An x band reflectarray antenna using concentric rings and a cross

An X Band Reflectarray Antenna Using Concentric Rings and a Cross An X Band Reflectarray Antenna Using Concentric Rings and a Cross Tran Nguyen Thi Nhat Le1, Hoang Dang Cuong1, Nguyen Thanh Tam2, Minh[.]

An X-Band Reflectarray Antenna Using Concentric

Rings and a Cross

Tran Nguyen Thi Nhat Le 1 , Hoang Dang Cuong 1 , Nguyen Thanh Tam 2 , Minh Thuy Le 3 , Nguyen Quoc Dinh 1*

1 Le Quy Don Technical University, Ha Noi-City, Viet Nam, E-mail: trannhatle.k47@gmail.com, cuongtdc2@gmail.com, dinhnq@lqdtu.edu.vn

2 Technical College of Communications, Ha Noi-City, Viet Nam, E-mail: nguyenthanhtam85287@gmail.com

3 School of Electrical Engineering, Hanoi University of Science and Technology, Ha Noi-City, Vietnam, E-mail: thuy.leminh@hust.edu.vn

(*Corresponding author: Nguyen Quoc Dinh)

Abstract - In this work, an X-band reflectarray antenna

using two concentric rings and a cross is proposed Three types

of unit cells are designed and analyzed to improve the phase

sensitivity of the unit cell A center-feed reflectarray antenna

with dimensions of 205.7 mm x 205.7 mm is then designed It

performs an excellent gain of 26 dBi at 10 GHz It also achieves

a good aperture efficiency of 67%, a sidelobe level of -15.6 dB,

and a 1-dB bandwidth of 12%

Keywords - Reflectarray, ring, cross, wideband, phase

sensitivity

I INTRODUCTION The reflectarray antenna (RFA) was firstly introduced in

1963 [1] However, until the late 1980s, it was paid more

attention thanks to the development of the printed antennas

Compared to the conventional reflectors, the microstrip

reflectarray has many distinctive advantages, such as high

efficiency, planar, low cost, low profile, easy fabrication, and

capability of reconfiguration [2-4] RFAs also tackle the high

loss in the feeding network in the phase arrays by air feeding,

especially at millimeter-wave Moreover, they do not require

numerous RF transmit/receive modules which are expensive

and cause the systems more complicated Hence, it could be a

candidate for a replacement of the phased arrays antennas in

satellite systems, radars, point to point terrestrial links [5-7]

However, RFAs have a limited bandwidth, which is inherited

from microstrip antennas [8] Moreover, the air feeding

structure causes differential spatial phase delay, which

degrades their bandwidth, especially for moderate and large

sized reflectarrays [8] The bandwidth of a RFA is affected by

two components: the bandwidth of the unit cells and the

structure of the reflectarray that cause the spatial dispersion

As the spatial dispersion is a principle limitation, many

researchers have mainly focused to improve the BW of the

unit cell which is mainly depended on the phase curve, and

changes rapidly around the resonant frequency, leading to the

phase sensitivity If the phase sensitivity is too high, the

etching tolerance can cause phase errors To lower the phase

sensitivity, a thicker substrate can be adopted, but, it leads to

a low phase-shift range and high cost to the antenna Recently,

various techniques have been adopted to broaden the BW of

the unit cells, which are structures in [9-11], or

multi-layer [12, 13] Besides, the reflectarrays using sub-wavelength

elements achieve a larger bandwidth [14-17]

In this work, the authors present a reflectarray antenna

with 256 elements, operating at X-band Three types of unit

cells are analysed and from them, the third unit cell is

proposed to improve the BW of the reflectarray This novel

multi-structures unit cell consists of two concentric rings

inside and a cross outside A foam layer is added to reduce the

phase slope By this way, the phase slope is significantly decreased, but the phase range is still kept at 3600

The paper is divided into four sections Section II shows the progress of analyzing and designing the three unit cells and the performances of these unit cells Section III presents the configuration of the proposed reflectarray antenna and simulation results The conclusion of this work is given in section IV

II UNIT CELL DESIGN

To improve the phase sensitivity of the unit cell for the reflectarray, three types of unit cells are designed, simulated and analyzed The topologies of them are illustrated in Figure

1 Figure 1a presents three types of unit cell (front views and side views) with their dimensions, which are analyzed Figures 1b and 1c illustrate the procedures of changing the size of the first unit cell and second and third unit cells respectively As presented in Figure 1a, all three types of the unit cells are etching on the substrate Diclad 880 with permittivity ɛr = 2.2 The first unit cell is just a cross etching

on the substrate, while both cross and rings are adopted for the second and the third Compared to the second, the third unit cell is placed on a layer of ROHACELL with a dielectric constant of 1.06 The detailed dimensions of the three types of the unit cells are shown in Table I

