2010 2nd International Conference on Industrial and Information Systems

A Design of Phased Array Antenna Based on the Vivaldi Antenna

Chen Xianzhong123, Yin Yixin13, Hou Qingwen13 , Li Xiaoli13, Zhu Menghui13 , Liu Kangli13 1 Information Engineering School, University of Science and Technology Beijing, Beijing, 100083, China

2 State Key Laboratory of Millimeter Waves, Nanjing 210096, China 3 Key Laboratory of Advanced Control of Iron and Steel Process (Ministry of Education)

e-mail: cxz@ustb.edu.cn

Abstract-A new design process of a phased array antenna at 2GHz is presented by the simulation method. First a suitable antenna pattern for the radiator of Vivaldi is selected as a slot antenna with light weight, small volume, low cost and high gain. A kind of the analysis software of high frequency structure simulation CST MWS is optimized for the VSWR and radiation pattern. However, several antennas with certain arrangement are often used to make up antenna array to realize beam synthesis. 1 *10H-plane linear array and 16*10 planar arrays are designed and simulated in this paper. Since behavior of element is not directly relevant to their behavior in the array, mutual coupling effect is discussed after the radiator design. The influence on the radiation pattern caused by the number of the array, the distance of the adjacent elements and the phase and the amplitude of the source current are all analyzed, and a simulation result for the parameter design of antenna array is analyzed either.

Keywords-phased array antenna(PAA}; vivaldi antenna; VSWR; radiation pattern; CST MWS

I. INTRODUCTION

With the rapid development of modem radar technology, the requirements of the radar systems on their respective antennas are becoming more and more stringent. Phased arrays are often adopted in many phased array radar systems. This paper attempts to study the process of a simple phased array design with the simulation method.

Stripline-fed Vivaldi antenna as antenna radiator is described and designed fIrst in section II. Section III contains plots of beam scanning for PAA , discussions of the mutual coupling effect and sidelobe controlling, which play an important role in designing a simple 1* IOH-plane P AA. 16* 10 planar arrays are also introduced in this part.

II. DESIGN OF THE VIVALDI ANTENNA ELEMENT

The radiation characteristics of the phased array antenna depend on the design of the radiator and the arrangement of these antenna elements. Meanwhile, the selection of different forms of antenna elements also affects the structure of the phased array and the design of feed network. So, how to design the required antenna element is the No.1 priority for the P AA research.

Among the many different kinds of elements used in array systems, most can be considered either as wire antennas, or slots, even as a combination of these. The advantages and disadvantages of these elements are compared in TABLE I, providing theoretic reference for choosing and further studying PAA element. Taking full advantage of the wideband, directive radiation, input impedance stability, high efficiency, simple structure, low

978-1-4244-8217-7110/$26.00 ©201O IEEE

334

cost and easy integration features, Vivaldi antenna is the best choice as a radiator in our design.

TABLE!.

Charact-eristics

bandwidth gain cost mutual

coupling

feeding

processing

other

COMPARISON OF SEVERAL COMMON RADIATORS' RADIATION PERFORMANCES

The Name of Radiator

patch dipole waveguide slotted Vivaldi linear array slotline

narrow narrow wide wide low low high high low low high low

big small big big

microstrip microstrip coupling aperture

stripline motivatio low-cost printing Printed-

photolithog- technolo- high precision circuit

raphy gy CNC cutting

high gain, low sidelobe level

A. Introduction of Vivaldi Antenna

Vivaldi antenna was pioneered by Gibson in 1979. "The Vivaldi Aerial is a new member of the class of aperiodic continuously scaled antenna structures and, as such, it has theoretically unlimited instantaneous frequency bandwidth. This aerial has significant gain and linear polarisation and can be made to conform to a constant gain versus frequency performance." [1] Besides, with the excellent performance as antenna element, the Vivaldi aerial can be used as the elementary component of the polarization and the bipolarization array, applying to broad-band antenna array or wide scanning angle phased­array [2].The antenna can be also used for all kinds of receiving system.

(a) (b)

Figure 1. (a)Exploded view of single element.(b)portion of infinite stripline-fed Vivaldi notch-antenna array.

