Broadband Polarization Rotation Reflective Surfaces and Their

IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 64, NO. 1, JANUARY 2016

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Broadband Polarization Rotation Reflective Surfaces and Their Applications to RCS Reduction Yongtao Jia, Ying Liu, Member, IEEE, Y. Jay Guo, Fellow, IEEE, Kun Li, and Shu-Xi Gong

Abstract—A novel broadband polarization rotation (PR) reflective surface (PRRS) with a high polarization conversion ratio (PCR) is proposed, which can reflect the linearly polarized incident wave with 90◦ PR. The proposed PRRS consists of a periodic array of square patches printed on a substrate, which is backed by a metallic ground. By connecting the square patch with the ground using two nonsymmetric vias, a 49% PR bandwidth is achieved with a high PCR of 96%, which is a significant improvement from the state-of-the-art 29% PR bandwidth. Moreover, the frequency responses within the operation frequency band are consistent under oblique incident waves. Furthermore, another ultra-wideband PRRS with a periodic array of quasi-L-shaped patches is proposed, which increases the PR bandwidth further to 103%. In addition, the designed PRRS is applied to wideband radar cross section (RCS) reduction. Different arrangements of the unit cells of the PRRS are proposed and their effects on RCS reduction are investigated. To validate the simulation results, prototypes of the PRRSs are fabricated and measured. The measured results are in good agreement with the simulated ones. Index Terms—Polarization rotation (PR), radar cross section (RCS), reflective surface.

I. I NTRODUCTION

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WING to its wide applications in radar section reduction and circularly polarized antennas, there has been a growing research interest in polarization rotators [1]–[5]. Generally speaking, polarization rotators can be categorized into transmission and reflection types. For the transmission type, one can exploit either the birefringence effect of anisotropic metamaterials [6]–[8] or the optical activity of chiral metamaterials [9]–[11]. Most of these structures operate at a narrow single or two separated narrow bands [6]–[9], [11]. In [10], an ultra-thin chiral metamaterial slab stacked with twisted complementary split-ring resonators (SRRs) for highly efficient broadband polarization transformation is proposed. Its conversion efficiency reaches up to 96% with a bandwidth of 24%. For the reflection type, the plasmon resonances and asymmetric

Manuscript received July 08, 2015; revised October 09, 2015; accepted November 14, 2015. Date of publication November 23, 2015; date of current version December 31, 2015. This work was supported by the National Natural Science Foundation of China under Grant 61372001, Grant 61401336, and Grant 61471278. Y. Jia, Y. Liu, and S.-X. Gong are with the Science and Technology on Antenna and Microwave Laboratory, Collaborative Innovation Center of Information Sensing and Understanding, Xidian University, Xi’an 710071, China (e-mail: [email protected]). K. Li is with the National Laboratory of Antenna and Microwave Technology, Xidian University, Xi’an 710071, China. Y. J. Guo is with the Global Big Data Technologies Center, University of Technology, Sydney, N.S.W. 2007, Australia (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TAP.2015.2502981

high-impedance surface are utilized to realize the polarization rotation (PR) in [12]–[19]. A mushroom-like artificial magnetic conductor (AMC) with the vias offset from the center of the patches is used to generate three separated narrow PR frequency bands [12]. A PR reflective surface (PRRS) based on the substrate integrated waveguide (SIW) is proposed in [13]. Three narrow PR frequency bands are achieved and two of them are combined together to form a wider one, whose fractional bandwidth reaches up to 9.5%. New results on broadband PRRSs have been increasingly published in recent years. The SRR is used to form a PRRS, which achieves a polarization conversion ratio (PCR) greater than 56% with a bandwidth of 77% in [14]. In [15], the SRR is combined with a metallic disk and the bandwidth of the PRRS reaches up to 82% with the PCR being higher than 80%. An ultra-wideband polarization conversion metasurface using double-head arrow structure was designed in [16], whose fractional bandwidth is as large as 118%. Unfortunately, its PCR is less than desirable as the lowest PCR within the working bandwidth is only 50%. In [18], a PRRS with two grounded vias loaded along the diagonal direction of the AMC patch is presented. By properly adjusting the distance between two vias, which are symmetric to the center of the patches, a 29.1% PR band is obtained. It should be pointed out that the studies mentioned above have not included the characteristics of PRRSs under oblique incident waves. The PRRSs proposed in the above literature either work at a few separated narrow PR frequency bands or do not have high enough PCRs over the PR band. There are a number of applications for PRRS, one of which is to reduce the radar cross section (RCS) of metallic bodies and sheets. In the past few years, many research activities to reduce the RCS of metallic targets have been reported. The AMC structures are the most commonly used lossless metamaterials for RCS reduction [20]–[23]. Within the operation frequency of an AMC, the reflections from the AMC and a perfect electric conductor (PEC) share the same magnitude but have opposite phases. By combining the AMC and PEC in a chessboard-like configuration, these two reflections cancel each other under a normal incident plane wave and reradiate the energy in other directions, thus reducing the RCS in the normal direction [20]. To overcome the bandwidth limitation, two kinds of AMCs with 180◦ phase difference over a wide frequency band can be combined to form the chessboard-like configuration [21], [22]. As a result, a broadband RCS reduction is achieved. Phase gradient metasurface (PGM) can also be used for wideband RCS reduction. It can provide additional wave components along the two in-plane directions simultaneously, leading to surface wave conversion, deflected reflection, or diffused reflection. Taking