The unit cells are simulated utilizing a Floquet cell [18] provided by the CST Microwave Studio simulation environment It evaluates the reflection characteristics of unit cells in a virtually infinite array which allows calculating the mutual coupling between elements The operating principle of Floquet port is described in Figure 2 The unit cell is located

at the end of a rectangular cuboid, which is in the same as the unit cell The plane wave is impinged to the aperture of the unit cell and the wave then reflects back to the wave port with

a set of reflection phase and amplitude Explanation of the cell boundary condition and simulation process can also be found

in [19] In reflectarray design, every element owned a private phase, which is calculated from the position of the element and the direction of the radiated beam The required phase-shift range of a unit cell for a reflectarray is more than 360o

To obtain a required phase for an element, varying the size

of elements is typically adopted beside other methods such as element rotation and delay line [19] In this work, to ease fabricating, the size of the elements l is adjusted from 5.0 mm

to 8.5 mm to achieve the phase-shift ranges for elements in the reflectarray Note that to obtain the phase curves, just the size

of element l is adjusted while w1, w2, s1, s2, r1 are constants for all of types of the unit cells Therefore, for the second and third unit cells, r2 is changed proportionally to l

The progresses of changing size of the first unit cell, the

second unit cell and the third unit cell are demonstrated in

more detail in Figures 1b and 1c respectively The

characteristics of three unit cells are simulated by the CST and

plotted in Figure 3

In Figure 3, the phase-shift range of the first unit cell is

just around 325o, not satisfying the phase-shift range

requirement To expand the phase-shift range, the authors

adopted resonant multi-structure using two rings and a cross

as type 2 in Figure 1a This second unit cell obtains a

phase-shift range of 375o

However, the trade-off is the significantly high phase slope, around 700o/mm while the figure for the first

o

equation(1):

(degree mm/ ) l



  

  where ψ is the reflection phase, l is the size of the

(a)

(b)

Fig 3 The reflection coefficients (a) the reflection phases and phase slopes (b) of three unit cells at 10GHz

(b) (a)

5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 -400

-300 -200 -100 0 100 200 300

l [mm]

8.5 GHz 9.0 GHz 9.5 GHz 10.0 GHz 10.5 GHz 11.0 GHz 11.5 GHz

Fig 4 The reflection phases (a) the reflection coefficients (b) of the third unit cell from 8.5 GHz to 11.5 GHz

5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 -0.7

-0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1

Type 1 Type 2 Type 3

l [mm]

5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 -2.0

-1.5 -1.0 -0.5 0.0

l [mm]

8.5 GHz 9.0 GHz 9.5 GHz 10.0 GHz 10.5 GHz 11.0 GHz 11.5 GHz

Fig 2 The unit element: (a) Three types of unit cells (front views and side

views) with their dimensions, (b) The procedure of adjusting the size of

the first unit cell, (c) The procedure of adjusting the sizes of the second

and third unit cells

Fig 2 The model of the floquet port

TABLE I

D ETAIL D IMENSIONS OF T HE U NIT C ELL

0.2 0.2 0.2 0.2 2.4 1.575 1.5

Unit: mm

Unit: mm

Diclad880 Diclad880

w1 s

1

s 2

r1

r2

L

w 1 s 1

s 2

r1

r2

L

h1 a

Diclad880 h1

a

w 2

h1

Type 1

Type 2 and type 3

(a)

(b)

(c)

h2

To decrease the phase slope or phase sensitivity, a foam

layer-ROHACELL is bonded between the ground layer and

Diclad substrate layer as type 3 in Figure 1a As a result, the

phase slope of this third unit cell drops to 200o/mm and the

phase-shift range is still more than 360o

The reflection coefficients of three unit-cells in Figure 3a

are higher than -0.7 dB It is acceptable for reflective unit cells

Among them, the first unit cell has a higher reflection

magnitude but it can not be used for the reflectarray because

the phase-shift range is less than 360o Although the second

and the third unit cells have similar average reflection

coefficients, the third is selected for designing the reflectarray

due to the significantly lower phase slope

The detailed performance of the third unit cell in the band

from 8.5 GHz to 11.5 GHz is presented in Figure 4 The

reflection coefficients of elements with sizes of more than 7.5

mm are significantly decreased at the upper band (10.5 GHz,

11 GHz, and 11.5 GHz) At the lower band (9.5 GHz, 9 GHz,

8.5 GHz), although the reflection coefficients show excellent

results, the phase-shift ranges are not enough 360o From these

results, it can be estimated that the reflectarray using this unit

cell has a gain drop at frequencies further from the center

frequency

III REFLECTARRAY CONFIGURATION DESIGN

In this work, a reflectarray is constructed from 256

elements with the dimension of 205.7 mm x 205.7 mm The

unit elements are arranged with a distance of 12.86 mm

(equivalent to 0.42 wavelength at 10 GHz) The feeder is a

pyramidal horn, which has dimensions of 42 mm x 38 mm and

a peak gain of 13 dBi at 10 GHz Figure 5 illustrates the

geometrical parameters of a reflectarray antenna The

phase-shifts of elements are calculated by equation (2) [19]:



where k0 is the free space wavenumber at the center frequency;

