Hindawi Publishing CorporationInternational Journal of Antennas and PropagationVolume 2013, Article ID 295767, 7 pageshttp://dx.doi.org/10.1155/2013/295767
Research ArticleLadder Arrangement Method for Stealth Design ofVivaldi Antenna Array
XiaoXiang He, SaiSai Huang, Teng Chen, and Yang Yang
College of Electronic and Information Engineering, Nanjing University of Aeronautics and Astronautics, 29 Yudao Street,Baixia District, Nanjing 210016, China
Correspondence should be addressed to SaiSai Huang; [email protected]
A novel stealth design method for X-band Vivaldi antenna arrays is proposed in this paper by ladder arrangement along radiationdirection. Two-element array, eight-element array, and 3 × 7-element array are investigated in this paper. S parameters, RCSs,and radiation patterns are studied, respectively. According to the ladder arrangement of Vivaldi antennas presented, 16.3 dBsmmaximal RCS reduction is achieved with satisfied radiation performance. As simulated and measured, results demonstrate that theeffectiveness of the presented low RCS design method is validated.
1. Introduction
Generally speaking, RCS reduction of antenna will affect theradiation performance and increase the antenna system com-plexity. Li and Liu [1] reduced RCS maximally by 27 dBsmat a specific frequency with two short-circuit pins loaded ineach microstrip unit and cut two H-shape slots. However,this method reduces the RCS only at a relatively narrowfrequency band and angle range. Another powerful method,investigated by Jang et al. [2], is to use the EBG structure,and about 10 dBsm RCS reduction is obtained outside of theoperating band, which also can be acquired with frequencyselective surface (FSS) on radome. In the operating band,the RCS with EBG structure is almost unchanged. Liu etal. [3] use fractal slot on microstrip patch antenna and theRCS can be reduced and the radiation characteristics canbe maintained. There are still some other methods such asholly-leaf-shaped [4] design or using a phase-switched screen(PSS) [5] boundary, which can reduce RCS in operating band.Vivaldi antennas are widely used in fire control system, radar,communication, and electronic countermeasures (ECM) [6]fields due to their high gain, less physical dimension, andbroad bandwidth. Therefore, the stealth design of Vivaldiantennas [6–11] is widely investigated. Based on the differenceof antenna current distribution in the radiating and scattering
status, He et al. [12] proposed a novel stealthy X-band Vivaldiantenna with maximally 19.2 dBsm RCS reduction.
2. Stealth Design Scheme
The stealth design method proposed in this paper is todecrease RCS of an antenna array while maintaining theantenna’s radiating performance. For the array element is anend-fire Vivaldi antenna, we give two schematic diagramsin which ladder arrangement is employed with a quarterwavelength step in radiation direction (see Figure 1).
As we can see from Figure 1(a), when the antenna radi-ates, the electromagnetic waves radiated by lower elementwill propagate an additional quarter wavelength relative tothat of the upper one, and 90∘ phase delay will occur, whichwill not much affect the radiating performance and can becompensated with feeding networks. However, when a planewave is incident to the antenna along the radiation direction,the electromagnetic wave of the lower element will propagatehalf wavelength more than that of the upper element asshown in Figure 1(b). The electromagnetic wave of the twotypes of antenna element generates 180 degrees of phasedifference and can cancel each other out. As a result, the lowRCS antenna array is achieved. In particular, the structure
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𝜆/4
(a)
(𝜆/4) × 2
(b)
Figure 1: The path of scattering and radiating wave with ladder arranged array.
Figure 2: View of the fabricated stealth antenna.
scattering can be decreased with cancellation theory and themode scattering will not be enhanced at least.
In this paper, the stealthyVivaldi antenna proposed byHeet al. [12], is employed as the array element.The photograph isshown in Figure 2, and the detailed parameters can be foundin [7, 12].
3. Results and Analysis
A novel ladder arrangement of a two-element array is shownin Figure 3(a). Height difference between the two units is setto be 7.5mm or a quarter wavelength according to 10GHz.Considering a plane wave incident from the direction 𝜃𝑖 =0 and 𝜙𝑖 = 0, RCS of the two-element array is obtainedwith simulation (for RCS is too low for ordinary chamberto measure, all the RCS results are simulated in this paper),
as shown in Figure 3(b), and the maximal RCS reduction isabout 9.5 dBsm at 12.5 GHz. According to radar function [13]:𝑅4
max = 𝑃𝑡𝐺2
𝜎𝜆2
/((4𝜋)2
𝑆/𝑁), we can know that the detectiondistance of the stealthy antenna array will be decreased by38% consequently. Besides, radiation pattern is also presentedin this paper, as shown in Figure 4; the gain of ladder arrayis reduced in the required direction and the main lobe isshifted about 35∘ degrees away from the radiation orientationneeded. When the exciting sources are adjusted 90 degreesby feeding network, the direction of the main lobe is almostcoinciding with that of the parallel-placed array. The returnloss of the stealthy two-element array is measured withVector network analyzer. The bandwidth is not narrowed bystealth design as shown in Figure 5(a).Themutual coupling isimproved by 18.47 dBsm maximally as shown in Figure 5(b)for the enlarged distance between the phase centers of twoelements, which is an additional advantage of our presentedmethod.