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Fig. 1. Topological structure of the unit cell of the broadband PRRS with a high PCR.

advantage of these characteristics of the PGM, wideband RCS reduction is achieved in [24]. However, there are few research studies on RCS reduction using PRRS, which is the focus of the reported research. In this paper, a novel broadband PRRS based on the AMC structure with a high PCR is presented. Different from the structure proposed in [18], the two grounded vias are nonsymmetric to the center of the unit cell. Its PCR is higher than 96% with a fractional bandwidth of 49%. In addition, based on the design of the proposed PRRS with a high PCR, an ultra-wideband PRRS with a larger PR bandwidth of 103.6% and PCR being higher than 50% is proposed using a quasi-L-shaped patch instead of the square one. Furthermore, we apply the PRRS to reduce the RCS of the metallic sheet in a wide frequency band. Different configurations of the unit cell of the PRRS are proposed and their effects on RCS reduction are compared and discussed. The PRRSs are studied using the full-wave EM simulation software HFSS and validated experimentally. This paper is organized as follows. Section II describes the topological structure of the broadband PRRS with a high PRC and presents its design procedure. In Section III, a PRRS with a larger PR frequency bandwidth based on the PRRS designed in Section II is presented and discussed. Then, Section IV presents and discusses two different configurations of the PRRS for wideband RCS reduction of a metallic target. Finally, this paper is concluded in Section V. II. N OVEL PRRS W ITH A H IGH PCR This section presents the design of a novel broadband PPRS with a high PCR. The unit cell of the proposed PRRS is depicted in Fig. 1. It consists of a square metallic patch printed on the substrate backed by a metallic ground plane, and two grounded vias loaded in a diagonal direction. It is noteworthy that the two vias are nonsymmetric to the center of the unit cell of the PRRS and the square patch, which is very critical for obtaining a high PCR across the whole operation frequency band. The period p of the unit cell of the PRRS is 6.0 mm; the width w of the patch is 4.0 mm; the diameters D of the two vias are 0.6 mm; and the offsets of the two vias to the center of the unit cell dt1 and dt2 are 2.5 and 1.2 mm, respectively. The patch is printed on a dielectric substrate with a permittivity of 2.65 and a thickness t of 2.0 mm (tan δ = 0.001). In the following, Ref Inc Inc we define rT E/T M = ETRef M /ET E and rT E/T E = ET E /ET E to represent the reflection ratio of TE-to-TM, and TE-to-TE polarization conversions, where the ETInc E represents the electric

Fig. 2. Simulated magnitude of the reflection coefficient rTE/TM of the PRRS with two symmetric vias.

