Highly reliable & adaptive smart phased array antenna with gain adjustment and beam forming capabilities for space application

1. Highly Reliable & Adaptive SMART Phased Array
Antenna with Gain Adjustment & Beam Forming
Capabilities for Space Applications
Syed Azhar Hasan
Institute a/Space Technology (SUPARCO)
Karachi, Pakistan
syed_azhar_hasan@yahoo.com
Abstract- This paper discusses design & measurement
techniques for highly reliable, adaptive & SMART electronically
steerable phased array antenna with gain adjustment & beam
forming capability for space applications. Complexity of feeding
network for phased array antenna has been reduced by many
folds with the use of 16 independently coaxial fed circularly
polarized truncated corner patch elements in sequentially
rotated configuration providing an excellent axial ratio of <0.1
dB for entire array highly recommended for satellite
communication. Independent element digital control enables the
options for beam steering through phase shifters, gain
adjustment, & combinations of multiple adaptive beam
generation discussed in detail. The chains from transmitter to
antenna for beam steering network have also been discussed.
Reliability analysis through antenna measurements show
degradation in gain is less due to failure of elements as compared
to other antennas. Phased array antenna has been simulated &
optimized on Ansoft HFSS vll.
I. INTRODUCTION
Microstrip & phased array antennas have been widely used
in between IOOMHz to 50GHz due to advantages provided by
them over other antennas in this frequency range. Phased
array means array of antenna elements providing control of
the beam direction and pattern shape including side lobes.
SMART electronically steerable adaptive phased array can
automatically steer its beam through digital control without
moving the antenna physically. Another objective of phased
array antenna is to provide beam control at a fixed frequency
& within range of bandwidth in frequency independent
manner. There are basically four basic electronic scanning
techniques: phase, time delay, frequency & electronic feed
switching [1). Antenna used in our application has proper
digital control, independent element control, through which
gain adjustment & beamforming is possible. It is basically
smart antenna having both switched beam and adaptive array
capability due to the presence of digital switching enabling
each element to be independently turned on and off resulting
in switched beam operation [2). However the presence of 360
degrees phase shifters enables the option of adaptive array as
the gain can be adjusted in the direction of receiver through
phase shift & beam shape can also be adapted as per switching
required to generate particular gain in particular direction.
Applications in which microstrip antennas are used are as
978-1-4244-6908-6/10/$26.00 ©2010 IEEE
124
follows: satellite communication, radar communication,
telemetry & telecommand, military, commercial, remote
sensing, biomedical radiators & many others [1). Microstrip
antennas or space communication require critical analysis in
three areas (a) Vibration (b) temperature (c) power handling
capability.
II. DESIGN, SIMULAnON & DEVELOPMENT
Design & simulation for electronically steerable phased
array antenna has been done on Ansoft HFSS vII. Individual
elements have been coax fed, in order to eliminate the
complexity of the network, circular polarization for
independent element has been achieved through truncation of
individual patch, and however for the whole array rotated
truncated comer configuration has been adopted [4], [5], and
[6]. Excellent results for gain as well as axial ratio have been
achieved. Inter element distances has been carefully selected
to reduce grating lobes & mutual coupling between elements
& comes out to be approximately 0.55)., of operating
frequency [7], [8], [9). Antenna has been developed on low
loss substrate Rogers 5880LM with a dielectric constant of 2.2
and having excellent electrical properties [1]. Developed
antenna is light weight as weight does not exceed I50gm.
Electronically steerable phased array antenna is developed
using sophisticated & high performance PCB manufacturing
machine LPKF Protomat.
•
Fig.I. Top view of simulated 16 element rotated truncated comer
electronically steerable phased array antenna.
