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Design of a High Gain and Low Sidelobe Coaxial

Collinear Antenna ArrayQiang Wang, Xiao-Lin Yang, Yan-Tao Li, Zhao-Bo Li, Si-Tao Chen

Institute of Physical Electronics of University of Electronic Science and Technology of China

Chengdu, 610054, China

Abstract-A high-gain and low-sidelobe coaxial collinear(COCO) array antenna is developed in the UHF band. Thebandwidth of the array element reaches to 5.8% and thegain is 6.054dB. To suppress the grating lobes, a newmethod of spacing array element is considered andconfirmed. A 4215 coaxial collinear array antenna isdesigned with this method. The grating lobes of the arrayis down to -30dB. The gain of finally array is 38dB,sidelobe level is less than -30dB, the Half-Power BeamWidth (HPBW) is 2.16 degrees in E-plane, and 2.12degrees H plane.

Index Terms-COCO antenna, high-gain, low-sidelobe,grating lobe

I. INTRODUCTION

The coaxial collinear (COCO) antenna which was first

proposed by B. B. Balsleyh and W. L. Ecklund[1] is a kind of

high gain omni-directional antenna. For its simple structure

and easy fabrication, it is widely used in radar and

communications systems. The mesosphere-stratosphere-

troposphere(MST) radar at Poker Flat, AK, is made of 256

separate coaxial collinear antennas constructed from coaxial

cable[2]. The Jicamarca radar observatory in Peru also

incorporates a large array containing 1536 separate coaxialcollinear antennas constructed of aluminum tubing [3]. An

array of n n-unit coaxial collinear antennas is made of n2

dipoles and has n feed points, compared to a nn dipole array

made of n2 dipoles, with n

2 feed points [4]. This greatly

reduces the complexity in feed network design and the

structure is relatively simple.

As the COCO antenna is composed of multiple units with

each length of , so for each COCO antenna, the lengthwill be greater than a wavelength. According to the grating

lobe formation conditions, grating lobes will be formed in the

coaxial arrangement array composed of COCO antenna. A

baffle technique was proposed by Josefsson, where radiation

occurs between two parallel plates to eliminate the gratinglobes [5]. In this letter, a new method to suppress the grading

lobe is presented. Simulation results show that this method is

feasible.

II. DESIGNANDRESULT

The structure of the COCO antenna is shown in Fig.1. It

consists of pieces of coaxial line connected together and the

length of each coaxial line is . The inner and outerconductors of the two adjacent subsections are stagger

connected. When the excitation current flowing through the

coaxial cable, the phase will change 180, and because of the

stagger-connected inner and outer conductors, the phase

would change another 180.So that it makes the phase of one

unit and the next unit same, and theoretically their amplitude

is approximately same. Then it realizes that the radiation of

every unit is added at the same phase on the far field. So every

coaxial-cable subsection is not only transmission line but also

radiator. The short line with the length of at both end ofthe antenna will reduce the return loss.

Tab.1 shows the gain of the COCO antenna with differentnumber of units and the length of each unit . It can beseen that the more of the unit number, the higher gain will be

obtained. In the same time, the dimension will be longer.

Reasonable number of the units is chosen to increase radiation

performance of the array. When the unit number is 4, the array

will achieve the highest gain in the same space.

2/g

4/g

Fig.1. The structure of the COCO antenna with 6-unit

Unit Number 2 4 6 8 10

Unit Gain(dB) 2.567 6.054 7.642 8.312 9.049Unit Length(mm) 875.4 1307 1802.6 2266.2 2729.8

COCO Number 22 15 11 8 7

Array Gain(dB) 14.73 18.04 17.71 16.54 16.48

Tab.1. The gain of the COCO antenna with different number of units

0.35 0.40 0.45 0.50 0.55-50

0

50

100

150

Antennaimpedance

Frequency(GHz)

Real Part

Imaginary Part

Fig.2. The impedance of the antenna

CST MICROWAVE STUDIO is used to design the

antenna. The proposed COCO antenna is consisted of 4 units.

The impedance curve of the antenna is shown in Fig.2. As can

be seen from Fig.2, the impedance of the antenna is 102 ohms

and the reactance is approximately 0 at the frequency 450MHz.

Selected the 75 ohm coaxial cable feed can match the antenna

better at the center frequency. The simulated return loss of the

COCO antenna is presented in Fig.3. The antenna is designedThis work was supported by Natural Science Foundation of China(10804016)

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to operate over a frequency from 439MHz to 465MHz. It has

an impedance band width of 5.8%.

0.35 0.40 0.45 0.50 0.55-20

-15

-10

-5

0

Returnlosses(dB)

Frequency(GHz)

Fig.3. Return loss of the antenna

For a linearly polarized antenna, its performance is often

described in terms of the E-plane and H-plane patterns [6].