field of the TE-polarized incident EM wave, whereas ETRef E and represent the electric field of the TEand TM-polarized ETRef M reflected EM wave, respectively. A. Principle Analysis of the PRRS and Its Design Procedure As reported in the previous works, an asymmetric highimpedance surface can be exploited to realize the PRRS. In [18], an AMC structure with two grounded vias loaded along the diagonal direction is reported. The two vias are symmetric to the center of the unit cell. Two narrow resonant frequency bands are generated and a wide PR frequency band is formed by adjusting the distance between the two vias. To understand the effect of the positions of the vias, we first make the two vias and the patch symmetric to the center of the unit cell. We make the width w of the patch 5.6 mm while keeping the other parameters the same as given in Fig. 1. It can be observed from Fig. 2 that the two resonant frequency points move closer to each other when parameter d is decreased, which is in agreement with the analysis in [18]. Then, we keep the topleft grounded-via unchanged at 2.5 mm and move the other one in a diagonal direction. As shown in Fig. 3(a), a third resonant point appears when the grounded via moves toward the center of the unit cell. In Fig. 3(b), it can be seen that when the two grounded vias are both in the upper left corner of the unit cell, the third resonant point appears. However, it is noted that when the grounded via approach the center of the unit cell, the two resonant frequency points at a lower frequency join together, which is similar to that of the symmetric one shown in Fig. 2. We can infer from Figs. 2 and 3 that it is the nonsymmetric distribution of the grounded vias that generates one more PR frequency point compared with the symmetric structure. In order to produce a wider PR band with a high PCR, the width of the square patch is reduced and the final design is optimized as shown in Fig. 1. Fig. 4 gives the magnitudes of the reflection coefficients of the proposed PRRS. It shows there are three peak values that are approximate to 1. This means that at the three frequency points (11.094, 14.098, and 16.798 GHz), almost all of the TE-polarized incident waves are converted to the TM-polarized ones. Moreover, the minimum value of rT E/T M among the three peak values is 0.979, which exhibits high PCM stability during the operation frequency of the proposed PRRS.

JIA et al.: BROADBAND PRRS AND THEIR APPLICATIONS

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Fig. 5. Simulated PCR of the proposed PRRS.

Fig. 6. Simulated reflection phase of rTE/TM and rTE/TE of the proposed PRRS. Fig. 3. Simulated magnitude of the reflection coefficient rTE/TM of the PRRS with two nonsymmetric vias (a) at different sides and (b) same side of the center of the unit cell of PRRS along the diagonal direction.

Fig. 4. Simulated reflection coefficients rTE/TM and rTE/TE of the proposed PRRS.

2 2 The PCR is defined as PCR = rT E/T M /(rT E/T M + 2 rT E/T E ). Fig. 5 shows the simulated PCR of the proposed PRRS. It can be seen that the PCR is higher than 96.4% among the three resonant frequency points. The fractional bandwidth of the PCR higher than 96.4% reaches up to 49% (from 10.74 to 17.72 GHz). Fig. 6 shows the reflection phase of the proposed PRRS under the TE-polarized incident wave. The reflection phases of rTE/TE are zero at the three PR frequency points (11.094, 14.098, and 16.798 GHz). That is to say, the proposed PRRS exhibits high-impedance characteristics at these three points. Since the incident wave is TE polarized and normal to the surface of PRRS, its electric field is parallel to the y-axis. Furthermore, the AMC can reflect the normally incident wave in phase. The y-direction induced current can be generated by the incident

electric field Ey . Due to the asymmetric structure of the PRRS, a potential difference in the x-direction can be introduced by the nonuniform distribution of the surface impedance along the xaxis. As a result, some current will move toward the x-direction, which will generate an electric field Ex along the x-direction. When the current moving along the x-direction is equal to that of the y-direction, the reflected field will be orthogonal to the incident field with the same magnitude. The simulated surface current distributions on the metallic parts of the unit cell at the three polarization points are given in Fig. 7. In Fig. 7(a), the total surface current on the ground is stronger than that on the square patch. On the contrary, as shown in Fig. 7(b) and (c), the total surface current on the square patch is stronger. What they have in common is that the intersection angle between the direction of the total surface current induced on the patch and ground exhibits a 45◦ angle off of the y-axis in all three PR points, which indicates a 90◦ PR. B. Simulated and Experimental Results In order to investigate how the parameters influence the reflection rotation characteristics of the proposed PRRS, a series of parameter analysis are performed. First, dt1 has been changed with the other parameters fixed. It can be seen from Fig. 8 that with the decrease of dt1 , the bandwidth of the PRRS narrows down. The third resonant frequency point moves to a significantly lower frequency and the second one slightly shifts to a higher frequency. When dt1 is 2.1 mm, the second and third resonant frequency points are combined together and there are only two resonant frequency points left. Then, the parameter dt2 has been investigated. In Fig. 9, we can see that the first resonant frequency point shifts to a higher frequency and the other two resonant frequency points remain unchanged or move

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Fig. 7. Simulated surface current distributions on metallic parts of the unit cell at the three resonant points: (a) 11.094, (b) 14.098, and (c) 16.798 GHz.

