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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.