In this paper, a novel design strategy that integrates good radiation and broadband low radar cross section (RCS) characteristics based on the concept of metasurface is proposed. The metasurface element adopts an etched cross patch and it directly behaves as a radiating structure. After that, a metasurface-based thinned array antenna A1 and a checkerboard metasurface antenna A2 are designed. The -10 dB operating bandwidth of these two antennas is 13.08–14.92 GHz (13.1%). Compared with the conventional rectangular grid array, A1 and A2 have similar radiation performance along with in-band and out-of-band RCS reduction (RCSR) in any polarized normal incidence. Reasons and merits of different arrangements are analyzed. Simulated and measured results verify the effectiveness of the design strategy.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Metamaterials refer to a class of artificial structures comprised of periodic or quasi-periodic arrangement of subwavelength units . Since it possesses many novel physical properties that conventional materials do not have, like negative refraction, beam control, and inverse Cherenkov radiation , a variety of microwave devices have been invented including perfect metamaterial absorbers (PMA) , polarization converters , and metamaterial lens  during the recent twenty years. Metasurface, as the two-dimensional equivalence of metamaterial, has attracted increasing attention due to its powerful manipulation of electromagnetic waves [6–8]. Moreover, its merits of low profile, easy fabrication and conformability makes it have enormous application potential and development prospect.
Antenna is a transmission device, which also acts as a transducer between guided wave and free space. Microstrip antenna has the natural characteristic of low profile, so it is easy to be conformal with the platform. Many researches have demonstrated that its integration with metasurface can improve the radiation performance of antenna along with reduce antennas’ influence on aerodynamic performance of the platform [9–12]. Moreover, current researches on metasurface antennas have expanded from reflective type to transmission one [13,14]. High gain [15,16], low side-lobe level (SLL) [17,18], broadband  and other extraordinary characteristics have also been explored in terms of metasurface antennas.
Nowadays, the low RCS platform design has been attached great importance in modern war. As an essential part of the wireless communication system, antenna is the main contributor of the overall RCS . Especially on some platforms with special needs, the simultaneous regulation of radiation and scattering is urgent in need. In the early stage of the research, absorbers  or diffuse scattering metasurfaces  were placed around the microstrip antenna for RCS reduction. And a gain enhancement was achieved meantime in . Besides, a low RCS antenna was designed by integrating one-dimensional high impendence surface to suppress surface wave . Holographic metasurface could not only be regarded as the radiation parasitic patch, but was also applied to suppress RCS by redirecting the wave propagation . Later in [25–27], the integration of radiation and scattering was realized in a true sense. Many novel configurations and strategies were proposed to advance the wireless communication systems and stealth technology.
The researches mentioned above have indeed inspired our thought to some extent, while there are still two problems urged to be solved. One is that in-band RCS reduction has a few sustainable solutions. At present, out-of-band RCS can be largely reduced by the use of FSS radomes , but in-band stealth problem is still a relatively tougher one without affecting antenna radiation performance. The other is that once utilizing the theory of phase cancellation, periodic structures are needed, resulting in an increased aperture size and there are few researches on quasi-periodic ones. Thinned array antenna, which is achieved through the elimination of some elements from the regular uniform antenna array according to a certain proportion, or the connection of those elements to the matched load, can not only reduce the weight and cost of the array, but also obtain the narrow beam equivalent to the full array . Thus, it has been successfully applied in satellite and radar systems. As a result, it has great potential in integrating with metasurfaces.
In this paper, a novel design strategy that integrates good radiation and broadband low RCS characteristics in one metasurface is proposed. Different from previous designs, which arrange metasurface units around the antenna, the proposed method is inspired by thinned array so as to integrate radiation and scattering performances simultaneously in one surface. To verify the design strategy, two topologies with different quasi-periodic cell arrangements are presented as examples. Each element acts as a microstrip antenna when fed by a coaxial probe. When illuminated by normal incident waves, the metasurface array antenna exhibits a broadband low RCS characteristic due to the 180° phase difference between the metasurface cell and the dielectric cell. Hence, two antennas we designed realize the RCS reduction in a wideband along with low side-lobe in the radiation performance brought by the thinned array distribution.
2. Results and discussions
2.1 Unit design and array antenna distributions
As a non-rotational symmetric structure, the unit is composed of a top etched cross shaped metal patch layer, a middle dielectric layer and a bottom metal layer. The dielectric is F4B, whose dielectric constant is 2.65, and loss tangent is 0.001. The unit periodic is P=10 mm, and its thickness h=3 mm.Using ANSYS HFSS software to optimize the parameters, values are tabulated in Table 1. The radiation characteristics of the element are analyzed under the radiation boundary and the lumped port feeding mode. The reason why that linear polarized unit cells are employed is because the slot along y-axis has little effect on cross-polarized phase in broadband. As shown in Fig. 1, the impendence bandwidth of |S11| is 13.1% (13.08–14.92 GHz), and the realized gain in the working frequency band is 5.26–6.62 dBi. It is also observed that the deepest resonant frequency lies in 14.0 GHz, so λ0 is defined as the wavelength in the free space at 14.0 GHz.
