Open Access
Add to BibSonomy
Abstract
In this paper, a novel design method that integrates broadband agile radiation and controllable scattering performance in one surface is proposed for an active metasurface. Each unit cell of the metasurface is incorporated with tunable devices, such as an electronic or micro-electro-mechanical-system (MEMS) varactor, to obtain frequency or spatial electromagnetic (EM) wave modulation. Besides, a probe is also adopted in the metasurface element so that the element can function as an antenna once fed by a transmitter. In this way, integrated broadband radiation and low radar cross section (RCS) are achieved for this type of active metasurface antenna (AMSA). To illustrate this method, an AMSA with a simple patch structure and lumped varactors, as an example, is demonstrated both numerically and experimentally. The results show that scanning radiation beams are obtained across a broad frequency band. Meanwhile, broadband flexible scattering performance is also realized by tuning the embedded varactors. Thus, a broad working band, reconfigurable radiation pattern, and low RCS are attained simultaneously for the AMSA. Good agreements between simulations and measurements further prove the effectiveness of the proposed method, which may have potential applications in stealth devices. Moreover, this method and the design strategy can be easily extended to other frequency range.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Metasurfaces are one kind of two-dimensional metamaterials composed of artificially periodic or quasi-periodic structures. They have attracted considerable interests due to their powerful manipulation of the amplitude, propagation direction or polarization of electromagnetic (EM) waves. According to the specialized functions, metasurfaces can be classified into various types. The polarization manipulation of EM waves is usually accomplished by polarization conversion metasurfaces which convert the polarization of incident wave into its orthogonal state or others [1–3]. As for amplitude manipulation, perfect metasurface absorbers, as one typical metasurface, can achieve perfect absorption of incoming wave and have been widely investigated to suppress the reflected energy [4–6]. In recent years, researchers also focus on the controlling of EM waves propagation using phase-shifting metasurfaces. For example, when two kinds of reflective metasurfaces with 180° reflection phase difference are combined in a planar chessboard configuration, the incoming radar wave would be split into four reflected beams in the diagonal directions [7–10]. In this way, the common scattering peak mostly occurred in the broadside of a metal plate can be obviously reduced. Besides, tailored reflection or refraction, such as diffusion scattering and anomalous reflection, can be designed at will by elaborately arranging phase-shifting metasurfaces [11–15]. It is noticed that most of these researches are about passive metasurfaces whose performance is fixed once the design is completed. It is believed that active metasurfaces with tunable properties suit the increasingly complex EM environment better. Fortunately, some pioneering active metasurfaces using reconfigurable techniques have also been proposed to produce flexible performances such as working frequency tunability [16–20], steerable polarization states [21–23], and dynamic scattering control [24–26] etc. These excellent works successfully verify the feasibility of active metasurface and provide enlightening ways for the subsequent designs. Nowadays, active metasurfaces with more powerful functionalities and their applications are still very challenging.
Stealth antennas have important applications in many low-observable platforms. The difficulty of stealth antenna design lies in the balance of radiation and scattering performance. Lots of efforts have been spent to reduce antennas’ radar cross section (RCS) and some effective methods including shaping, slotting, loading impedance or probe and so forth have been proposed [27–31]. Actually, the antenna RCS reductions in most of the aforementioned designs are obtained at the cost of acceptable radiation sacrifices. Recently, metasurfaces have also been applied to reduce antenna RCS [32–37]. Different from the above methods, the usage of metasurfaces can control the scattering field with the radiation performance kept or even improved, which is very promising for military applications. However, it is worth noting that most of these metasurfaces are loaded around or above antennas, which inevitably increase the volume and the design complexity.
In this paper, we propose the concept of active metasurface antenna (AMSA), which integrates tunable radiation and controllable scattering performances simultaneously in one surface. To verify the design concept, an AMSA is presented as an example. Each element in the AMSA acts as a reconfigurable microstrip antenna when excited by a coaxial probe. When illuminated by radar waves, the AMSA also exhibit agile scattering performance and thus low-RCS characteristic is realized. Besides, agile operating frequency and multiple functions for both radiation and scattering can be obtained by simply tuning the varactors embedded in each unit cell. Prototypes were also fabricated and tested, and the results verify the effectiveness of the proposed method.