The second example is an eight-element array shown inFigure 6(a). This is an asymmetric structure and the heightdifference is also set to be 7.5mm.The S
55(fifth element from
the left of the ladder arrangement array) is also simulatedand compared with that of the parallel arrangement array. Asshown in Figure 6(b), active S
55of the two types of array in
whole X-band from 8GHz to 12GHz is less than −10 dBsmand meets the engineering application demand. We can seefrom Figure 7(a) that the direction of the main lobe of theeight-element ladder array is offset 6∘ compared to that ofthe parallel array. This is mainly due to the asymmetricalarrangement in the X direction. The offset of the main lobealso reduces by 2.7 dBsm at 0∘ of maximum gain in theH-plane. This problem can also be compensated by phaseshifting network, as illustrated in Figures 7(a) and 7(b), whichis the same as that of the two-element ladder arraymentionedabove. We find that the ladder arrangement proposed in thispaper is very effective in radiation performance from twoexamples above. We also can find from Figure 7(b) that shift-phase ladder array method can restrain back lobe which
International Journal of Antennas and Propagation 3
(a)
7 8 9 10 11 12 13
Parallel placed Ladder placed
−25
−30
−35
−40
−45
−50
−55
−60
−65
Frequency (GHz)
RCS
(dB
sm)
(b)
Figure 3: (a) Two-element ladder arranged array and (b) RCS of two-element array.
Figure 4: (a) E-plane pattern of two-element arrangement (10GHz). (b) H-plane pattern of two-element arrangement (10GHz).
is very important in most applications. Consider a planewave incident from the direction 𝜃𝑖 = 0 and 𝜙𝑖 = 0 tothe two types of array; RCSs of two arrays are both lessthan −20 dBsm as shown in Figure 8; the RCS is signifi-cantly reduced with the proposed stealth design method.Within the X-band, RCS reduction is about 9.54 dBsm at10GHz and 7.8 dBsm average reduction in the whole X-band which is very important in stealth design of associateplatform.
As mentioned in the previous paragraph, due to theasymmetry arrangement of the antenna array, the main lobeof the gain will be shifted, which can also be compensatedby symmetrical arrangement despite the phase compositionfeeding network discussed above. We design a symmetryarray with the middle 3 × 3 elements 7.5mm higher thanthe rest of the other elements, as shown in Figure 9. Ascan be seen from Figures 10(a) and 10(b), the main lobe isalong 0∘ now and the radiation pattern is improved excluding
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−5
−10
−15
−20
−25
−30
0
Frequency (GHz)6 8 10 12 14 16 18
Parallel placed Ladder placed
S 11
(dB
sm)
(a)
−25
−30
−35
−40
−45
−50
−70
−55
−60
−65
Frequency (GHz)6 8 10 12 14 16 18
Parallel placed Ladder placed
S 12
(dB
sm)
(b)
Figure 5: (a) Measured S11
of two-element array. (b) Measured S12
of two-element array.
(a)
−5
−10
−15
−20
−25
−30
−35
−40
−45
Frequency (GHz)7 8 9 10 11 12 13
Parallel placed Ladder placed
S 11
(dB
sm)
(b)
Figure 6: (a) Ladder arrangement of eight-element. (b) Active S55
of eight-element array arrangement.
the gain reduction. Feeding network with phase-shiftingfunction can also be employed to increase themaximumgain.As a result, the radiation performance of symmetrical ladderarrangement is also excellent. The scattering characteristic of3×7-element ladder array is also compared with that of 3×7-element parallel array as shown in Figure 11 with the angle ofthe incident wave being set to be 𝜃𝑖 = 0, 𝜙𝑖 = 0. It is veryclear that RCS of 3×7-element ladder antenna array has beengreatly reduced.Themaximal declination at 10.5 GHz reaches16.3 dBsm and the average RCS reduction is 8.9 dBsm in theX-band. To sum up, from the three examples mentionedabove and the associated RCS reduction listed in Figure 12,
as long as there is a reasonable stair-like layout, we achievea prospective stealth design method of Vivaldi antenna arraywith the same prominent radiation performance.
4. Conclusion
In this paper, a novel stealth design method of antennaarray is proposed and applied to Vivaldi antenna. Maximum16.3 dBsm RCS reduction is achieved with satisfied radiationperformance. The proposed design method is effective andpotential for stealth design of other end-fire antenna arrayssuch as quasi-Yagi antenna array and Fermi antenna array.
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Figure 10: Pattern of 10GHz. (a) E-plane pattern of 3 × 7 unit array. (b) H-plane pattern of 3 × 7 unit array.
7 8 9 10 11 12 13
−5
−10
−15
−20
−25
−30
−35
Frequency (GHz)
Parallel placed Ladder placed
RCS
(dB
sm)
Figure 11: RCS with the frequency of 3 × 7 element.
Conflict of Interest
Theauthors of this paper donot have any conflicts of interests.
Acknowledgment
This paper is supported by Nanjing University of Aeronauticsand Astronautics Research Funding NZ2013208.
Frequency (GHz)7 8 9 10 11 12 13
−5
−10
−15
−20
20
15
10
5
0
Two-elementEight-element3 × 7-element
RCS
redu
ctio
n (d
B sm
)
Figure 12: RCS reduction of different antenna arrays.
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