Design of Ultra broadband Radar Cross Section Reduction Surfaces using Artificial Magnetic Conductors

Amit Kumar Singh and Anjini Kumar Tiwary

Department of Electronics and Communication Engineering,

Birla Institute of Technology, Mesra, Ranchi – 835 215

[email protected], [email protected]: A comprehensive approach aimed to reduce radar cross section using checker board surface when compared to PEC over an ultra-broadband frequency range is discussed. Using the unique phase property of electromagnetic band gap structure to change the direction of scattered field of target, a planar surface comprises of two AMC’S has been designed. A single band unit cell of AMC1 and dual band unit cell of AMC2 has been investigated. An ultra-broadband reduction in RCS is achieved by adopting the methodology of designing the two artificial magnetic conductor (AMC’s), i.e., to have a reflection phase of 00 at separate frequencies, suitably chosen to achieve reduction in RCS in the desired frequency band. The checker board configuration is formed by two different AMC’S with 1800±370 phase difference over more than 99% frequency bandwidth (3.6 GHz to 10.7 GHz). A 10 dB reduction in RCS is observed over the frequency range of 3.78 GHz to 10.7 GHz, which is more than 95% of frequency bandwidth. The maximum Radar cross section reduction is 32.5 dB. The predicted and simulated data show an excellent agreement. Modeling and simulation are done in Microwave Studio by Computer Simulation Technology (CST).

Keywords: Artificial Magnetic Conductor, Electromagnetic Band gap, Perfect Electric Conductor, Radar Cross Section, Perfect Electric Conductor.

I. Introduction

Electromagnetic waves are reflected if they meet an electrically leading surface. If these waves are received again at the place of origin, then that means an obstacle is in the propagation direction. The principle of radar works by transmitting an electromagnetic (EM) wave and detecting a reflected echo from the target. If there is a target present with sufficient signal-to-noise ratio (SNR), then detection is declared. Every target has a signature. In the radar world, this signature is known as the radar cross section (RCS). The RCS is the collective scattering from all the contributors on the platform. Development of modern stealth platform gives tough challenge to reduce RCS of the carrier. Many methods have been employed in the past to reduce RCS such as adding Radar Absorbent Material, changing target’s geometry in order to direct the scattered waves away from the reflected path. On the other hand, one can easily find out that with some special reflection coatings printed on an object the reflected EM wave can further be reduced. As mentioned above, geometry alteration is another widely used option for RCS reduction. The primary objective in geometry alteration is to redirect scattered waves away from the backscattered direction. However, it may compromise performance in terms of aerodynamics. One feasible solution is to utilize artificial impedance surfaces which can redirect scattered waves without altering the geometry of the target. It has been shown in 3 that when a checkerboard patterned artificial impedance surface is designed using an AMC structure in combination with a PEC structure destructive interference is created due to the 180_ phase difference between the reflected fields from each of the two elements. This enables RCS reduction by the checkerboard surface as compared to that of a PEC surface of the same size; scattered fields are redirected in four directions (in the four quadrants). The angular directions of these four lobes can be determined approximately using array theory according to 6, 7. The checkerboard design results in thin surfaces with very low RCS as compared to bulky, lossy materials or Salisbury surfaces 8. Others have applied a similar approach to reduce the radar signature and increase the gain of antennas 9–13. In 14, However, it is well known that AMC technology is band limited 28, 29; thus the 10-dB RCS reduction bandwidth is also band confined. Using a combination of two properly designed AMC structures, instead of a combination of AMC and PEC, a more broadband RCS reduction can be achieved 15

II. DESIGN OF PROPOSED CHECKERBOARD STRUCTURE

This section describes about the design configuration of proposed checkerboard surface, which uses a combination of single and dual band Artificial Magnetic Conductor (AMC). For all the simulation purpose, CST MWS software is used. Here, AMC structures is designed on Rogers RT/duroid-5880 substrate whose dielectric constant = 2.2, thickness = 6.35 mm and followed by a PEC ground plane. Following are the design steps for the proposed checkerboard structure.