2. Active metasurface antenna (AMSA) and its working principle
Active metasurface antenna (AMSA) is one type of modified metasurface which can function as both a tunable antenna and a controllable metasurface. As shown in Fig. 1, the tunability of the unit cell is implemented by embedding tunable devices such as electronic or Micro-Electro-Mechanical System (MEMS) varactors. Note that the embedded tunable devices along x and y axes, which are depicted by color symbols in Fig. 1, may be different. In order to function as an antenna, a probe is also inserted in the cell and thus it can be feed in the radiating mode. By controlling the varactors along the y direction, as seen in Fig. 1, the operating frequency of the AMSA can be continuously tuned in a desired frequency band. Consequently, broadband radiation property (depicted in Fig. 2(a)) is obtained. Moreover, favorable beam scanning performance, as shown in Fig. 2(b), is also expected when an array is constructed and proper phase shifters are used. To summarize, frequency and radiation pattern tunability can be obtained simultaneously. It is worthwhile to point out that the simple square patch shown in schematic Fig. 1 can be readily replaced by other structures to meet different needs.
Fig. 1 A cell topology of active metasurface antenna (AMSA). The color symbols represent tunable devices such as electronic or MEMS varactors.
Fig. 2 Illustration of the AMSA design concept. (a,b) Radiation properties: frequency reconfigurability and scanning radiation patterns. (c,d) Examples to demonstrate the flexibility of manipulating scattering fields.
In another aspect, the AMSA can regulate the scattering field distribution for different polarized incidence independently. For x-polarized incident wave, the AMSA can be regarded as a conventional metasurface since the feeding is added along y direction. By controlling the varactors along x direction in each unit cell independently, desired reflection phase distribution can be achieved in the whole surface and the reflected beams can be scattered to arbitrary directions. As two examples, Figs. 2(c)-2(d) display the anomalous reflection and chess-board reflection cases for clarity. While for y-polarized incidence, the scattering field consists of both structural mode and antenna mode components. It is noted that the antenna mode scattering is the dominant component due to the sub-wavelength unit cell of the AMSA. So the in-band RCS of the AMSA can be significantly reduced due to the absorption of the match load behind the antenna. While for the out-of-band range, the incident waves are totally reflected, and then the AMSA can be regarded as an ordinary metasurface just like the case of x polarization. Note that the two polarizations are independently controlled. Thus the AMSA demonstrates flexible performance under different polarized incidences.
3. Examples and simulations
3.1 An active metasurface antenna (AMSA)
To validate the design concept, an AMSA is presented as an example here. As shown in Fig. 3, the AMSA comprises 8 × 8 unit cells. Each cell is composed of a square metal patch printed on a grounded FR4 substrate with the thickness of 3 mm and dielectric constant of 4.3. The periodicity of the cell is 20 mm and the patch width is 14 mm. Each edge of the patch is connected to the ground through a varactor and a metallic via in the middle. For clarity, the capacitances of the varactors along x and y directions are denoted as Cx and Cy, respectively. The unit cell is excited with a 50Ω-coaxial probe in y direction, as seen in Fig. 3. The feeding point is 3.5mm-off the cell center.
3.2 Broadband and tunable radiation performance
Firstly, the radiation performance of the AMSA shown in Fig. 3 is investigated. When the feed ports in each unit cell are excited, the simulated reflection coefficients using HFSS are shown in Fig. 4(a). As can be seen, the operating frequency decreases gradually from 3.8 GHz to 2.27 GHz when Cy varies from 0.2 pF to 1.0 pF, which verifies the frequency reconfigurability of the AMSA. Figures 4(c)-4(e) show the 3D radiation patterns at corresponding resonant frequencies. It is observed that well-behaved broadside radiation beams and low side lobe levels (SLLs) are obtained in all cases.