The Vivaldi aerial can be formed various kinds of broadband antennas with different feeding structions. A common structure of Vivaldi antenna feeding with strip line is shown in Figure lea). Stripline-fed Vivaldi antennas are comprised of: I) a stripline-to-slotline transition; 2) a stripline stub and a slotline cavity; and 3) a tapered slot.

lIS 2010

The impedances of the slotline cavity and the tapered slot radiator combine at the transition to yield an equivalent series impedance on the feed line. The strip line stub can be represented by a series reactance. The resistance and reactance of the antenna impedance yield insights into the effects of various design parameters.

B. Parameters of a Vivaldi Antenna

Figure 2. Definition of parameters of Vivaldi antenna element with circular cavity and radial stub.

The design parameters of a Vivaldi notch-antenna are defined in Figure I(b) and 2.They can be classified into three categories: substrate parameters (relative dielectric constant, sr, and thickness, t), array grid parameters (H­plane spacing, a, and E-plane spacing, b), and antenna element parameters, which can be subdivided into the stripline/slotline transition, the tapered slot, and the strip line stub and slotline cavity. The stripline/slotline transition is specified by Wsr (strip line width) and WSL (slotline width). The exponential taper profile is defmed by the opening rate and two points P I (z I, y I) and P2 (z2, y2)

Y = ±(CI *eRx + C2) (1)

Taper length L is z2-zi and the aperture height H is 2( y l - y2 ) + WSL . The parameters related to the strip line stub and slotline cavity shown in Figure 2 are as follows: RR, radius of radial strip line stub; AR, angle of radial strip line stub; DsL, diameter of circular slotline cavity; La, offset of slotline cavity from the ground plane; Lrc, distance from the transition to the slotline cavity; LrA, distance from the transition to the taper.

C. Simulation of a Vivaldi Antenna

As shown in Figure 3, an optimized Vivaldi antenna model at 2GHz is emulated by using electromagnetic simulation software CST MWS.

/

(a) (b)

Figure 3. (a) Emulation model of a Vivaldi antenna.(b)sectional view.

Referring to Figure 2, the geometry size of optimized Vivaldi antenna is listed as follows: t=1O.4, E r=2.2, La=35mm, DsL=45mm, WsL=3mm, Lrc=7.5mm, R=O.O I , H=104mm , b= I40mm , Wsr=3.5mm , RR=35mm, AR=90° , L= I80mmo

335

Figure 4 shows VSWR of the emulation model of the antenna element, and as we can see, almost all VSWR are less than two in frequency range 1.1�2.3GHz, reflecting good reflection and radiation characteristics. At the point of 2GHz, VSWR equals to I.367(less than two), indicating good system matching and meeting the system application requirements.

I····· i) \ t , ,! • f !,

\ .... .. .. . : ......

/:"" --- ! , .. .. . I""" "" "

"-

, '-. ---- -;--e--o

O_l 0.4 05 0.6 0.' 0_8 0.9 I t 1 U 1.3 1.4 IS 1.6 11 1" 1.9 II] 2.1 U 23 FreQ.HI'"CY/�

Figure 4. VSWR of the emulation model.

In addition, the radiation diagram is analyzed as shown in Figure 5.The gain of the designed antenna is 7.6dBi, higher than 1.4dBi at the same frequency band in comparison with patch antenna. Figure 5 (a) and (b) illustrate that the width of main lobe of the radiation pattern is 69.5°, 69.88° in XOZ and YOZ plane, respectively, and reveal perfect symmetry performance, which is favorable for array design.

(a)

: �gm. l J':':::OOO_' -5 . •... ..... .. j ........ ··· i ···· ........ ] ... ... . .. ··· i·· · ········t· ..... .... .

-10 . . . .•.•... . 1 . . . . -·_··· 1 -··· ········ 1······· .. . .. j . . .. . . . .. . . ! ......... .

-15 _ .j . . ... j ... . j -�t� + 1 :0671 1 r3: l --do 100

Thelta!DiIO'_

(b)

Figure 5. The radiation diagram of antenna:(a)XOZ-plane.(b)YOZ­plane.

It has been found that there is big relationship between structural parameters of antenna element and its radiation characteristics. For example, VSWR of the slot aerial is related to the antenna itself, and limited by the coupling of slot line and strip line. A detailed discussion of the effects of individual parameters on VSWR is presented in [3].