A.Design of single band unit cell artificial magnetic conductor

Square patch has been taken as the single band AMC in the proposed check board design as it provides an excellent broad frequency band 1. Once the size of the unit cell is fixed as shown in Fig. 1(a), it has only one design parameter, the length of the patch (x), which influences the frequency of the first resonance, as shown in Fig 1(b).

2749550660400012065018034000

(a) (b)

Fig 1(a) and (b): Unit cell of AMC1 and its Reflection phase diagram

Fig. 1(b) shows the impact of the different patch length on the resonance frequency. It depicts that on increasing patch length resonance frequency shift towards lower value of frequency as shown in Tab

Table 1: Resonance frequency corresponding to different patch length

Patch length (x) (mm) Resonance frequency (GHz)

4.96 7.6

6 7

7 6.5

8 5.8

B. Design of dual band unit cell artificial magnetic conductor.

Different methodologies have been examined for multiband artificial magnetic surface 2. Dual band AMC is designed by using two square patches that has been amalgamated as shown in Fig 2. In the proposed design the outer length (v) of patch is responsible for first resonance as shown in Fig 3(b), and inner length (u) is responsible for second resonance as shown in Fig 3(a).

143891018923000

344297051562000 Fig 2: Unit cell of AMC 2

-88905143500

(a) Fig 3 (b) Fig 3: Impact of (a) u and (b) v on the reflection phase of unit cell of AMC 2

Table 2: Effect of length (in mm) on Resonance frequency (in GHz)

Inner length

(u) Outer length

(v) Fu

Fv

7 13 9 3.2

6 12.5 9.8 3.6

5 12 10.8 4

4 11.5 11.4 4.3

It is necessary to control the second resonant frequency without altering the first resonant frequency, and vice versa, so that a robust control of the bandwidth of the 10-dB RCS reduction can be realized. These requirements can be met by a very basic design which is a combination of an inner patch and an outer ring, as shown in Fig 2. In the proposed amalgamated design, a square patch and a square ring are considered. The outer ring of dimensions (v) being the larger element, is primarily responsible for the first resonance, while the smaller inner patch (of dimension u) for the second resonance.

C.Design of AMC1 and AMC2

Design of two AMC1 and AMC2 has been proposed as shown in Fig 4 in order to satisfy phase difference of (1800±370) 1. It is expected to have 10 dB RCS reduction in the frequency range satisfying phase difference (1800±370).

2970530571500288290508000

(b)

Fig 4 (a) and (b) : Design of AMC 1 and AMC 2.

Here, the AMC structures are designed on a standard Rogers RT/duroid-5880 dielectric substrate (thickness = 6.35 mm, dielectric constant = 2.2) backed by a PEC ground plane.

The dimension of unit cell of AMC’S is given in Table 3.

Table 3: Dimensions for AMC1 and AMC2

Design parameter Dimension

X 4.98

U 4.20

V 11.50

101219048387000The dimension listed in Table 3 satisfies the phase difference condition for a 99.30% fractional bandwidth in the frequency range 3.6 GHz to 10.7 GHz.

Fig 4. Phase reflection as a function of frquency for both AMC1 and AMC2

To achieve broadband RCS reduction using checkerboard surfaces, one should first optimize the checkerboard surface with two single band AMCs. Once it is optimized, one of these AMCs should be converted to dual band AMC to achieve greater RCS reduction bandwidth.

Therefore, a design to achieve broadband RCS reduction using checkerboard surfaces is to combine single and dual band AMCs. The frequencies at which the minimum reflection phase difference of 1430 occurs (necessary for at least 10-dB RCS reduction) are indicated in Fig 4.by black vertical line. In fig 18 Phase reflection of AMC 1 and AMC 2 and satisfy 99.30% bandwidth for RCS reduction.

III. ANALYSIS OF CHECKERBOARDSURFACE

In this section we will analyze the mechanism of checker board surfaces. It is based on the principle that the scattered fields will cancel and form different scattering patterns when the phase difference of the reflected fields between the two structures is180 0. RCS reduction can be approximated by array theory 3. Checker board structure consists of four elements, two AMC1 and two AMC2 and arrange as shown in Fig 5. Here, it is worth noting that the size of AMC super cell doesn’t affect the RCS reduction performance in the normal direction. However, it determines the angular direction of the maxima (?) of the scattered fields, which can be determined using basic array theory. Therefore, the size of the AMC supercells should always be selected incorporating the above factor.