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2. A. Architecture ofPhased Array Antenna Network
Architecture for phased array antenna includes RF
transmitter with its output connected to equally balanced 1:16
output Wilkinson power divider. Then at each output of
Wilkinson power divider a high power 360' controlled
electronic phase shifter is used providing individual element
scanning capability however provided the correct phase these
elements can be used to generate high gain beam [10]. After
phase shifters power amplifiers are used, these power
amplifiers regenerate the power lost at Wilkinson power
divider output and phase shifters in form of insertion losses
through the gain of the amplifier. Output of power amplifiers
get connected to 16 independent coax fed elements of
circularly polarized electronically steerable phased array
LATCH (EN) PHASE SHIFTER (Vent)
LATCH (EN) PHASE SHIFTER (Vent)
LATCH (EN) PHASE SHIFTER (Vent)
LATCH (EN) OAC PHASE SHIFTER (Vent)
LATCH (EN) PHASE SHIFTER (Vent)
LATCH (EN ) PHASE SHIFTER (Vent)
LATCH (EN) PHASE SHIFTER (Vent)
LATCH (EN) PHASE SHIFTER (Vent)
LATCH (EN) PHASE SHIFTER (Vent)
LATCH (EN) PHASE SHIFTER (Vent)
LATCH (EN) PHASE SHIFTER (Vent)
LATCH (EN) PHASE SHIFTER (Vent)
LATCH (EN) PHASE SHIFTER (Vent)
LATCH (EN) PHASE SHIFTER (Vent)
LATCH (EN) PHASE SHIFTER (Vent)
LATCH (EN ) PHASE SHIFTER (Vent)
antenna. EIRP equation for whole system comes out to be: FigJ. Digital Network diagram for electronically steerable phased array
EIRP = TXoutpucLosses+ Gain+ Gantenna (1)
PHASE SHIfTER PQWERAMPlIFIER
PHASE SHIFTER POWERAMPLIFIER
PHASE SHIFTER POWERAMPLIAER
PHASE SHIFTER POWERAMPLIFIER
PHASE SHIFTER PQWERAMPlIFIER
PHASE SHIFTER POWERAMPLIFIER
PHASE SHIFTER POWERAMPLIFIER r 16 ELEMENTS
1:16 CIRCULARLY
I
TXOUTPUT
P
WILKINSON
H PHASE SHIFTER POWERAMPLIFIER r
POLARIZED PHASED
POWER POWER H PHASE SHIfTER POWERAMPLIFIER r ARRAY ANTENNA
DIVIDER
PHASE SHIFTER PQWERAMPIIFIER
PHASE SHIfTER POWERAMPLIFIER
PHASE SHIFTER POWERAMPliFIER
PHASE SHIfTER POWERAMPLIFIER
PHASE SHIFTER POWERAMPLIFIER
PHASE SHIfTER POWERAMPLIFIER
PHASE SHIfTER POWERAMPLIFIER
Fig.2. Architecture diagram for electronically steerable phased array antenna
B. Digital Network For SMART electronically steerable
phased array antenna
Digital Network for phased array antenna is responsible for
electronic steering of the antenna beam by providing
appropriate voltage to get certain phase shift. It is also
responsible for gain adjustments for the antenna & is also
responsible for switching of antennas & taking decision which
elements to switch on & which elements to switch off. Hence
digital network makes an antenna a SMART ANTENNA
having both switched beam and adaptive array configuration
simultaneously through algorithms [2].
125
antenna
The brain of digital network is the onboard computer
having algorithms fed into it giving command to latch which
enables & disables and act as a switch for each element to turn
on or off as per requirement. DACs are used to provide
acceptable voltage levels to phase shifters as inputs so they
may provide a certain required phase change, as the phase
shifters are voltage controlled. Phase error depends on the
levels of voltage that DACs can provide and phase shifters can
support.
III. RESULTS FOR SIMULAnON, TESTING &
MEASUREMENTS
A. Gain measurementsfor beamforming network
The gain measurements for beamforming network can be
made without actually changing the phase shifts of phase
shifters so their involvement is not considered here as it will
not scan the beam in other directions. Below is the table
containing the list of gain measurements for different
configurations of ON antenna elements with gain variation,
for covering gain values of all possible combinations.
TABLE!