Fig. 4 Simulated radiation performance of the AMSA. (a) Reflection coefficients with different Cy. (b) Scanning radiation patterns in yoz plane at 2.8 GHz with Cy = 0.6 pF. 3D radiation patterns at (c) 3.8 GHz with Cy = 0.2pF, (d) 2.78 GHz with Cy = 0.6pF, and (e) 2.27 GHz with Cy = 1.0pF.
Moreover, if Cy in the whole AMSA is fixed and the cells in each row are fed with gradient input phase along y direction, beam scanning performance can also be expected. Figure 4(b) further plots the simulated radiation patterns in yoz plane with the phase difference Δφ between adjacent rows changes from 0° to ± 45°. It clearly shows that desirable scanning beams are obtained as expected. To sum up, the proposed AMSA achieves tunable radiation properties in broad frequency band.
3.3 Dual-polarized agile scattering and low RCS performance
Different from traditional array antennas, the proposed AMSA can exhibit various scattering functions by tuning the capacitances of varactors. As examples, three working states denoted as State 1, State 2, and State 3, respectively, are presented to illustrate the flexible manipulation of scattering field. Figure 5 shows the corresponding capacitance distributions. For brevity, each cell is coded and represented by colors in both axes. The code in the top right corner represents varactors along y direction, while that in the bottom left corner represents varactors along x direction. The corresponding capacitances Cx and Cy are detailed in Table 1. Take State 1 for example, all the top right corners are represented by code ‘0’, as shown in Fig. 5(a), which means that all the y-oriented varactors have capacitance of Cy = 0.6 pF. While the bottom left corners are represented by two different states, indicating the x-oriented varactors have two values, i.e. Cx = 0.5 pF for code ‘0’ and Cx = 0.7 pF for code ‘1’, respectively.
Fig. 5 Two-dimensional varactor capacitances diagrams for three representative designs. For brevity, each cell is coded and represented by different colors in both axes. (a) State 1 in which chessboard configuration is formed along x-axis. (b) State 2 in which a certain phase gradient is formed along x-axis. (c) State 3 in which two different phase gradients are formed along x- and y-axes, respectively.
Table 1. Varactor capacitance and the corresponding codes (unit: pF)
Figures 6(a) and 6(b) show the simulated RCS versus frequency of State 1 under x- or y-polarized incidence, respectively. For comparison, the RCS of a same-sized metal plate is also given. It is observed that obvious RCS reductions in both polarizations are obtained for the AMSA. As shown in Fig. 6(a), maximum RCS reduction of 12 dB occurs at 2.78 GHz under y-polarized incidence. It is noted that this frequency is exactly the resonant frequency of the AMSA operating in antenna mode. While for x polarization, two RCS reduction peaks of 30 dB and 23 dB occur at 2.69 GHz and 2.88 GHz, respectively, as seen in Fig. 6(b). To understand the RCS reduction mechanism, the 3D far-field scattering beams at these three frequencies are depicted in Figs. 6(c)-6(e), respectively. Figure 6(c) shows that weak scattering is obtained and the peak is still pointing to the normal direction. As analyzed in section II, this is mainly caused by the absorption of the match load. While at 2.69 GHz and 2.88 GHz, the backward scattering wave is split into four symmetrical beams, resulting in a significant suppress of the backward reflection. This working principle coincides with conventional chess-board low-RCS surfaces [8].
Fig. 6 Simulated scattering performance of State 1. RCS versus frequency under (a) y- and (b) x-polarized incidence. 3D scattering beams at (c) 2.78 GHz under y-polarized incidence, (d) 2.69 GHz and (e) 2.88 GHz under x-polarized incidence.
For State 2, as depicted in Fig. 5(b), Cy is fixed to be 0.6 pF, and Cx is set to be 0.43 pF, 0.56 pF, 0.61 pF and 0.66 pF for coding state ‘set to be 0.43 pF, 0.56 pF, 0.61 pF and Actually, here Cx are elaborately chosen so that a certain phase gradient can be produced in the x direction. In this case, the normal incidence will be deflected to θ = 30° in xoz plane theoretically [14]. To verify this point, the simulated scattering performance of State 2 is shown in Fig. 7. Similar to State 1, obvious RCS reduction resulted by absorptivity is also observed at 2.78 GHz under y-polarized incidence, as shown in Figs. 7(a) and 7(c). While for x-polarized incidence, the far-field scattering beams and near-electric-field distribution are shown in Figs. 7(b) and 7(d), respectively. Both figures clearly show that anomalous reflection occurs in xoz plane at θ = 28°, which agrees reasonably well with the theoretical prediction.