356870768350031000707683500

(a) (b)

Fig 5: Combination of AMC1 and AMC2 forming a square checkerboard

(a) 3-D view (b) Top view

The checkerboard surface can be considered as a planar array with a progressive phase shift of around 180?. Array factor can be represented by equation 1 for this ground plane. The phases of the two AMC structures, as well as the phase difference between them, are plotted in Fig 4.

AF=n=1Nej(n-1)(klxsin? cos?+?x)* k=0nej(n-1)(kly sin? sin?+?y) (1)

Where, ?x and ?y represents progressive phase shift in the x and y direction respectively. Ix and ly represents distance between the element in the x and y direction respectively.

Principal maxima directed along ?0, ?0 can be found using equation

tan?0=?ylx/?xly (2)

sin?2=(?xklx)2+ (?ykly)2 (3)

QUOTE tan?o=?ylx/?xly

The RCS reduction of a scattering surface, compared to perfect electric conductor is given by

RCS reduction = 10log|Es|2/ 10log|Ei|2 (4)

For a dual AMC check board structures, RCS reduction is given by the equation.

RCS reduction = 10 logB1ejQ1+B2ejQ2/2 (5)

Where B1 and B2 are the reflection coefficient amplitudes of the two AMC structures, and Q1 and Q2 are their reflection phases.

Ratio of scattered field and the incident field is called reflection coefficient of the surface. Each AMC’S occupy half area of the check board surface as shown in Fig. 5. So reflection coefficient of check board surface is the average reflection coefficient of the two AMC structure. Hence equation 5 provides a good calculation for RCS reduction for a dual band compared to that of a PEC plane as it doesn’t include the edge effect. Therefore, at least 10 dB RCS reduction can be achieved using a checkerboard surface when the reflection phase difference between the two constitutive artificial magnetic conducting elements is (180°± 37°).

Fig 6: RCS response of Checkerboardsurface for normal incidence.

Fig 6. shows the RCS response of Checker board surface for normal incidence, here the response shows the fractional bandwidth A 10-dB RCS reduction occurs, in the ideal case of an infinite checkerboard surface, within 1800 ± 370 phase difference between the two AMC’s structures. The RCS reduction of the checkerboard surface, compared to the RCS of a metal plate, can be approximated by (5). Thus, the predicted RCS reduction as a function of frequency is illustrated in Fig.6, and it exhibits a 10-dB reduction over a wider frequency band of 3.78-10.7 GHz, which is 95.5% in good agreement with the predicted response as discussed in Table 4.

Table 4

Design Frequency Band (GHz) 10 dB RCS reduction B.W (Simulated) 10 dB RCS reduction B.W (Measured)

36 14.5 – 21.8 N/A 40.22%

37 13.2 – 24.2 58.5% N/A

38 5.1 – 7.6 N/A 32%

39 9.5 – 15.8 N/A 50%

1 3.75 – 10 91% 91%

Purposed design Fig. 5 3.78-10.7 95.5% N/A

In Table 4: a comparison between the 10-dB RCS reduction bandwidth of the proposed checkerboard and the other known wideband checkerboard designs is presented. It is apparent that our proposed novel checkerboard design has the superior 10-dB RCS reduction bandwidth.

IV. CONCLUSION

The checkerboard surface, combining two different designs of AMC’S structures (AMC1 and AMC2), were designed and simulated with CST Microwave Studio. A brief discussion on the proper selection of AMCs to achieve Ultra-broad bandwidth is given. It is showed that a wider 10-dB RCS reduction bandwidth (3.78-10.7 GHz) of over 95.5% is obtained by combining two different types of AMC’S structures comprises of the square and amalgamated square surfaces. The phase reflection curve of two AMC’s shows excellent agreement with 10-dB RCS reduction. The analytical expression of (5) provides a good design guideline of 10-dB RCS reduction of a dual AMC checkerboard surface compared to the corresponding PEC, and it has been verified with simulations of the subject designs.

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