POSSIBLE COMBINATIONS & GAIN MEASUREMENTS FOR SMART
PHASED ARRAY ANTENNA
8
9
10
II
12
13
14
15
16
12870
11440
8008
4368
1820
560
120
16
I
E c= 65536
14.3 ±O.I
14.8 ±O.I
15.2 ±O.I
15.6 ±O.I
16.05 ±O.I
16.4 ±0.05
16.7 ±0.05
17 ±0.05
17.28 0
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3. 8
9
10
II
12
13
14
15
16
12870
11440
8008
4368
1820
560
120
16
1
Ec= 65536
14.3 ±0.1
14.8 ±0.1
15.2 ±0.1
15.6 ±O.I
16.05 ±O.I
16.4 ±0.05
16.7 ±0.05
17 ±0.05
17.28 0
B. Adaptive phased array antenna gain possibilities
By utilizing the concept of beamforming network &
applying phases to phase shifters, we can increase by the
possibilities of outcomes by many folds. This helps the
antenna to adapt its beam according to the requirements. For
360 ' digital controlled phase shifters number of bits
determines the levels of phase shifts possible. This
phenomenon is illustrated with the help of table mentioned
below:
TABLE II
ADAPTIVE BEAM COMBINATION POSSIBILITIES FOR SMART
PHASED ARRAY ANTENNA
No of Bit
Bits Possible
outcomes
No N/A
phase
shifters
2 4
5 32
6 64
8 256
10 1024
Possible
Combinations
(C)
65536
262144
2097152
4194304
16777216
67108864
Possible
Permutations
(P)
5.687403955x10
13
2.27496 I 582xl0
14
1.8 I 9969266xl0
15
3.63993853 I xlO
ls
1.4559754 I 2xl0
16
5.82390 165x l 0
16
Although all these are combinations are theoretically
possible however implementation & testing of all these
possibilities is not scope of our application.
C. Phased array antenna gain measurement results
Gain for the phased array antenna comes out to be 17.28 dB
and has been measured and tested in anechoic chamber
facility. Grating lobes were not present due to proper inter
element spacing and side lobes have been very low indicating
high performance results as well. Measurement Results for
gain are well matched to simulation results in Ansoft HFSS
vII.
126
dB(GainTotal)
l-dB(Ga01Total)1
2o ,--------------------,
10 +---------r--r--------�
C
.g,-1O +--------+-/----I-i-"c------�
iil
"0
-20 +--�-+_-__r--f-----+_I_-�-_+-�
-30 -j----"----------------=-----j
�0 L-
------------------�
deg{The'a,
Fig.4. Gain measurements for electronically steerable phased array antenna
D. Phased Array antenna axial ratio measurement results
Circular polarization is very critical in satellite
communication & particularly axial ratio is very important
criteria for determining circular polarization as it takes place
when phase difference between electric field components is
90) and are equal in amplitude. Circular polarization is useful
when considering seeing through rain conditions in cases of
radars and satellite communication [3]. An excellent axial
ratio< O.IdB has been achieved through two techniques, first
truncating independent elements and then using elements in
rotated truncated comer configuration further improving the
axial ratio for the entire phased array. Measurement Results
for gain are well matched to simulation results in Ansoft
HFSS vII.
40
35
30
025
.�
"ii 20
·x
�
!g 15
10
o
dB(AxiaiRatioValue)
I-dB{AxiaIRaliov.kJ·,1
� )
/ I
r V I
u
f
) V .. �V V
� � : � � � � � � � 0 � � � � � � � : � �
deg(The,a,
Fig.5. Axial ratio measurements for electronically steerable phased array
antenna
E. Phased array antenna reflection coefficient (s-ll)
measurements
The reflection coefficient is the easiest to measure among
measurements for phased array antenna and is done through
network analyzer. We have used Rohde & Schwarz network
analyzer for our testing. Bandwidth of approximately 150MHz
is achieved which is sufficient for our application.
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4. dB(S(WavePort1.WavePort1)) I - dB(S(WavoPort1 .WavePortl »I
-5 +_-----��----------���
.."