Figure 4 shows the simulated 2-D E-plane at mid-frequency. It

can be seen that the element pattern appears zero in the anglesof 59.7, 120.3 and 180. The H-plane pattern of the antenna

is omni-directional.

According to the grating lobe formation conditions:

(1)Where is the angle of maximum radiation direction,

is the wavelength in the air and is the distance between two

adjacent elements. When is 90 degrees and the antennalength is greater than a wavelength, the antenna array will

appear grating lobes, as shown in Fig.4. If the position of the

grating lobes is adjusted to the zero point of element pattern,

effective suppressing of grating lobe can be achieved.

0 30 60 90 120 150 180-60

-50

-40

-30

-20

-10

0

dB

Theta/Degree

Fig.4. Element pattern and array factor pattern

If all antenna elements are in the same phase andamplitude, the optical path differencebetween two adjacent

elements as shown in Fig.5 is:

(2)Where is the optical path difference between two

adjacent elements, and are the optical path between theelements and the far-field region. When point P is in the far-

field region, and , is much larger than . So that: (3)

When (where k is integer), the radiation of everyelement is added at the same phase on the far field and the

grating lobe is formed. Choose =1 and =30.3, then it canbe educed that =1322mm. The array factor pattern shown in

Fig.4 with 15-element and space between adjacent elements is

1322mm.

2r

1r

Fig.5. Geometry of a two-element array positioned along thez-axis

By CST Microwave Studio, a COCO antenna array with415 elements is designed and simulated. Horizontal spacing

between adjacent elements is 0.7 wavelengths and the element

number is 42. Vertical spacing between adjacent elements is

1322mm and the element number is 15. A floor is placed away from the antenna array to make the antenna

unidirectional radiation.

0 2 4 6 8 10 12 140.0

0.2

0.4

0.6

0.8

1.0

R

elativeamplitude

Element Number

(a)Longitudinal current distribution

0 5 10 15 20 25 30 35 400.0

0.2

0.4

0.6

0.8

1.0

Relativeamplitude

Element number

(b)Transverse current distribution

Fig.6. Normalized current distribution of the array

One of the major advantages of array antennas is that the

array excitation can be closely controlled to produce

extremely-low-sidelobe patterns or very accurate

approximations of chosen radiation patterns [7]. Taylor Line

Source Synthesis is chosen to suppress the sidelobe level. The

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normalized longitudinal and transverse current distribution is

shown in Fig.6.

-1 80 -15 0 - 120 - 90 - 60 - 30 0 30 6 0 9 0 1 20 1 50 180-80

-70

-60

-50

-40

-30

-20

-10

0

Gain(dB)

Theta/Degree

(a)E-plane pattern

- 18 0 - 150 - 12 0 -9 0 -6 0 - 30 0 3 0 60 90 12 0 15 0 18 0-80

-60

-40

-20

0

Gain(dB)

Theta/Degree

(b)H-plane pattern

Fig.7. The pattern of the array antenna

The pattern of the antenna array is shown in Fig.7. The

gain of the array can reach to 38dB. It can be seen in Fig.7 that

sidelobe levels of the antenna array is down to -30dB.

III. CONCLUSION

In this paper, the COCO antenna and its array has beendesigned and optimized. Based on the length of COCO

antenna is greater than a wavelength, we propose a method of

reducing the impact of gating lobe. Simulation results show

that this method is feasible. The gain of finally array is 38dB,

sidelobe level is less than -30dB, the Half-Power Beam Width

(HPBW) is 2.16 degrees in E-plane, and 2.12 degrees in H-

plane.

REFERENCE

[1] B. B. Balslay and W. L. Ecklund. "A portable coaxial collinear antenna".IEEE Trans. Antenna Propagat, vol.AP-20, pp. 513-516, 1972.

[2] B. B. Balsfey, W. L. Ecklund, D. A. Carter, and P. E. Johnston. "TheMST radar at Poker Flat, Alaska".Radio Sci., vol. 15, pp. 213-223, Mar.-Apr. 1980.

[3] G. R. Ochs, "The large 50 Mc/s dipole array at Jicamarca radarobservatory". NBS Rep. 8772, Boulder, CO, Mar. 1965.

[4] Thierry J.Judasz, Warner L.Ecklund, Ben B. Balsley.The CoaxialCollinear Antenna: Current Distribution from the Cylindrical antennaEquation. IEEE Transactions on Antennas and Propagation, vol.AP-35,no.3, pp.327-331, March 1987.

[5] L. Josefsson, "A waveguide transverse slot for array applications," IEEETrans. Antenna Prop, vol. AP-41, no. 7, pp. 845-850, July 1993.

[6] Hertz, H. "Electrical Waves", London, Macmillan and Co, 1893.[7] Robert J. Mailloux. "Phased Array Antenna Handbook". Norwood:

Artech House, Inc.1994.