Fig. 8. Simulated magnitude of the reflection coefficients rTE/TM and rTE/TE of the proposed PRRS for the case of different dt1 decreasing from 2.5 to 2.1 mm.

Fig. 9. Simulated magnitude of the reflection coefficients rTE/TM and rTE/TE of the proposed PRRS for the case of different dt2 decreasing from 1.4 to 1 mm.

slightly with the decrease of dt2 . As shown in Fig. 10, when the parameter w decreases, both of the first two resonant frequency points move to a higher frequency while the third one remains unchanged. To obtain a relatively high and stable PCR over the operation frequency band, the values of the parameters are finally determined as given in Section II. The stability of the PRRS under the oblique incident waves has not been investigated in most of the previous works. As shown in Fig. 11, when the incident angle ϑ changes from 0◦ to 30◦ , the frequency response is reasonably stable. When the incident angle continues to increase, the magnitudes of the reflection coefficients of the proposed PRRS are no longer stable. However, the reflection coefficient is still larger than 0.8 when the incident angle increases to 60◦ .

Fig. 10. Simulated magnitude of the reflection coefficients rTE/TM and rTE/TE of the proposed PRRS for the case of different w decreasing from 4.2 to 3.8 mm.

Fig. 11. Simulated magnitude of the reflection coefficients rTE/TM and rTE/TE of the proposed PRRS under oblique incident waves.

In order to validate the simulated results, a prototype with 80 × 50 cells (480 mm × 300 mm) is fabricated and measured as shown in Fig. 12. Due to the restrictions of the test conditions, the reflectance of the proposed PRRS is not measured in the frequency range higher than 18 GHz. It can be seen that the measured reflection coefficient matches well with the simulated one.

III. N OVEL U LTRA -W IDEBAND PRRS A wider PR frequency band is urgently needed in many applications. Therefore, a broadband PRRS with high PCR is proposed and analyzed in Section II. On the basis of the PRRS presented in the previous section, another PRRS with a larger


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Fig. 12. Simulated and measured magnitudes of the reflection coefficients rTE/TM of the proposed PRRS.

Fig. 14. Simulated reflection coefficients rTE/TE of the ultra-wideband PRRS. (a) Magnitude. (b) Phase. Fig. 13. Topological structure of the unit cell of the ultra-wideband PRRS.

PR bandwidth has been designed by replacing the square patch by a quasi-L-shaped one. Fig. 13 gives the topological structure of the proposed ultrawideband PRRS. The structure is similar to that shown in Fig. 1. However, the operation frequency of the new structure shifts to a higher frequency, which cannot be measured in our equipment. Thus, we have to enlarge the unit cell of the PRRS. The permittivity of the substrate is 2.65. Other parameters are as follows: W1 = 9.1 mm, W2 = 4.1 mm, W3 = 1.3 mm, G1 = 2.5 mm, G2 = 0.4 mm, dt3 = 2 mm, dt4 = 4 mm, D1 = 1 mm, and t1 = 3 mm. In this section, we use another method to analyze the principle of the proposed ultrawideband PRRS. When the intersection angle between the electric field and the x-axis is 45◦ and 135◦ , the magnitude and phase of the reflection coefficients rTE/TE of the ultra-wideband PRRS are shown in Fig. 14(a) and (b), respectively. From Fig. 14(a), it can be seen that the magnitude of the two curves are nearly the same. Most of the energy is reflected and the polarization of the reflected wave is the same as that of the incident wave. Fig. 14(b) shows the reflection phases of the two incident waves. There are five frequency points (6.4, 10.5, 16, 16.7, and 18.1 GHz) where the reflection phases of the two curves are approximately either 0◦ or 180◦ and the phase differences between the two curves are nearly 180◦ . Taking the first frequency point (6.4 GHz), e.g., we decompose the electric vectors of the incident and reflected waves as shown in Fig. 15. We assume that the electric field of the incident wave Ei is parallel to the x-axis. Then, Ei can be decomposed into two