�-10t---------�------+-----4
�
�
�-15t-----------�---+------4
(J)
:>
-� +------------���-----�
-25 "--___________________........J
Frequency (GHz)
Fig.6. S-II parameter measurements for electronically steerable phased array
antenna
IV.PHASED ARRAY ANTENNA RELIABILITY
ANALYSIS
Reliability is an extremely important parameter when
considering an antenna or any other component for space
application [11]. However phased array antenna's reliability
depends on phase shifters, power amplifiers, cables &
connections made. The array gain is increased by 3dB if the
number of elements is doubled; similarly if the elements are
reduced to half then gain will be reduced by 3dB. As per
normal link margin analysis it is recommended to have link
margin of 3dB to ensure safe transmission of data and ensure
safe communication. Reliability of phased array antenna can
be verified by turning off certain switches at a time so that
gain degradation verifY that up till how many elements failed
the link will still remain established.
20
18
2
o
Phased ArrayAnlenna R.liabil�y Analysis I -Gain degradation (dB)I
/
I
7
/
7
/
�
----------
-----
o 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
t�o of OFF Antenna Elements
Fig.7. Reliability & Failure Analysis for electronically steerable phased array
antenna
127
This graph shows that as number of OFF antenna element
increase the gain degradation also increases, however till half
of the elements fail we can ensure that the link will be
established easily as per 3dB margin analysis. This shows
antenna subsystem is highly reliable as link will be established
even if 8 out of 16 elements fail.
V. CONCLUSION
The SMART phased array antenna mentioned in the paper
has the capability of electronic beam steering i.e. adaptive
beam scanning along with independent switched beam
generation capability such that fixed beam and independent
scanned beams can be generated simultaneously while
adjusting gain through the use of digital network involved in
the architecture. Spillover loss can be eliminated as elements
can be independently phase scanned to increase efficiency.
Mentioned is a highly reliable configuration as compared to
others configurations as gain degradation is not catastrophic.
However phased array antennas have high data rates & fast
reaction times as compared to other configurations making
them the ultimate choice in modem & futuristic space
applications
ACKNOWLEDGEMENT
I would like to thank Institute of Space Technology for
providing us the opportunity to work on this extensive
research project. I would also like to thank Pakistan Space &
Upper Atmosphere Research Commission for providing us the
opportunity to use their anechoic chamber facility for Antenna
measurement analysis. In the end I would also like to thank
my family for supporting me at every moment, without them
it could not have been possible.
REFERENCES
[I] u. Bahl & P. Bhartia, Microstrip antennas, 1981
[2] Constantine A Balanis, Antenna Theory Analysis & Design, 3rd Edition.
[3] Hubregt J Visser, Array & Phased array Antenna Basics.
[4] A 32-GHz Microstrip Array Antenna for micro spacecraft Application by
1. Huang.
[5] Y. Lu, D. G. Fang, and H. Wang , "A wideband circularly polarized 2*2
sequentially rotated patch antenna array".
[6] Wonkyu Choi', Cheolrig Pyo. and Jaeick Choi, "Broadband circularly
polarized Comer-truncated Square Patch Array Antenna".
[7] C.-H. Liang, L. Li, and x.-J. Dang,"lnequality condition for grating lobes
of planar phased array".
[8] Thinh Q. Ho*, Charles A Hewett, Lilton N. Hunt, "Lattice Spacing Effect
on Scan Loss for Bat-Wing Phased Array Antennas".
[9] M. G Bray* , D. H. Werner, D. W. Boeringer and D. W. Machuga, "
Thinned Aperiodic Linear Phased Array Optimization for Reduced Grating
Lobes During Scanning with Input Impedance Bounds".
[10] 1. Ehmouda, Z. Briqech, and A Amer , "Steered Microstrip Phased
Array Antennas".
[11]Afromeev, A S.;Kaplun, V. A;Kuzmenko, T. P.;Nakonechnyi, A N.;Sap
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accelerated simulation".
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