Fig. 7 Simulated scattering performance of State 2. 3D scattering beams under (a) y- and (b) x- polarized incidence at 2.78 GHz. (c) RCS versus frequency under y-polarized incidence. (d) Near-electric-field distribution in xoz plane at 2.78 GHz under x-polarized incidence.
It is worthwhile to point out that only in-band RCS reduction is obtained for y-polarized incidence in the above two states. Another example, State 3, is also presented to show the out-of-band RCS reduction of the AMSA under y-polarized incidence. To achieve this goal, two different phase gradients along x and y directions are designed by choosing proper Cx and Cy, as seen in Fig. 5(c) and Table 1. The corresponding far-field scattering beams and near-electric-field distributions are shown in Figs. 8(a)-8(d). As seen from the numerically results, the normal incidences are redirected to θ = 31° in xoz plane (x polarization) and θ = 23° (y polarization) in yoz plane, respectively. Thus the common large backscattering is also effectively reduced. These results further verify the powerful manipulation of the scattering field and the low-RCS characteristic of the proposed AMSA.
Fig. 8 Simulated scattering performance of State 3 at 2.78 GHz. 3D scattering patterns under (a) x- and (b) y-polarized incidence. Corresponding near-electric-field distributions in (c) xoz and (d) yoz planes.
4. Fabrication and measurements
Finally, three samples are fabricated and measured to verify the simulations. Figure 9(a) shows the picture of one prototype. For proof-of-principle, MuRata 0402 GRM capacitors with different capacitances are welded in each unit cell. The detailed capacitances are shown in Table 2. Restricted by the available commercial capacitors, the capacitances used in these samples do not exactly correspond to the above three states (seen in table. 1 and Table 2).
Table 2. Detailed capacitances for different samples (unit: pF)
The tunable radiation performance is measured first. These results are also compared with the re-simulated ones using the practical capacitance employed. Figure 10(a) shows the reflection coefficients of Sample 1 and Sample 2. It is observed that the measured resonant frequency changes from 2.8 GHz to 3.9 GHz when Cy varies from 0.6 pF to 0.2 pF. These frequencies are 0.07 GHz and 0.1 GHz higher than the simulations. Figure 10(b) depicts the co- and cross-polarized radiation patterns at 2.8 GHz in yoz plane. As can be seen, the measured SLL is ‒15 dB and the cross-polarization is lower than ‒22 dB. Thus well-behaved main beam and low SLL are obtained. Moreover, the reasonably good agreements between simulations and measurements verify the tunable and broadband radiation performance of the proposed ASMA.
Fig. 10 Measured radiation performance. (a) Reflection coefficients. (b) Normalized radiation patterns of Sample 1 at 2.8 GHz.
To validate the agile scattering performance of the proposed ASMA, another two prototypes, Sample 1 and Sample 3, were measured in anechoic chamber using free space method [24]. A same-sized metal board is also measured for calibration. The measurement setup is shown in Fig. 9(b). Note that the angle α here is approximately 10°. Figures 11(a)-11(d) show the results of Sample 1 and Sample 3. As shown in Figs. 11(a)-11(b), both samples exhibit remarkable reflection reductions around 2.78 GHz for y polarization. When the incidence is x-polarized, as seen in Fig. 11(c), the measured reflection reduction of Sample 1 exhibits two dips at 2.71 GHz and 2.95 GHz, which is 0.03 GHz or 0.07 GHz higher than the simulations. When the receiving horn is rotated to θ = 40°, the measured results are shown in Fig. 11(d). It shows that a crest appears around 2.8 GHz, indicating the reflection is redirected to θ = 40° at this frequency. All these results are consistent with the analysis in section III.