III. DESIGN OF PHASED ARRAY

A. Analysis of Mutual Coupling Between Elements

In a finite array, all of the currents and fields differ from element to element in magnitude, phase, and distribution, and these differences vary as a function of

frequency and array scan angle. This difference between the performance of isolated antennas and elements in arrays is due to mutual coupling effects, which can be represented efficiently by transmission parameters S2, 1 obtaining from CST MWS.

The mutual coupling effect brings about vector sum of coupling energy between array antenna elements changing with the scan angle, which may result in larger power reflection in specific frequency and direction, and makes radiation pattern resulting in false beam segment. Thus, mutual coupling analy-sis is crucial to the design of P AA.

Two adjacent elements are aligned in Figure 6 (a), the spacing d between the port is l40mm. The value of S2, 1 in Figure 6(b) at 2 GHz is less than -23.8dB under this condition. Results show that mutual coupling effect is smaller when the distance is larger.

(a)

Figure 6. (a) two adjacent elements aligned in line. (b)S2,1.

On the other hand, Figure 7 (a) shows another alignment of two adjacent elements model and corresponding S2,1 is calculated in Figure 7 (b ).As a result, the mutual coupling can be lowered primarily by increasing the spacing d between antenna elements. It shows that at 2GHz, the transmission parameter is less than -15dB d is smaller than 45mm.

(a) �--�----,c------r---,-----r------, d 1"

ro �o ---l����---�, s--rmfi,--�-� F��y/Gt-tz

(b)

.. d-2'!! rl <

, d .. "'C

, ,. d---,c

Figure 7. (a) two adjacent elements aligned vertically. (b)S2,1.

In combination with the contents above and engineering experience, S2,1 need to be less than -15dB at least in order to compose a array. Therefore, the spacing between elements must be more than 140mm in vertical alignment and more than 45mm in line arrangement.

336

B. Cotroll of Side lobe Level ofPAA

One of the major advantages of array antennas is that the array excitation can be closely controlled to produce extremely-Iow-sidelobe patterns or very accurate approxima-tions of chosen radiation patterns. Many intricate procedures have been developed for synthesizing useful array factors.

As is known to all, the sidelobe level of antenna array excited by the current with identical amplitude and phase can only get -13dB theoretically, which would be higher in reality. In many cases, high sidelobe level is harmful because it will generate useless infection, especially causes echo for radar. Moreover, with the changes of side lobe level, directivity of antenna is reduced, and the width of the main lobe is widened. For most applications, both low­sidelobe level and narrow beam-width are required. The procedure commonly referred to as Dolph-Chebyshev synthesis equates the array polynomial to a Chebyshev polynomial and produces the narrowest beam width subject to a given (constant) sidelobe level and vice versa. Taking a 1 * 10H-plane array applying vertical arrangement with the 100mm spacing as example, the following is to account for the radiation pattern difference between two exciting current mentioned above.

.....

(a) (b)

Figure 8. exciting current witb identical amplitude and phase.(b) Dolph-Chebyshev Synthesis.

In regards to exciting current with identical amplitude and phase, there are much sidelobe and the level is - l ldB as we can see in Figure 8(a). Nevertheless, using Dolph­Chebyshev synthesis, the radiation pattern features a mainlobe at a given sidelobe level, -30dB, as illustrated in Figure 8(b). According to reference [4], we can get that Chebyshev incentive amplitude of each unit in a 1 * 1 0 array distribution is :

1: 1.67:2.60:3.41 :3.88:3.88:3.41 :2.60: 1.67: 1 Compared with Figure (a) and (b), it turns out that the

radiation pattern of array has lower sidelobe level, and a good effect on suppressing grating lobe in order to concentrate antenna radiating energy and give prominence to main beam by using Dolph-Chebyshev synthesis. The beam width is about 10 degrees corresponding to side lobe level for -29.9 dB. Therefore, it is more suitable to apply Dolph-Chebyshev synthesis before phase synthesis.

