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The Design of A Focused Sparse Microstrip
Antenna Array
Guilin Sun and Qi Zhu
Key Laboratory of Electromagnetic Space Information, Chinese Academy of Sciences
Department of EEIS, University of Science and Technology of China
Abstract: Focused antenna array is of interest in many applications for its unique
characteristics, by which microwave energy can converge on a determinate spot close
to the antenna aperture in the near-field region. One of the important applications of
focused antenna array is remote sensing (non-contact sensing); focused array can
focus the microwave energy on the target point to get desired parameters [1][2].
Microwave-induced hyperthermia is another important application. The power
deposition is required to be strictly confined on cancerous tissues without heating the
adjacent healthy tissues [3]. RFID reader also is an important application [4][5], by
the use of focused array, it becomes conveniently to limit the interference between
nearby portals and reduce reading errors due to multipath phenomena.
In this paper, the design of focused sparse array has been discussed. Sparse array
technique has been used to depress the sidelobe level (SLL) in the focal plane when
the averaged distance between adjacent antennas is beyond one wavelength.
Numerical result reveals that sparse array technique can depress SLL in focal plane
effectively without extending the focal spot size. Finally, a focused array sparse array
composed of 16 microstrip antennas working at X band has been designed as an
example. Simulated results demonstrate the efficiency of spare array technique on
depressing SLL in the focal plane.
Keywords:
focused array; sidelobe; sparse array; near-field
References:
[1] Mirjana Bogosanovi´c, Allan G. Williamson, “Microstrip antenna array with a
beam focused in the near-field zone for application in noncontact microwave
industrial inspection,” IEEE Trans ON Instrumentation and Mesurement, vol. 56, no.
6, Dec 2007.
[2] K. D. Stephan, J. B. Mead, D. M. Pozar, L. Wang, and J. A. Pearce, “A near field
focused microstrip array for a radiometric temperature sensor,” IEEE Trans. Antennas
Propag, vol. 55, no 4, Apr 2007.
[3] J. Hautcoeur, F. Colombel, X. Castel, M. Himdi and E. Motta Cruz, “Near-field
focused array microstrip planar antenna for medical applications,” IEEE ANTENN
WIREL PR, vol. 13, 2014.
[4] A. Buffi, A. A. Serra, P. Nepa, H-.T. Chou and G. Manara, “A focused planar
microstrip array for 2.4 GHz RFID readers,” IEEE Trans. Antennas Propag, vol. 58,
no. 5, May 2010.
[5] Romain Siragusa, Pierre Lemaître-Auger, and Smail Tedjini, “Tunable near-field
focused circular phase-array antenna for 5.8-GHz RFID applications,” IEEE ANTENN
WIREL PR, vol. 10, 2011.
Guilin Sun received the B. S. degree in EEIS from University of
Science and Technology of China, Hefei, Anhui, in 2011. He is
currently studying for the Ph. D degree in EEIS at University of
Science and Technology of China. His research interests include
focused antenna array, wireless power transfer and implantable
antenna.
Qi Zhu received the B. S degree and M. S degree in physics from
Hefei Univ. of Tech. in 1989 and 1992, and received Ph. D. in
airplane from Nanjing Univ. of Aeronautics and Astronautics. In
1998, He joined University of Science and Technology of China
(USTC), as an Associate Professor and now he is working for USTC
as a Professor. His research interests are in the area of microwave
and millimeter-wave technology, electromagnetic theory.
Guilin Sun,Qi Zhu Dept. of EEIS, UNIVERSITY OF SCIENCE & TECHNOLOGY OF CHINA
The Design of A Focused Sparse
Microstrip Antenna Array
Outline
Research background
Three types of focused antenna arrays
Design and simulation of focused sparse array
Conclusion
Research background
Focused antenna array can converge microwave energy
on a determinate spot in the near-field region.
Application:
Remote sensing (non-contact sensing)
Microwave-induced hyperthermia
RFID reader
Research background
(c)Microwave-induced hyperthermia[3] (b)2.45GHz RFID reader[2] (a)remote sensing[1]
[1] Bogosanovic, et. "Microstrip antenna array with a beam focused in the near-field zone for application in
noncontact microwave industrial inspection." IEEE Transactions on Instrumentation and Measurement 56.6
(2007): 2186-2195.
[2] Buffi, A., et al. "A focused planar microstrip array for 2.4 GHz RFID readers." IEEE transactions on
antennas and propagation 58.5 (2010): 1536-1544.
[3] Tofigh, Farzad, et al. "Near-field focused array microstrip planar antenna for medical applications." IEEE
Antennas and Wireless Propagation Letters 13 (2014): 951-954.
Research background
In all applications, focal spot size and sidelobe level (SLL)
in focal plane are strictly confined.
focal spot size( beamwidth between 3dB points in focal plane)[1]
[1] Sherman, John. "Properties of focused apertures in the Fresnel region." IRE Transactions
on Antennas and Propagation 10.4 (1962): 399-408.