Fig. 11 Measured reflection performance. The reflection reduction of (a) Sample 1 and (b) Sample 3 under y-polarization. (c) The reflection reduction of Sample 1 under x-polarization. (d) The normalized reflection pattern of Sample 3 under x-polarization.
To summarize, all the measured results in Figs. 10-11 are in accordance with the simulations. The observed discrepancies between simulations and measurements are mainly attributed to the inaccurate chip capacitors modeling and substrate parameters deviations. Other factors, such as imperfect soldering, measurement setup and accuracy, fabrication and assembling errors, contribute to the discrepancy as well. From above results, good radiation and wave-manipulation capabilities of the proposed AMSA are confirmed. Moreover, these results also prove the effectiveness of our multi-functional AMSA design method. It is worth mentioning that the scattering filed control is independent for both polarizations, and more powerful scattering fields can be obtained by elaborately choosing the varactors capacitance.
5. Summary
This paper presents a novel design method to realize multi-functional active metasurface antenna (AMSA) with integrated agile radiation and controllable scattering performance. To obtain radiation property, feeding structure is added to each unit cell of the AMSA. Thanks to the integration of tunable devices, the operating frequency of the AMSA can be tuned to cover a broad band. Thus wideband and agile radiation beams are achieved. Meanwhile, when illuminated by radar waves, the AMSA can flexibly manipulate the reflected waves by simply adjusting the capacitance of the integrated varactors. Multiple scattering functions such as wave absorption and anomalous reflection are presented, and these functions can be freely combined to realize low RCS characteristic. The integrated broadband radiation and low RCS performance of AMSA are verified both numerically and experimentally. In addition, the proposed method can be further extended to achieve more powerful functions such as diffusion scattering and polarization conversion to meet different needs. It is worthwhile to point out that this method and the design strategy can also be scaled to lower or higher frequency range.
Funding
National Natural Science Foundation of China (Grant No. 61701523, 61801508, 61471389 and 61671464); the Natural Science Foundational Research Fund of Shaanxi Province, China (Grant No.2018JM6040); and Postdoctoral Innovative Talents Support Program of China (Grant No. BX20180375).
References
1. X. Gao, X. Han, W. P. Cao, H. O. Li, H. F. Ma, and T. J. Cui, “Ultrawideband and high-efficiency linear polarization converter based on double V-shaped metasurface,” IEEE Trans. Antenn. Propag. 63(8), 3522–3530 (2015). [CrossRef]
2. H. Shi, A. Zhang, S. Zheng, J. Li, and Y. Jiang, “Dual-band polarization angle independent 90 degrees polarization rotator using twisted electric-field-coupled resonators,” Appl. Phys. Lett. 104(3), 034102 (2014). [CrossRef]
