An Ultra-wideband and Polarization-independent Metasurface for RCS Reduction

In this paper, an ultra-wideband and polarization-independent metasurface for radar cross section (RCS) reduction is proposed. The unit cell of the metasurface operates in a linear cross-polarization scheme in a broad band. The phase and amplitude of cross-polarized reflection can be separately controlled by its geometry and rotation angle. Based on the diffuse reflection theory, a 3-bit coding metasurface is designed to reduce the RCS in an ultra-wide band. The wideband property of the metasurface benefits from the wideband cross polarization conversion and flexible phase modulation. In addition, the polarization-independent feature of the metasurface is achieved by tailoring the rotation angle of each element. Both the simulated and measured results demonstrate that the proposed metasurface can reduce the RCS significantly in an ultra-wide frequency band for both normal and oblique incidences, which makes it promising in the applications such as electromagnetic cloaking.

Scientific RepoRts | 6:20387 | DOI: 10.1038/srep20387 metasurface can significantly reduce the RCS from a bare metal plate in an ultra-wide frequency band for both normal and oblique incidences. The co-polarized RCS reduction is more than 10 dB in 7.9-20.8 GHz for normal x-and y-polarized incident waves. Under oblique incidences, the bandwidth decreases slightly due to phase aberrations. Nevertheless, the proposed metasurface still performs well in the operating band.

Results
Unit cell design. The front structure of the unit cell is depicted in Fig. 1. The structure, composed of a symmetric split ring and a cut wire, is patched on the substrate F4B (thickness h = 3.0 mm, dielectric constant ε = . 2 65 r , loss tangent δ = . tan 0 009). Other dimensions of the structure are listed in Fig. 1, where α represents the open angle of the symmetric split ring, β is the rotation angle of the unit cell. α and β will be mainly considered in the following discussions. For the sake of analysis, u-and v-axes are introduced here along 45° direction with respect to x and y directions.
The simulations of the unit cell are accomplished with commercial software CST Microwave Studio, with periodic boundary conditions in x and y directions and floquet ports in z direction. Fig. 2 shows the simulated reflection of the unit cell for normal x-polarized incidence, with α to be 90° and β to be 45°. In the figure, R xx and R yx represent the reflections of the co-and cross-polarized waves, respectively. It can be seen that cross-polarized reflection is strong over a broad band under the incidence of normal x-polarized waves. Actually, the ultra-wideband property results from multiple resonant modes on the unit cell. The symmetric split ring supports symmetric and anti-symmetric modes excited by electric-field components along v-and u-axes [31][32] , whereas the cut wire supports multi-order modes excited by electric-field components along v-axis. As the combination of the two structure, the unit cell can realize multiple resonances at different frequencies, leading to the broadband performance of the linear polarization conversion. Figure 3 gives the cross-polarized reflection coefficients for different α values and fixed β (β =°45 ). As shown in Fig. 3(a), when the open angle varies from 25° to 145°, the amplitudes of cross-polarized reflections are greater  Scientific RepoRts | 6:20387 | DOI: 10.1038/srep20387 than 0.8 in a broad band. From Fig. 3(b), it can be seen that the linear response of the phase is not affected, except for a constant phase shift due to different α values. The phase shift covers a range of more than 180° over a wide frequency when α changes from 25° to 145°. Due to symmetry, a mirror structure (with respect to y-axis) of the unit cell can produce the same cross-polarized reflection except for a 180° phase shift. As a result, with both structures, full control of the phase of cross-polarized reflection can be achieved while amplitude remains substantially constant. Figure 4 shows the cross-polarized reflection coefficients for different β values and fixed α (α =°90 ). It is clearly that the amplitude of cross-polarized reflection can be continuously controlled in a broad band by adjusting the rotation angle while the phase remains constant.
In general, both the phase and amplitude of cross-polarized reflection can be controlled within a wide band by separately tailoring the open angle and rotation angle of the unit cell. This greatly facilitates the complete control of light propagation and the realization of RCS reduction.
Metasurface design and simulations. The operating mechanism of our proposed metasurface is to diffuse the scattered energy into many directions using random distribution of the reflection phases, and as a result, dramatically reduce the backward scattering [33][34] . The random distribution of the reflection phases is designed using eight types of unit cells, which conforms to a 3-bit coding scheme extended from ref. 35. The eight types of unit cells have phase responses of 0, π/4, π/ 3 8, π/2, π/ 5 8, π/ 3 4, and π/ 7 8, which mimic the '000' , '001' , '010' , '011' , '100' , '101' , '110' and '111' elements, respectively. The dimensions of the eight basic elements, as given in Fig. 5(a), are derived from the relation between the open angle α and the cross-polarized reflection phase at 14 GHz. Then by coding the elements with a random sequence, the whole metasurface is constructed, as shown in Fig. 5(b). When x-polarized waves are normally incident on the metasurface, the polarization of a substantial component of the scattered field is orthogonal to that of the incident wave ( Fig. 6(a)). The reason is that both the symmetric and anti-symmetric modes are excited since x-polarized waves have both u-and v-components simultaneously, maximizing the conversion between x and y polarization. However, when the metasurface is illuminated by normal v-polarized incident waves, only a small component of incident waves are converted to the cross polarization, as shown in Fig. 6(b). Therefore, the 3-bit coding metasurface shown in Fig. 5(b) is polarization dependent.  In order to realize RCS reduction for arbitrary linear polarized waves, the rotation angle of each element is tailored randomly in the range of 0°-360° to construct a random 3-bit coding metasurface (see Fig. 7). As discussed previously, the phase property of cross-polarized wave is not influenced by the change of the rotation angle. Fig. 8(a) shows the monostatic RCS of a bare metal plate and the metal plate coated with the metasurface for normal x-and y-polarized incident waves. It can be seen that the backward scattering from a metal plate can be reduced significantly by covering it with the designed metasurface. As demonstrated in Fig. 8(b), the RCS is suppressed by 8 dB from 7.9 to 21 GHz for both x-and y-polarized incident waves. Fig. 8(c) shows monostatic RCSs of the metasurface for four transmit-receive polarization cases under normal incidences, where the RCS of the metal plate is also given for comparison. In the figure, σ xx , σ yy represent co-polarized RCS (with the incident and backscattering waves in the same polarization), and σ xy , σ yx represent cross-polarized RCS (with the incident and backscattering waves in different polarizations). It is found that both co-and cross-polarized RCS of the metasurface are much less than the monostatic RCS of the metal plate. Furthermore, the co-polarized RCS of the metasurface is close to the cross-polarized RCS, indicating that about half of incident energy is converted to cross polarization. It is worth mentioning that the proposed metasurface shows good polarization conversion performance for both x-and y-polarized incident waves, which demonstrates the polarization independence of the metasurface. As depicted in Fig. 8(d), the co-polarized RCS reduction is over 10 dB in 7.9-20.8 GHz, which greatly decreases the detection possibility of the target by linear polarization radar.
Experimental results. To further verify the design, a sample (240 mm × 240 mm) of the metasurface is fabricated, as depicted in Fig. 9(a). To obtain the RCS reduction, both the scattering coefficients from a bare metal  plate and the metasurface sample are measured. The experiment setup is illustrated in Fig. 9(b), where two antennas are connected to a vector network analyzer (Agilent N5245A). When rotated by 90°, the horn antennas can be reconfigured between transverse-magnetic (TM) and transverse-electric (TE) modes, so that both the co-and cross-polarized scattering can be measured. In addition, the scattering properties of the metasurface under oblique incidences are evaluated by varying the incident angle θ from 0° to 30°. Due to the limitations of experimental conditions, the measurement is conducted only in the range of 6-18 GHz. The measured scattering coefficients from a bare metal plate and the metasurface sample at different incident angles are illustrated in Fig. 10. As expected, the resulting co-polarized scattering of the metasurface has a magnitude comparable to that of the cross-polarized scattering for different incident angles, verifying the polarization conversion property of the  metasurface. After some mathematical manipulating, the co-polarized RCS reduction is achieved for both x and y polarizations as shown in Fig. 11, from which it can be seen that the metasurface can effectively decrease the co-polarized RCS from the bare metal plate. For normal incidence, over 10 dB co-polarized RCS reduction is achieved over a broad band from 8 GHz to 18 GHz. As the incident angle grows, the bandwidth decreases a little due to the phase aberrations. However, the proposed metasurface still performs well in the operating band.

