Multistep Cylindrical Structure Analysis at Normal Incidence Based on Water-Substrate Broadband Metamaterial Absorbers

1 Introduction

The low observability of submarines and other kinds of ships is very crucial to their survivability. However, the submarine’s periscope necessarily increases its observability by above-water sensors in the complex ocean environment [1]. Therefore, there have been various attempts to develop low-observable technologies and radar cross-section (RCS) reduction, such as stealth technology [2], [3], [4], [5], [6], [7], [8].

On the aspect of shape design, Liao et al. [9] developed a wideband RCS reduction measure applicable to rod-like targets. It was thought that this could be integrated with radar absorbing materials to expand the RCS reduction region in both frequency and angular domains. On the other hand, typical microwave absorbers, such as metamaterial absorbers, can minimise the reflection of microwaves and can therefore be used in realising stealth technologies. For example, Kakimi presented a method for broadband terahertz (THz) wave confinement and interaction by a photonic crystal slab, which involved the trapping and capture of the waves as they propagate through free space [10]. Then, based on emerging metamaterials, single-, dual-, multi-, and broadband absorbers could be flexibly realised [11]. Furthermore, the water-resonator-based metasurface is a soft material, which can be bent into a large curvature to transform into complex-shaped structures. Meanwhile, the permittivity of the naturally occurring water is strongly frequency-dispersive at microwave frequencies [12], and therefore water is considered a promising substance for tunable electromagnetic metamaterials and has gained considerable attention recently.

A new design of water-resonator-based metasurface was proposed by Song et al. [13]. Then, Pang et al. [14] proposed a temperature-tunable water-substrate broadband metamaterial absorber (WBMA). However, little information was provided about the analysis of a multistep cylindrical structure at normal incidence based on water-substrate broadband metamaterial absorbers.

In this paper, we propose a new multistep cylindrical structure based on WBMAs to achieve substantial RCS reduction at normal incidence over a wide frequency band. The proposed complex structure, which reduces the RCS by greater than 10 dB over a frequency range of 4.58–18.42 GHz, is fabricated. This large bandwidth is the main advantage of the proposed complex structure, which is significantly better than similar designs reported previously.

2 Theory and Design

2.1 Unit Cell Design of Water-Substrate Broadband Metamaterial Absorbers

One of these unit cells is a symmetrical sandwich structure, which includes two Fr4 layers, a water layer and a metal layer. There are nine cylindrical holes in the first Fr4 layer, as shown in Figure 1. Consequently, water can be filled easily between the second Fr4 layer on the metal plate at the bottom and the first Fr4 layer serving as a lossy source of microwave.

Figure 1:

3D schematic diagram and top view of the water-substrate broadband metamaterial absorber.

To achieve RCS reduction over a large bandwidth, the full-wave simulator CST Microwave Studio software is employed for optimising the unit cell design parameters. The optimum dimensions of these unit cells are a=30 mm, h=2.3 mm, w=0.6 mm, f=0.5 mm, r=1 mm, and d=0.5 mm.

For the sake of simplification, the metal is assumed to have an electric conductivity of 5.8×10⁷ S/m, and the complex form of the permittivity of the Fr4 is taken as 4.3(1−j0.025) in our simulation. The metal thickness is assumed to be 1 mm. The complex form of the dielectric constant of water as a function of the frequency at 20 °C is described by the Debye formula [5]. The absorption is calculated from the reflection coefficient because of the bottom metal with an electrically large thickness. Figure 2 shows the reflectivity and absorption efficiency of this unit cell versus the frequency. As can be seen, an absorption peak exists at the frequency f=15.2 GHz with absorption efficiency of 99.9 % and with absorption efficiency >90 % in the frequency band 8.1–19.2 GHz.

Figure 2:

Reflectivity and absorption spectra of the water-substrate broadband metamaterial absorber optimised at 20 °C.

2.2 Multistep Cylindrical Structure Configuration

As shown in Figure 3, the proposed broadband RCS reduction multistep cylindrical structure configuration consists of n+1 cylinders. The radius and the length of the m^th cylinder are denoted as R_m and L_m. For the sake of simplicity, we set n=3. After using a similar design procedure as in [9], the geometric parameters of the different step orders are derived and presented in Table 1. The resonant frequency is set at 10 GHz, assuming that the required RCS suppression is in the X-band.

Figure 3:

Geometry of a metal cylinder and a metal multistepped cylindrical structure at normal incidence.

Table 1:

Geometric parameters of the multistep cylindrical structure.

Multistep	Radius (m)	Length (m)
1	0.1	0.15
2	0.1075	0.363
3	0.115	0.351
4	0.1225	0.136

Besides, the WBMA is so malleable that it can be bent into a constant curvature to stick to each cylindrical surface. Then, we regarded the metamaterial absorbers as an impedance surface and substituted the complex impedances into the surfaces of the four-step cylindrical structure.

To evaluate the bandwidth of the proposed complex structure, the simulated monostatic RCS for normal incidence versus the frequency from the multilevel fast multipole method (MLFMM) solver in FEKO is presented in Figure 3. The RCS board values are normalised by a metallic cylinder with physical dimensions similar to those of the proposed board to show its ability for RCS reduction.

3 Measurement Results and Discussion

To validate the design idea and simulations presented in this letter, a four-step cylindrical structure based on WBMAs has been fabricated.

Then, this combination is tightened by plastic bolts on the edge, and two tubules are added to fill the prototype with water. In the experiment, we used distilled water, which theoretically has similar electromagnetic parameters as those used in the CST simulation. These antennas cover the operation bandwidth range of 3.1–24 GHz. The monostatic behaviour of the four-step cylindrical structure based on WBMA was compared by the results achieved from the simulation in the similar set-up.

Figure 4 shows the simulation and measurement results of four-step cylindrical structure based on WBMA. They are in general 10–50 dB lower than the baseline reference in the C-, X-, and Ku-bands. The measured bandwidth is 86.5 % (4.58–18.42 GHz) by more than 10 dB, which is in good agreement with the simulated one, as shown in Figure 4.

Figure 4:

Simulation and measurement results of the proposed board RCS reduction (in dB) against frequency.

4 Conclusion

A wideband RCS reduction design for suppressing the RCS peak near normal incidence was proposed for rod-like targets. The proposed design was integrated with the multistep cylindrical structure and WBMAs to expand the RCS reduction region in frequency. The designed structure showed a wideband RCS reduction of more than 86.5 % bandwidth of from 4 GHz to 20 GHz by more than 10 dB, which is a significant enhancement compared to similar designs in the literature. The measurement results showed excellent fit to the simulated curves, thus verifying the power and capacity of the proposed combination.