Ladder Arrangement Method for Stealth Design of Vivaldi Antenna Array

A novel stealth design method for X-band Vivaldi antenna arrays is proposed in this paper by ladder arrangement along radiation direction. 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 dBsm maximal RCS reduction is achieved with satisfied radiation performance. As simulated and measured, results demonstrate that the effectiveness of the presented low RCS design method is validated.


Introduction
Generally speaking, RCS reduction of antenna will affect the radiation performance and increase the antenna system complexity.Li and Liu [1] reduced RCS maximally by 27 dBsm at a specific frequency with two short-circuit pins loaded in each microstrip unit and cut two H-shape slots.However, this method reduces the RCS only at a relatively narrow frequency 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 the operating band, which also can be acquired with frequency selective surface (FSS) on radome.In the operating band, the RCS with EBG structure is almost unchanged.Liu et al. [3] use fractal slot on microstrip patch antenna and the RCS can be reduced and the radiation characteristics can be maintained.There are still some other methods such as holly-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, and broad bandwidth.Therefore, the stealth design of Vivaldi antennas [6][7][8][9][10][11] is widely investigated.Based on the difference of antenna current distribution in the radiating and scattering status, He et al. [12] proposed a novel stealthy X-band Vivaldi antenna with maximally 19.2 dBsm RCS reduction.

Stealth Design Scheme
The stealth design method proposed in this paper is to decrease RCS of an antenna array while maintaining the antenna's radiating performance.For the array element is an end-fire Vivaldi antenna, we give two schematic diagrams in which ladder arrangement is employed with a quarter wavelength step in radiation direction (see Figure 1).
As we can see from Figure 1(a), when the antenna radiates, the electromagnetic waves radiated by lower element will propagate an additional quarter wavelength relative to that of the upper one, and 90 ∘ phase delay will occur, which will not much affect the radiating performance and can be compensated with feeding networks.However, when a plane wave is incident to the antenna along the radiation direction, the electromagnetic wave of the lower element will propagate half wavelength more than that of the upper element as shown in Figure 1(b).The electromagnetic wave of the two types of antenna element generates 180 degrees of phase difference and can cancel each other out.As a result, the low RCS antenna array is achieved.In particular, the structure  scattering can be decreased with cancellation theory and the mode scattering will not be enhanced at least.
In this paper, the stealthy Vivaldi antenna proposed by He et al. [12], is employed as the array element.The photograph is shown in Figure 2, and the detailed parameters can be found in [7,12].

Results and Analysis
A novel ladder arrangement of a two-element array is shown in Figure 3(a).Height difference between the two units is set to be 7.5 mm or a quarter wavelength according to 10 GHz.Considering a plane wave incident from the direction   = 0 and   = 0, RCS of the two-element array is obtained with simulation (for RCS is too low for ordinary chamber to measure, all the RCS results are simulated in this paper), as shown in Figure 3(b), and the maximal RCS reduction is about 9.5 dBsm at 12.5 GHz.According to radar function [13]: we can know that the detection distance of the stealthy antenna array will be decreased by 38% consequently.Besides, radiation pattern is also presented in this paper, as shown in Figure 4; the gain of ladder array is reduced in the required direction and the main lobe is shifted about 35 ∘ degrees away from the radiation orientation needed.When the exciting sources are adjusted 90 degrees by feeding network, the direction of the main lobe is almost coinciding with that of the parallel-placed array.The return loss of the stealthy two-element array is measured with Vector network analyzer.The bandwidth is not narrowed by stealth design as shown in Figure 5(a).The mutual coupling is improved by 18.47 dBsm maximally as shown in Figure 5(b) for the enlarged distance between the phase centers of two elements, which is an additional advantage of our presented method.
The second example is an eight-element array shown in Figure 6(a).This is an asymmetric structure and the height difference is also set to be 7.5 mm.The S 55 (fifth element from the left of the ladder arrangement array) is also simulated and compared with that of the parallel arrangement array.As shown in Figure 6(b), active S 55 of the two types of array in whole X-band from 8 GHz to 12 GHz is less than −10 dBsm and meets the engineering application demand.We can see from Figure 7(a) that the direction of the main lobe of the eight-element ladder array is offset 6 ∘ compared to that of the parallel array.This is mainly due to the asymmetrical arrangement in the X direction.The offset of the main lobe also reduces by 2.7 dBsm at 0 ∘ of maximum gain in the H-plane.This problem can also be compensated by phase shifting network, as illustrated in Figures 7(a) and 7(b), which is the same as that of the two-element ladder array mentioned above.We find that the ladder arrangement proposed in this paper is very effective in radiation performance from two examples above.We also can find from Figure 7 is very important in most applications.Consider a plane wave incident from the direction   = 0 and   = 0 to the two types of array; RCSs of two arrays are both less than −20 dBsm as shown in Figure 8; the RCS is significantly reduced with the proposed stealth design method.Within the X-band, RCS reduction is about 9.54 dBsm at 10 GHz and 7.8 dBsm average reduction in the whole Xband which is very important in stealth design of associate platform.
As mentioned in the previous paragraph, due to the asymmetry arrangement of the antenna array, the main lobe of the gain will be shifted, which can also be compensated by symmetrical arrangement despite the phase composition feeding network discussed above.We design a symmetry array with the middle 3 × 3 elements 7.5 mm higher than the rest of the other elements, as shown in Figure 9.As can be seen from Figures 10(a  the gain reduction.Feeding network with phase-shifting function can also be employed to increase the maximum gain.As a result, the radiation performance of symmetrical ladder arrangement is also excellent.The scattering characteristic of 3 × 7-element ladder array is also compared with that of 3 × 7element parallel array as shown in Figure 11 with the angle of the incident wave being set to be   = 0,   = 0.It is very clear that RCS of 3 × 7-element ladder antenna array has been greatly reduced.The maximal declination at 10.5 GHz reaches 16.3 dBsm and the average RCS reduction is 8.9 dBsm in the X-band.To sum up, from the three examples mentioned above and the associated RCS reduction listed in Figure 12, as long as there is a reasonable stair-like layout, we achieve a prospective stealth design method of Vivaldi antenna array with the same prominent radiation performance.

Conclusion
In this paper, a novel stealth design method of antenna array is proposed and applied to Vivaldi antenna.Maximum 16.3 dBsm RCS reduction is achieved with satisfied radiation performance.The proposed design method is effective and potential for stealth design of other end-fire antenna arrays such as quasi-Yagi antenna array and Fermi antenna array.

Figure 1 :
Figure 1: The path of scattering and radiating wave with ladder arranged array.

Figure 2 :
Figure 2: View of the fabricated stealth antenna.

Figure 5 :
Figure 5: (a) Measured S 11 of two-element array.(b) Measured S 12 of two-element array.

Figure 7 :
Figure 7: Pattern of 10 GHz.(a) E-plane pattern of eight-element arrangement array.(b) H-plane pattern of eight-element arrangement.

Figure 8 :
Figure 8: RCS with the frequency of eight-element arrangement .

Figure 12 :
Figure 12: RCS reduction of different antenna arrays.