Novel Broadband Reflectarray Antenna

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Novel Broadband Reflectarray Antenna

with Windmill-Shaped Elementsfor Millimeter-Wave Application

Hua Li & Bing-Zhong Wang & Ping Du

Received: 11 December 2006 /Accepted: 14 March 2007 / Published online: 28 March 2007# Springer Science + Business Media, LLC 2007

Abstract A novel broadband millimeter-wave reflectarray antenna composed of windmill-

shaped elements of variable lengths is proposed. Compared to the conventional single-layer

reflectarray elements, the windmill-shaped elements can realize much larger phase variation

range (over 600°), leading to broader bandwidth. Using this technique, a 15°-beam-steering

reflectarray operating at 30 GHz is designed. The computed results demonstrate the

agreement of the main beam steering with the design requirement, and a 1-dB gain

bandwidth close to 20% is obtained. The validity of the obtained results is verified bycomparing the ones generated by Ansoft High Frequency Structure Simulator (HFSS) with

those produced by Ansoft Designer. The antenna is useful for millimeter-wave applications.

Keywords Reflectarray antenna . Windmill-shaped element . Broadband . Millimeter-wave

1 Introduction

Reflectarray antennas are low profile reflectors consisting of a planar array of microstrip

patches, with a certain tuning to produce prescribed beam shape and direction when

Int J Infrared Milli Waves (2007) 28:339 – 344

DOI 10.1007/s10762-007-9218-8

This work was supported by the National Natural Science Foundation of China (No. 90505001), the

High-Tech Research and Development Program of China (No. 2006AA01Z275), and the Creative

Research Group Program of UESTC.

H. Li (*) : B.-Z. Wang : P. Du

Institute of Applied Physics, University of Electronic Science and Technology of China,

Chengdu 610054, People’s Republic of China

e-mail: [email protected]

B.-Z. Wang

e-mail: [email protected]

H. Li

Department of Applied Physics, University of Electronic Science and Technology of China,

Chengdu 610054, People’s Republic of China



illuminated by a primary source (Fig. 1) [1]. These antennas are rapidly becoming an

attractive alternative to the traditional parabolic reflector antennas and the array antennas in

some applications because of its advantages, such as a low profile and lower loss, especially

in millimeter wavebands [2]. Associated with the many advantages of the reflectarrays,

there are also some serious shortcomings that need to be resolved, particularly the issue of

limited bandwidth (about3∼5%). To achieve a wider bandwidth for a conventional

reflectarray, techniques such as using thick substrate for the patch and stacking multiple

patches have been employed [3, 4]. However, multilayer configurations are costly and

generally impractical, due to surface wave effects, at millimeter-wave frequencies such as

the Ka-band [5]. A thick substrate, however, has a large unattainable reflection phase range,

which has an adverse effect on gain and overall radiation efficiency [6].In this paper, a novel single-layer windmill-shaped element structure for millimeter-wave

application is proposed to improve the bandwidth. By simply adjusting the size of the

elements, a much larger phase variation range compared to the conventional single-layer

reflectarray elements is achieved, which gives contribution to a wider bandwidth. In order

to validate the phase data, we designed a 15°-beam-steering reflectarray operating at

30 GHz. The simulation results generated by Ansoft High Frequency Structure Simulator

(HFSS) and Ansoft Designer demonstrate that the main beam steering agrees with the

Feed

Reference plane

Element

Dielectric substrate

Metal ground

x

y

Fig. 1 Geometry of the reflectar-

ray antenna.

Windmill-shaped patch

Ground

t

a

(Side view)(Top view)

x

y

l

w

l

d

l

a

g

Fig. 2 Top and side views of the windmill-shaped element.

340 Int J Infrared Milli Waves (2007) 28:339 – 344



design requirement and the 1-dB gain bandwidth is close to 20%. This new reflectarray

antenna is very useful for millimeter-wave broadband applications.

2 Element structure and simulation

The reflectarray concept is based on the scattering characteristics of microstrip patches. So

the key technique in the design is how the individual elements are made to scatter theincident wave with the proper progressive phase shift necessary to produce a phase

coherent beam in a specified direction. Different solutions have been proposed in

literatures: microstrip patches with different resonant lengths [7, 8], patches of the same

size loaded with stubs of variable length, or identical patches with different angular rotation

[9 – 12]. The phasing method using variable size patches is a common choice in many

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

-700

-600

-500

-400

-300

-200

-100

0

100

R e f l e c t e d p h a s e

( d e g . )

Length l (mm)

t =1mm

t =1.5mm

t =2mm

Fig. 3 Reflected phase at normal

incidence versus windmill-

shaped patch length l for different

substrate thickness t a ¼ 6mm;ð er ¼ 1:05; w ¼ 0:75mm;

d ¼ 0:3mm; g ¼ 0:15mmÞ.

a side view

b top view

a

b x

z

θ

Fig. 4 Schematic plan of linear array excited by a normal incidence plane wave.

