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

zhuqi@ustc.edu.cn

guilin@mail.ustc.edu.cn

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.