3. G. S. Kong, G. Z. Wang, H. F. Ma, and T. J. Cui, “Broadband circular and linear polarization conversions realized by thin birefringent reflective metasurfaces,” Opt. Mater. Express 4(8), 1717–1724 (2014). [CrossRef]
4. N. I. Landy, S. Sajuyigbe, J. J. Mock, D. R. Smith, and W. J. Padilla, “Perfect metamaterial absorber,” Phys. Rev. Lett. 100(20), 207402 (2008). [CrossRef] [PubMed]
5. X. Shen, T. J. Cui, J. Zhao, H. F. Ma, W. X. Jiang, and H. Li, “Polarization-independent wide-angle triple-band metamaterial absorber,” Opt. Express 19(10), 9401–9407 (2011). [CrossRef] [PubMed]
6. X. Liu, T. Starr, A. F. Starr, and W. J. Padilla, “Infrared spatial and frequency selective metamaterial with near-unity absorbance,” Phys. Rev. Lett. 104(20), 207403 (2010). [CrossRef] [PubMed]
7. M. Paquay, J. C. Iriarte, I. Ederra, R. Gonzalo, and P. D. Maagt, “Thin AMC structure for radar cross-section reduction,” IEEE Trans. Antenn. Propag. 55(12), 3630–3638 (2007). [CrossRef]
8. W. G. Chen, C. A. Balanis, and C. R. Birtcher, “Checkerboard EBG surfaces for wideband radar cross section reduction,” IEEE Trans. Antenn. Propag. 63(6), 2636–2645 (2015). [CrossRef]
9. S. H. Esmaeli and S. H. Sedighy, “Wideband radar cross-section reduction by AMC,” Electron. Lett. 52(1), 70–71 (2016). [CrossRef]
10. W. G. Chen, C. A. Balanis, and C. R. Birtcher, “Dual wide-band checkerboard surfaces for radar cross section reduction,” IEEE Trans. Antenn. Propag. 64(9), 4133–4138 (2016). [CrossRef]
11. Z. W. Li, L. R. Huang, K. Lu, Y. L. Sun, and L. Min, “Continuous metasurface for high-performance anomalous reflection,” Appl. Phys. Express 7(11), 112001 (2014). [CrossRef]
12. S. Liu, T. J. Cui, Q. Xu, D. Bao, L. Du, X. Wan, W. X. Tang, C. Ouyang, X. Y. Zhou, H. Yuan, H. F. Ma, W. X. Jiang, J. Han, W. Zhang, and Q. Cheng, “Anisotropic coding metamaterials and their powerful manipulation of differently polarized terahertz waves,” Light Sci. Appl. 5(5), e16076 (2016). [CrossRef] [PubMed]
13. T. J. Cui, M. Q. Qi, X. Wan, J. Zhao, and Q. Cheng, “Coding metamaterials, digital metamaterials and programmable metamaterials,” Light Sci. Appl. 3(10), e218 (2014). [CrossRef]
14. N. Yu, P. Genevet, M. A. Kats, F. Aieta, J. P. Tetienne, F. Capasso, and Z. Gaburro, “Light propagation with phase discontinuities: generalized laws of reflection and refraction,” Science 334(6054), 333–337 (2011). [CrossRef] [PubMed]
15. X. Yan, L. Liang, J. Yang, W. Liu, X. Ding, D. Xu, Y. Zhang, T. Cui, and J. Yao, “Broadband, wide-angle, low-scattering terahertz wave by a flexible 2-bit coding metasurface,” Opt. Express 23(22), 29128–29137 (2015). [CrossRef] [PubMed]
16. B. Sanz-Izquierdo and E. A. Parker, “Dual polarized reconfigurable frequency selective surfaces,” IEEE Trans. Antenn. Propag. 62(2), 764–771 (2014). [CrossRef]
17. P. S. Taylor, E. A. Parker, and J. C. Batchelor, “An active annular ring frequency selective surface,” IEEE Trans. Antenn. Propag. 59(9), 3265–3271 (2011). [CrossRef]
18. W. H. Xu, Y. He, P. Kong, J. L. Li, H. B. Xu, L. Miao, S. W. Bie, and J. J. Jiang, “An ultra-thin broadband active frequency selective surface absorber for ultrahigh-frequency applications,” Appl. Phys. Lett. 118, 184903 (2015).