e. Beam Scanning for 1 * 10 PAA

During the design of array, it is important to choose the value of d, the spacing between adjacent elements, and it affects many radiation pattern parameters. If value of d is too big, mutual coupling effect will be small and wave beam will be narrow but easier to generate grating lobe and much sidelobes. Small spacing d results in big mutual

coupling effect and low sidelobe etc. Spacing d between elements in (3), where 17m is maximum beam direction

angle, is satisfied with low sidelobe and no grating lobe radiation diagram. Operating frequency fo is 2GHz, and

wavelength 1 is 150mm, so d equals 150mm. Taking "'1) max

mutual coupling effect into consideration, the element spacing d ranges from 140mm to 150mm in co-line arrangement, resulting much side lobe and small beam­scanning ranges, so vertical alignment is needed. Then, spacing d satisfies 45mm:::;; d < 150mm and 75mm:::;; d < 150mmusing Chebyshev Synthesiso As an

example, we choose d = 100mm in 1 * lO P AA reported here 0

d < A ::::;; A (3)

1+ I cos17rn I cos 17 = sin B*cos¢ (4)

'1/ = -kd cos 17m (5)

In (4)-(5), is angle from array axis to a point in space, is incentives phase of adjacent elements. Equation (5) shows the relationship between phase and scanning angle, that is the basic principle of P AA. TABLE II shows some certain scan angles.

TABLE II. CERTAIN SCAN ANGLES CORRESPONDING To 'P

e 0 5 10 IS 20 25 30 35 1J 90 85 70 65 70 65 60 55

IV'I 0 20.9 41.7 62.1 82.1 101.4 120.0 137.7

Figure 9 Illustrates Chebyshev radIatIOn patterns excited by table II. It is observed that theta angles representing scan angles are changing with 4' , which can realize the scanning without rotating antenna. That is working process of electricity scanning. Among 0 � 40 degrees for theta, sidelobe level is lower and scanning effect is well.

-. -�--:--. .

- .

. � . _- J -.

_ 1. __ .Ii. � -

:--��'- -

• 711 ' -- �

__ �L!�_O -:;{ -

. 7t, ' j

__ �L!:_O -:4 _

(f)25 0 (g)30 0 (h)35 0

Figure 9. I *IOPAA beam scanning.

D. 16*10 Planar Array

Linear array antenna array is the basic configuration and foundation of other complex arrays, such as planar array which is common in practical application. For example, in a furnace burden surveillance system, a 1 *16 linear array with spacing 0.8A can generally reflect the burden shape from the imaging results of Matlab simulation, as shown in Figure 10. In practice, applying 16*10 planar array can realize fuzzy imaging.

The model of 16*10 planar array is shown in Figure 11 (a), with 80mm spacing in X direction and 140mm

337

spacing in Y direction. The H-plane radiation pattern is excited by the current with identical amplitude and phase at 2 GHz in Figure 12(b), whose main lobe magnitude is 26.8dBi, angular width (3dB) is 5.90 and sidelobe level is -13.2dB 0

Figure 10. The simulation of surface roughness of burden. (IS, spacing:0.8Amin, maximum roughness: 8cm, maximum random phase:

360 degree, reference surface: -2m).

,.

(a) (b)

Figure II. (a) model of 16*10 planar array.(b)H-plane radiation pattern.

IV. CONCLUSION

Vivaldi antenna element and its composition of phased array antenna array are designed by using CST, that is aim to understand the principle of electricity scanning for PAA. The antenna element has good radiation and reflection characteristics in frequency band between 1�2.3GHz. We need to organize array for getting to achieve the indexs such as bandwidth, gain and radiation patterns. Furthermore, the bandwidth of the main lobe would be smallest under given level of the side lobe by using Dolph-Chebysheve, improving the performance of the antenna array. As a result, an expected radiation pattern with low side lobe, high gain, no grating lobe and narrow beam is achieved.

This work is partially supported by the National High Technology Research and Development Program 863 (2009AA04Z156), and Key Discipline Project of Beijing Municipal Commission of Education (xk I00080537).

REFERENCES

[I] Gibson, P. 1. The vivaldi aerial. In Pro. 9th European Microwave Conference, Brighton, U K, 1979: 101-106.

[2] Niu Junqian,Lv Shanwei.Vivaldi antenna and its applicationin broadband measuring system. Journal of Astronautic Metrology and Measurement.2004.I,vol. 24,no. 3: 20-23.

[3] Joon Shin, Daniel H. Schaubert. A parameter Study of Stripline­Fed Vivaldi Notch-Antenna Arrays. IEEE transducers on antennaas and propagation, May 1999, vol. 47, no. 5: 879-886.

[4] Lin Changlu,Nie Zaiping. Phase Array Antenna Handbook.Beijing: Electronic Industry Press .2002.

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