Ri is the spatial distance from the feeder (F) to the ith element,

R(xi, yi) is the phase of the ith element that creates a radiated

beam at (b, b) In this work, just a beam at (b = 0, b = 0) is

radiated, therefore:



0

( , ) 0 ( , )

x y









Thanks to the phase curve in Figure 3b, the reflection

phases of the elements, which are calculated from equation

(3), are convert to the corresponding sizes of the elements on

the x-axis Note that the phase curve in this case is of the third

unit cell Figures 6a, and 6b demonstrate the elements with

various sizes in the reflectarray The distance from the feeder

H is mainly depended on the reflectarray antenna aperture,

which is similar to the focal length of the conventional

reflectors Thanks to [20] and [21], H is calculated for an

optimized aperture efficiency and blocking effect In this

work, the reflectarray antenna aperture is D = 205.7 mm, the

optimized H/D ratio is 0.75, and therefore H =154.3 mm

Fig 7 The radiation pattern of the reflectarray antennas

-40 -20 0 20

Theta [deg.]

10 GHz (horn)

9.5 GHz (co-pol.)

10 GHz (co-pol.) 10.5 GHz (co-pol.)

9.5 GHz (x-pol.)

10 GHz (x-pol.) 10.5 GHz (x-pol.)

9.2 9.4 9.6 9.8 10.0 10.2 10.4 10.6 10.8 23.0

23.5 24.0 24.5 25.0 25.5 26.0

Frequency [GHz]

Fig 8 Gain against frequency of the reflectarray antennas

Fig 5 The geometrical parameters of a planar reflectarray antenna

θ b

φ b

(x i , y i )

F

R i

X

Y

Z H

θ 0

(x 0 , y 0 )

D/2 F’

D/2

r i

Fig 6 The front view (a) and the side view (b) of the

center-fed reflectarray

The performance of the reflectarray is shown in Figures

7, 8 and Table II As shown in Figure 7, the reflectarray

achieves a maximum gain of 26 dBi at 10 GHz while the

sidelobe level is around -16.5 dB The cross-polarization

level at 10 GHz is -36.9 dB Figure 8 shows the obtained

1-dB bandwidth is 12%, from 9.5 to 10.7 GHz As shown in

Table II, the gain drops more and more at frequencies

further the center frequency, which reflects the performance

of the third unit cell

A comparison with other reflectarray antennas using

concentric rings is listed in Table III The proposed antenna

has some good features The sidelobe level of the reflectarray

is -1.5 dB lower than others in Table III Moreover, it shows a

good peak gain at 10 GHz, which lead to an excellent aperture

efficiency of 67% compared to 39.1% in [21], 44% in [22],

33.6% in [23], and 32.2% in [14] However, the bandwidth of

the proposed reflectarray is only 12%, just better than the

reference [22] The aperture efficiency of the antennas is

calculated by equation (4):

2

4

a

G A







 (4) where G is the gain of the reflectarray,  is the wavelength of

the center frequency, A is the physical area of the reflectarray

antenna aperture

TABLE II

G AIN , SIDE LOBE , AND HALF POWER BEAMWIDTH

OF THE REFLECTARRAY ANTENNA

Frequency

(GHz) Max.Gain (dBi) Sidelobe level

(dB) beamwidth (°) Half power

TABLE III

A COMPARISION WITH OTHER REFLECTARRAY ANTENNAS USING RINGS

[22] [23] [24] [15] work This

Frequency

Aperture size

(mm) 135 195 300 430 205.7

1-dB gain

BW (%) 3.3 (-3dB BW) 17.8 15.4 14.2 12

Peak gain

Sidelobe (dB) -15 -15 - -15 -16.5

Cross-pol

Aperture

Effciency (%) 39.1 44 33.6 32.2 67

IV CONCLUSIONS

An X-band reflectarray antenna with two rings and a cross

is proposed in this work The proposed unit cell has multi-structures with a foam layer It achieves a low phase sensitivity with a phase slope of 200o/mm and a phase-shift range of more than 360° The reflectarray with 256 elements obtains a peak gain of 26 dBi at 10 GHz It also has a good aperture efficiency of 67% and a cross polarization of -36.9

dB The simulated results show its 1-dB bandwidth of 12%

ACKNOWLEDGMENT This research is funded by Hanoi University of Science and Technology under project number T2021-SAHEP-007

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