19. J. Zhao, Q. Cheng, J. Chen, M. Q. Qi, W. X. Jiang, and T. J. Cui, “A tunable metamaterial absorber using varactor diodes,” New J. Phys. 15(4), 043049 (2013). [CrossRef]
20. A. B. Li, Z. J. Luo, H. Wakatsuchi, S. Kim, and D. F. Sievenpiper, “Nonlinear, active, and tunable metasurfaces for advanced electromagnetics applications,” IEEE Access 5, 27439–27452 (2017). [CrossRef]
21. G. H. Du, C. Y. Yu, and C. J. Liu, “Frequency selective surface with switchable polarization,” Microw. Opt. Technol. Lett. 56(2), 515–518 (2014). [CrossRef]
22. W. Li, S. Xia, B. He, J. Z. Chen, H. Y. Shi, A. X. Zhang, Z. R. Li, and Z. Xu, “A reconfigurable polarization converter using active metasurface and its application in horn antenna,” IEEE Trans. Antenn. Propag. 64(12), 5281–5290 (2016). [CrossRef]
23. M. T. Nouman, J. H. Hwang, M. Faiyaz, K. J. Lee, D. Y. Noh, and J. H. Jang, “Vanadium dioxide based frequency tunable metasurface filters for realizing reconfigurable terahertz optical phase and polarization control,” Opt. Express 26(10), 12922–12929 (2018). [CrossRef] [PubMed]
24. H. Yang, X. Cao, F. Yang, J. Gao, S. Xu, M. Li, X. Chen, Y. Zhao, Y. Zheng, and S. Li, “A programmable metasurface with dynamic polarization, scattering and focusing control,” Sci. Rep. 6(1), 035692 (2016). [CrossRef] [PubMed]
25. B. Zhu, K. Chen, N. Jia, L. Sun, J. Zhao, T. Jiang, and Y. J. Feng, “Dynamic control of electromagnetic wave propagation with the equivalent principle inspired tunable metasurface,” Sci. Rep. 4(1), 4971 (2015). [CrossRef]
26. X. Wan, M. Q. Qi, T. Y. Chen, and T. J. Cui, “Field-programmable beam reconfiguring based on digitally-controlled coding metasurface,” Sci. Rep. 6(1), 20663 (2016). [CrossRef] [PubMed]
27. W. Jiang, Y. Liu, S. X. Gong, and T. Hong, “Application of bionics in antenna radar cross section reduction,” IEEE Antennas Wirel. Propag. Lett. 8, 1275–1278 (2009). [CrossRef]
28. Y. B. Thakare Rajkuma, “Design of fractal patch antenna for size and radar cross section reduction,” IET Microw. Antennas Propag. 4(2), 175–181 (2010). [CrossRef]
29. L. J. Zhou and F. Yang, “Radar cross section reduction for microstrip antenna using shaping technique,” in Proc. Int. Conf. Microw. Millimeter Wave Techn. 871–873 (2016).
30. D. R. Jackson, “The RCS of a rectangular microstrip patch in a substrate-superstrate geometry,” IEEE Trans. Antenn. Propag. 38(1), 2–8 (1990). [CrossRef]
31. D. M. Pozar, “Radar Cross Section of Microstrip antenna on normally biased ferrit substrate,” Electron. Lett. 25(16), 1079–1080 (1989). [CrossRef]
32. Y. H. Liu and X. P. Zhao, “Perfect absorber metamaterial for designing low-RCS patch antenna,” IEEE Antennas Wirel. Propag. Lett. 13, 1473–1476 (2014). [CrossRef]
33. T. Liu, X. Y. Cao, J. Gao, Q. R. Zheng, W. Q. Li, and H. H. Yang, “RCS reduction of waveguide slot antenna with metamaterial absorber,” IEEE Trans. Antenn. Propag. 61(3), 1479–1484 (2013). [CrossRef]
34. Y. Liu, K. Li, Y. Jia, Y. Hao, S. Gong, and Y. J. Guo, “Wideband RCS reduction of a slot array antenna using polarization conversion metasurfaces,” IEEE Trans. Antenn. Propag. 64(1), 326–331 (2016). [CrossRef]
35. Y. Jia, Y. Liu, Y. J. Guo, K. Li, and S. X. Gong, “Broadband polarization rotation reflective surfaces and their application on RCS reduction,” IEEE Trans. Antenn. Propag. 64(1), 179–188 (2016). [CrossRef]
36. Y. Liu, Y. Hao, K. Li, and S. Gong, “Radar cross section reduction of a microstrip antenna based on polarization conversion metamaterial,” IEEE Antennas Wirel. Propag. Lett. 15, 80–83 (2016). [CrossRef]
37. K. Li, Y. Liu, Y. T. Jia, and Y. J. Guo, “A circularly polarized high-gain antenna with low RCS over a wideband using chessboard polarization conversion metasurfaces,” IEEE Trans. Antenn. Propag. 65(8), 4288–4292 (2017). [CrossRef]