Int J Infrared Milli Waves (2007) 28:339 – 344 341



designs due to its simplicity. But the phasing range (about 340°) of such method is not able

to cover all the phases in 0°∼360° [13], which limits the antenna bandwidth. In order to

improve the phasing range, we design a new element structure as shown in Fig. 2. It is a

windmill-shaped metal patch etched with some rectangle slots on a grounded substrate. The

structure parameters are selected with a ¼ 6mm; er ¼ 1:05; w ¼ 0:75mm; d ¼ 0:3mm;

g ¼ 0:15mm, and l ¼ 0:1mm 1:5mm.

The phase of the reflected wave is controlled by changing the length of the arm and thesize of the center square slot. Varying such sizes changes the impedance of the windmill-

shaped element and therefore the phase of the reflected wave. The reflected-phase analysis

is carried out by using an equivalent unit cell-wave-guide approach (WGA) [14] with

Ansoft High-Frequency Structure-Simulator (HFSS) software. WGA assumes that a uniform

plane wave is normally incident on an infinite array of periodic structure, and a pair of perfect

magnetic conductors and a pair of perfect electric conductors form the four waveguide side

walls. The reflected-phase is obtained by the phase of reflected coefficient S 11.

-80 -60 -40 -20 0 20 40 60 80

-50

-40

-30

-20

-10

0

10

20

R a d i a t i o n

p a t t e

r n

( d B )

θ (deg.)

HFSS

Designer

Fig. 5 Computed E-plane radia-

tion patterns of the windmill-

shaped patch reflectarray.

20 25 30 35 40-15

-10

-5

0

5

10

15

20

G a i n ( d B )

Frequency (GHz)

Windmill-shaped patch reflectarray

Square patch refelctarray

Fig. 6 Gain against frequencyfor the windmill-shaped patch

reflectarray and the square patch

reflectarray at 30 GHz.




The phase of the reflected wave versus the length l is illustrated in Fig. 3. It can be seen

that very large phasing ranges are obtained. Even if the substrate thickness t is up to 2 mm,

the phasing range is still more than 600°.

3 Array design and simulation

In order to validate the phase data of the elements, we designed a 15°-beam-steering

reflectarray operating at 30 GHz. Figure 4 shows the basic geometry of this reflectarray

using 1×18 windmill-shaped patches of variable size. The center spacing between elements

is 0.6 l in the x-direction. The substrate thickness is selected with 2 mm. In general, the

feed may be positioned at an arbitrary angle and distance from the reflectarray, but is

assumed to be far enough from the reflectarray so that the incident field can be

approximated by a plane wave. In our design, we set that the incident field is a plane

wave coming from the normal direction (θ=0°) and linearly polarized along the x-axis. Tosteer the main beam off at a given angle θb relative to broadside, the required phase shift of

the nth element is given as [15]:

7 n ¼ 2:

1 na sin θb ð1Þ

According to (1) and Fig. 3, we can choose the dimensions of the elements.

Ansoft HFSS and Ansoft Designer are based on the Finite Element Method (FEM) and

the Method of Moments (MOM), respectively. They are used to produce the E-plane ( xOz

plane) radiation patterns, as shown in Fig. 5. Both results demonstrate that the main beam isat 15°, quite agreeing with the design requirement.

To make a comparison of gain bandwidth between the windmill-shaped patch

reflectarray and the conventional single-layer reflectarray, a same reflectarray composed

of 1×8 square patches is designed and simulated. Figure 6 shows the computed gain against

frequency for the two reflectarrays. As shown in this Figure, a 1-dB gain bandwidth of 20%

is achieved by the windmill-shaped patch reflectarray, which is wider than that for the

square patch reflectarray, namely 13.23%.

4 Conclusion

A novel broadband millimeter-wave reflectarray antenna composed of windmill-shaped

elements of variable lengths is proposed in this paper. The windmill-shaped element has the

advantage of realizing much larger phase variation range, compared to the conventional

single-layer reflectarray elements. Using this technique, a 15°-beam-steering reflectarray

operating at 30 GHz is designed. The computed results demonstrate the agreement of the

main beam steering with the design requirement and a 1-dB gain bandwidth close to 20%.

This new reflectarray antenna is very useful for millimeter-wave broadband applications.

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Novel Broadband Reflectarray Antenna

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