𝑎:the side length of a square focused aperture
λ: free space wavelength
F :focal distance from the aperture to the geometrical focal spot
∆𝑠= 0.8868𝜆𝐹/𝑎
Challenge
:
Research background
Increasing the number of antennas
Increasing the complexity of feeding
network and cost
Increasing the distance between antennas
Result in high SLL, even grating lobes
Research focus:
Research background
Enlarge the focused array using fewer elements.
Control the sidelobe level in focal plane when the
space between adjacent elements is beyond one
wavelength.
Three types of focused antenna arrays
X
Y
Z
(0,0,F)
(x ,y ,0)ii
A focused microstrip antenna array is positioned in the XY plane, the
focal distance is F, the polarization of all elements is parallel to x
direction, the coordinate of the i’ th element is (𝑥𝑖 , 𝑦𝑖 , 0).
Three types of focused antenna arrays
Analysis reveals that periodicity distribution of antennas leads to high
SLL when distance between antennas is beyond one wavelength.
Sparse array technique is adopted in order to destroy the periodicity
and achieve low SLL.
To ensure the electric field radiated from each element in phase at
focal point, corresponding compensating phases φi must be added to
their excitation
𝜑𝑖 = 2𝜋( 𝑥𝑖2 + 𝑦𝑖
2 + 𝐹2 − 𝐹)/λ
Three types of focused antenna arrays
To prove the effect of sparse array, three arrays were designed
and the field distributions in focal planes were calculated.
The overall dimension of sparse array is the same as
two other arrays.
No. Array type Element space Excitation amplitude
1 uniform array Uniform
2 Chebyshev array Chebyshev
3 sparse array Sparse distribution Uniform
Antenna elements: 16 Focal distance F:4λ Frequcency: 10.0GHz
Three types of focused antenna arrays
(2) Chebyshev array (3)Sparse aray (1)Uniform array
The normalized power density in the focal plane of three
arrays is shown below.
Three types of focused antenna arrays
No. Array type SLL
1 uniform array -5.88dB
2 Chebyshev array -5.2dB
3 sparse array -10.44dB
Chebyshev array is not efficient to control SLL when distance
between adjacent elements is beyond one wavelength.
It is obvious that the sparse array can control SLL in focal
plane effectively without extending the focal spot size.
Design and simulation of focused sparse array
A focused microstrip array using sparse array technology
was designed and simulated.
Parameters:
1. Working frequency: 10.0GHz
2. Antenna elements: 16
3. Overall dimension: 140𝑚𝑚 × 140𝑚𝑚
4. Focal distance F: 100mm
Design and simulation of focused sparse array
No. 1 2 3 4 5 6 7 8 5.3 2.3 2.9 27.0 34.7 -59.5 58.4 -55.2
19.5 53.4 -28.4 41.5 -55.3 36.5 -17.6 -14.8
No. 9 10 11 12 13 14 15 16
-45.5 -25.3 -30.1 37.4 11.0 -6.1 39.6 -24.3
-55.5 -21.8 -7.7 -22.0 0.6 -43.7 14.3 17.4
To achieve low sidelobe level, the coordinates of 16
microstrip antennas were optimized by genetic algorithms,
which were listed in following table.
TABLE The coordinates of antenna elements (unit: mm)
Design and simulation of focused sparse array
Initialize Population
Evaluate Fitness
Output Results
Satisfy constraints
Selection
Crossover and Mutation
Yes
No
genetic algorithm
Initialize population:
the coordinates of 16 antennas in XY plane
Fitness function:
The maximum SLL in the focal plane
Constraints:
SLL is below -10dB or cycle index is
beyond 50
The classical genetic algorithm was
adopted as shown in the left picture.
Design and simulation of focused sparse array
Simulated structure of the array
Microstrip antenna
Feeding networkInput Port
patch
feeding line
substrate1
substrate2
pin
ground plane
The entire structure is comprised of three metallic layers (patch,
ground plane and feeding network) and two dielectric substrates,
the thickness of both substrates is 0.5mm and permittivity is 2.65.
Design and simulation of focused sparse array
8.0 8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0
-25
-20
-15
-10
-5
0
S1
1 (
dB
)
freq (GHz)
Simulated returns loss of the focused sparse array
The return loss of the array at 10.0GHz is below -20dB, we
adjusted the excitation phase of each element by changing the
length of microstrip transmission line.
Design and simulation of focused sparse array
-120 -100 -80 -60 -40 -20 0 20 40 60 80 100 120
-25
-20
-15
-10
-5
0
no
rma
lize
d p
ow
er
de
nsity(d
B)
x(mm)
(a) electric field intensity in the focal plane (b) normalized power density on x axis(Z=F)
The electric field intensity in focal plane when 1W power was
input is shown; the maximum of sidelobes only is -10.25dB. The
spot size is 28.7mm.
CONCLUSION
The design and performance of a near-field focused
sparse array composed of 16 microstrip antennas
working at X band is introduced.
Sparse array technique is an effective method to
control SLL in the focal plane when the distance of
adjacent elements is beyond one wavelength.