From Fig. 2(a), it can be figured out that the fundamental mode of the patch is (1, 0) mode , and the current intensity is strong on the edge of metallic patch as well as the etched bars parallel to the y-axis, which means the current flows as assumed. Figure 2(b) shows the radiation pattern of the unit in 14.0 GHz, which indicates the element has a well-behaved radiation characteristic.
Because thinned array arrangement is utilized, the grounded substrate between antenna element is a good participant for designing an out-of-phase unit. Therefore, the grounded substrate is regarded as “0” unit, and the element can be seen as “1” unit for phase cancellation. Reflection performances of “0” and “1” unit are shown in Fig. 2(c) and (d). According to the scattering cancellation principle to achieve 10 dB RCS reduction, the phase difference satisfies 180°±37° in the interval of 8.8 GHz–18.0 GHz under the y-polarized incident wave as Fig. 2(d) depicts.
Thus, the condition to avoid the emergence of grating lobe can be indicated as
Here, θ refers to 0°. The unit tile periodicities in x and y directions are dx = dy = 0.467λ0 which are smaller than the value of 0.732λ0. So, the grating lobe suppression is easier for thinned arrays to satisfy. Based on the units above, three configurations of array antennas are designed as Fig. 3 shows. All of them have the same aperture of 60 mm×60 mm (2.80λ0×2.80λ0) with the unit numbers of N1=18, N2=20 and N3=36, respectively.
A1 is designed in the form of metasurface-based thinned array antenna and A2 is designed as a checkerboard coding metasurface-based one. A conventional rectangular grid with the same diameters as A1 and A2 is set as a reference array antenna (RA), so that the strength of the thinned array can be compared and analyzed in the following section.
2.2 Simulation analysis of the radiation performance of metasurface array antennas
Figure 4 depicts the simulated reflection coefficients of the three array antennas. The working frequency band of RA. A1 and A2 is 12.12–13.99 GHz, 12.23–13.77 GHz, and 12.47–14.01 GHz, respectively. As shown in Fig. 5, the radiation performances of the three arrays are simulated. Figure 5(a) plots the realized gain in the broadside direction. As can be seen, the maximum gain of A1, A2 and RA is 18.2 dBi, 16.9 dBi and 18.5 dBi, respectively. The gain of A1 is quite close to RA within the working band, and A2 has no more than 2 dB lower gain compared with RA. As depicted in Fig. 5(b), they all have similar radiation performances in the frequency of 13.5 GHz, and the radiation beam of A1 has a good directivity with 19.8° of half-power beamwidth (HPBW). In addition, it can be noticed that A1 has a lower side-lobe than others due to its advantage of the thinned array. 3-D radiation patterns of A1, A2 and RA are shown in Fig. 5(c)-(e). It can be investigated that the energy which A1 radiates is more concentrated than others. Having understood the advantages of thinned array distribution when radiating, it can be concluded that the distances between adjacent cells in RA are 0.47λ0 so that the decreasing gain is not brought by the excuse of gate lobe. Furthermore, under the condition that all arrays have the same radiation aperture, A1 and A2 can realize similar radiation performance with fewer elements. And for the reason that A2 has an obvious in-band RCS reduction, it has no more than 2 dB gain difference compared with RA, rather than 3 dB in theory.
Therefore, the comparison shows that the radiation performances of array antenna A1 and RA are similar, and the realized gain difference between them is less than 0.8 dB in the operating frequency band. The distinction is that A1 and A2 decrease 50% and 44.4% number of the units and feed structures, respectively. In a word, thinned array antenna can realize high stable gain and more effective radiating with fewer units.
2.3 Simulation analysis of the scattering performance of metasurface array antennas
Figure 6 makes a contrast of monostatic RCS (MRCS) performance among A1, A2 and RA ranging from 12 to 20 GHz. A1 can reduce MRCS from 12.4–20.0 GHz in x-polarization and 12.0–20.0 GHz in y-polarization, while A2 reaches MRCS reduction from 12.3–20.0 GHz in x-polarization and 12.0–20.0 GHz in y-polarization, resulting in in-band and out-of-band RCS reductions simultaneously for both polarizations.
