain
: 12dB to 16 dB (Reconfigurable)
VSWR
: 2: 1 (Max)
HPBW
: ±80 (Az) and ±400 (El)
Inter element spacing
: 0.6λ
The layout of the 1X8 linear array as shown in Fig.4.19, as the number of diodes
are more, the momentum simulation in ADS is little complex, therefore the array
factor analysis has been carried out in MATLAB by considering single element
radiation characteristics. The Figures 4.20 and 4.21 shows the radiation pattern at
X-Band and S-Band respectively.
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Figure 4.19 ADS layout of 1X8 wheel antenna aarray
Figure 4.20 Simulated E
E-field pattern at S-band
band in array configuration
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Figure 4.21 Simulated E-field pattern at X-band in array configuration
4.7 ARRAY FABRICATION
1X8 RWA array has been fabricated using photo lithography technique in
FR4 substrate with 31mil thickness. The photograph is shown in Fig.4.23.
Figure 4.23 Photograph of fabricated 1X8 RWA array unit
The return loss measurement set-up as shown in Fig.4.24, while measuring the
antenna array instead of PIN diodes when diodes are in ON states 0Ω Chip
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resisters are used, when the diodes are in OFF states, resisters were removed from
the geometry leaving the gap of 0.5mm.
Figure 4.24 Photograph of return loss measurement setup
4.8 CONCLUSION
In this chapter, simulated and experimental data have demonstrated the
concepts of single element reconfigurable wheel antenna and its array by
switching OFF and ON of PIN diodes for multiple bands of frequencies. The
performance of RWA can be further improved by proper designing of driver
circuit in the antenna structure. The technique has taken the advantage of different
number of radiating lengths with the use of PIN diode switches, each
configuration resonating at different frequency, In array radiation pattern there is a
grating lobe within 35 deg for X-band, therefore the main beam can be steered
only within ±15 deg. For S-band there is no grating lobe as the inter element
spacing is less than a wavelength.
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