Discussions
In this paper, a metasurface for RCS reduction has been designed, fabricated and measured. The random 3-bit coding metasurface, consisting of eight types of unit cells with different open angles, is designed based on the diffuse reflection theory. The ultra-wideband RCS reduction feature of the metasurface benefits from both the wideband linear polarization conversion property and flexible phase modulation ability of the unit cell. Furthermore,  the polarization-independent feature of the metasurface is achieved by adjusting the rotation angle of each element. Both the simulated and measured results demonstrate that the metasurface can effectively reduce the RCS from the bare metal plate in an ultra-wide band. In comparison to previous approaches, our metasurface for RCS reduction has the advantage of easy fabrication, broad bandwidth and polarization independence of incident waves, which makes it promising for electromagnetic cloaking.

Methods
To obtain the RCS reduction, both the scattering coefficients from a bare metal plate and the metasurface sample are measured. The measurements are carried in a microwave anechoic chamber. The sample is placed as high as the horn antennas in the experiment. The distance between antennas and sample is chosen far enough to avoid the near field effect. Two standard linearly polarized horn antennas serve as transmitter and receiver, respectively. The antennas are connected to a vector network analyzer (Agilent N5245A), which has the function of a time domain gating. When rotated by 90°, the horn antennas can be reconfigured between transverse-magnetic (TM) and transverse-electric (TE) modes, so that both the co-and cross-polarized scattering can be measured. In the case of normal incidence, the transmitting and receiving horn antennas are placed adjacently. Both the transmitting and receiving horn antennas can move along the circumference trace to obtain the scattering at different incident angles.