Figure 6(a) suggests the RCS performance under x-polarized incident wave. It is observed that more than 6 dB RCSR is obtained from 14.7 GHz to 19.5 GHz (BW=28.1%) by A1, and from 12.9 to 19.1 GHz (BW=38.8%) by A2. The peak RCS reduction values are 16.4 dB and 16 dB, respectively. The RCSR is the combined result of phase cancellation along with the absorbing. For y-polarized wave, A1 has no less than 6 dB RCSR ranging from 15 to 15.9 GHz and from 17.3 to 19.7 GHz. A2 has 6 dB RCSR in the whole frequency band as shown in Fig. 6(b). At 17.4 GHz, A2 reaches the maximum RCSR up to 41.9 dB. It is noticed in the Fig. 2 that the unit has a structure resonant frequency around 18 GHz. A2 has a better performance in RCS reduction than A1 because it is constituted with periodic structures, avoiding the fringe effect. However, the quasi-periodic A1 can also reduce RCS with a peak value 18.5 dB.
Above all, it can be concluded that the array antennas proposed have the polarization-insensitive characteristic as shown in Fig. 6(c), the low broadband RCS performance when inclined polarization (θinc=0°, φinc=60°) under incidence confirms that the deduction is still kept. 3D scattering patterns are given at two typical frequencies in Fig. 6(d) and (e). The reflective energy from A2 is mainly divided into four directions while the energy from A1 is diffused into multiple directions, so it conveys that thinned arrangement is another method to achieve RCS reduction.
In order to illustrate the strength of A1, the bistatic RCSs at xoz plane and yoz plane are given in Fig. 7. The distributions lead A1 and A2 to manipulate the scattering field in different ways. As Fig. 7 shown, a focused scattering beam is obtained with the peak pointing to normal direction by RA, while A1 diffuses the energy into multiple directions, and A2 mainly reflects incident wave into four spatial directions. As the main merit of A1 in contrary to checkerboard structure A2, the normally impinging plane wave is scattered into random multiple directions. Thus, there is no evident power peak value over the space, leading to an effective suppression of backward reflection. After the analysis on the Fig. 6 and Fig. 7, we can draw the conclusion that the proposed array antennas have stable RCS reduction ability to any incident wave polarization.
The radiation and scattering performances of the proposed design are compared with similar works in Table 2. Compared with the F-P antenna  and the grooved ground antenna , the proposed design achieves a wideband RCS reduction by using an integrated strategy without increasing the antenna profile. In , only in-band RCS is reduced. Furthermore, the antenna proposed can reduce 50% number of array elements so as to reduce the weight and cost of the devices, while others cannot. Besides, in current literatures, there are many works on uniform arrays rather than quasi-periodic ones.
Overall, composed of same elements, the array antennas designed have comparable radiation gain under the same antenna aperture size. Compared with conventional full array, A1 and A2 have fewer cell numbers and enable to reduce in-band and out-of-band RCSs which cover the X-band and Ku-band simultaneously.
3. Experiments and results
To verify the correctness and feasibility of the above designs, two types of improved metasurface array antennas, along with the reference array antenna, are fabricated. The three types of array antennas’ radiation patterns and RCS performance are measured in an anechoic chamber, as demonstrated in Fig. 8. During the measurement, three power dividers are used for the excitation of arrays.
As shown in Fig. 9(a) and (b), the measured normalized radiation patterns of proposed antennas at 14 GHz are consistent with the simulations. The small discrepancy between measured and simulated patterns is attributed to substrate parameters deviations and fabrication errors. Generally, they indicate a high agreement between the experiment and the simulations, except for some directions where the signals are rather weak to detect. In addition, Fig. 9(c) and (d) show the measured RCSR under x- and y-polarized normal incident wave for the proposed antenna A1, A2 relative to RA. Restrict to the experiment condition, the RCSR of A1 and A2 is measured within 12.3–17.6 GHz. There is a consistency between simulation and measurement in Fig. 9 and Fig. 6: obvious in-band and out-of-band RCS reductions are obtained. Compared with the simulated monostatic RCSR, the trend of the overall curves is basically consistent, though it moves to the lower frequency slightly.
Thinned array antenna can save the cost of devices and simplify the antenna structure by using the same aperture size with fewer array elements to achieve higher resolution. Metasurface-based thinned array antennas with wideband low RCS are designed for this reason. The cell designed has the merit of good radiation ability and stable phase difference in scattering performance. The experiment compares the differences among three kinds of array antennas, analyzes the advantages of the thinned antenna arrangement and then fabricates and measures them. As a result, A1 and A2 are demonstrated wideband low scattering ability under any polarized normal incidence along with the comparable radiation performance. To conclude, this paper has a guiding significance to the design of array antenna on the stealth platform in the future.
Natural Science Basic Research Program of Shaanxi Province (No.2019JQ-103, No.20200108, No.2020022, No.2020JM-350); National Postdoctoral Program for Innovative Talents (No.2019M653960, No.BX20180375); National Natural Science Foundation of China (No.61801508, No.6217012409).
The authors declare no conflicts of interest.
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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