Fig. 15. Decomposition of the electric vectors of incident and reflected waves at 6.4 GHz.

components Eiu and Eiv . Since the reflection phase is 0◦ (180◦ ) when the electric field is parallel to the u (v)-axis [45◦ (135◦ ) off the x-axis], the reflected electric field Eru (Erv ) is equal to the incident electric field Eiu (−Eiv ). Meanwhile, the magnitude of the reflected fields is equal to that of the incident fields. Finally, the total reflected field Er can be obtained, which shares the same magnitude with Ei and is parallel to the y-axis. The same applies for the other four frequency points. Thus, all the energy of the incident wave is reflected with a 90◦ rotation at these five frequency points. The simulated reflection coefficients rTE/TM and rTE/TE of the ultra-wideband PRRS are given in Fig. 16. The five PR frequency points agree well with those points discussed above, which validate the analysis above. The PCR of the ultra-wideband PRRS is shown in Fig. 17. Its fractional bandwidth is 103.6% (from 5.98 to 18.84 GHz) with the PCR

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Fig. 16. Simulated magnitude of the reflection coefficients rTE/TM and rTE/TE of the ultra-wideband PRRS.

Fig. 19. Simulated and measured magnitudes of the reflection coefficients rTE/TM of the ultra-wideband proposed PRRS.

IV. A PPLICATION OF THE P ROPOSED PRRS FOR W IDEBAND RCS R EDUCTION A. Theoretical Analysis of the PRRS for RCS Reduction

Fig. 17. Simulated PCR of the ultra-wideband PRRS.

In the previous sections, two different kinds of PRRSs were presented. In order to observe the relationship between the PCR of the PRRS and its effects on RCS reduction, the ultra-wideband PRRS shown in Section III was used. In this section, two different configurations of the unit cells of the ultra-wideband PRRS are proposed and their effects on RCS reduction are compared and discussed. Both configurations consist of 8 × 8 cells. The RCS of a metallic ground backed substrate with the same permittivity and size as the proposed configuration is selected as the reference. The RCS of a target σ can be written as [25] σ = 4π limR→∞ R2

Fig. 18. Simulated magnitude of the reflection coefficients rTE/TM of the ultrawideband PRRS under oblique incident waves.

being higher than 50%. As shown in Fig. 18, when the incident angle ϑ changes from 0◦ to 30◦ , the frequency response of the ultra-wideband PRRS is reasonably stable in the frequency range 6–15.5 GHz. However, in the frequency range 15.5–18 GHz, the reflection coefficients are not stable. That is because the periodic of the unit cell of the ultra-wideband PRRS is larger than half of the wave length in the frequency range 6–15.5 GHz. A prototype with 40 × 25 cells (480 mm × 300 mm) is fabricated and measured to validate the simulated results. The measurement is also performed in the frequency range lower than 18 GHz due to the limitation of the test conditions. As shown in Fig. 19, it can be seen that the measured reflection coefficient is in good agreement with the simulated one.

|Es | |Ei |

2

2

(1)

where Ei and Es are the electric field intensity of the incident and scattered waves at the target, respectively; and R is the distance between the target and the detection radar. According to (1), we can analyze the RCS of the PRRS by the electric fields of the incident and reflected waves at the target. Configuration 1 is shown in Fig. 20(a). All of the unit cells are arranged in the same direction. When this structure is impinged by a normal incident wave with an electric field in an arbitrary direction, its RCS is discussed using the reflected electric field as shown in Fig. 20(b). Arbitrary electric field of the incident wave Ei can be decomposed into two components along the x- and y-axes, namely, Eix and Eiy , respectively. Since the ultra-wideband PRRS has the characteristics of PR, the electric fields Eix and Eiy will be reflected with a 90◦ rotation and the direction of the total reflected electric field Er is depicted in Fig. 20(b). Since PRRS just changes the polarization rather than the magnitude of the electric field of the incident wave, the total RCS of the proposed configuration will not be reduced compared with that of the reference. However, the co-polarization and cross-polarization RCS of Configuration 1 is changed. When θ is 0◦ , the electric field Er is orthogonal to Ei . As a result, the co-polarization RCS will be significantly reduced, whereas the cross-polarization RCS is increased. When θ is 45◦ , the electric field Er is parallel to Ei . Hence, both the coand cross-polarizations RCS will remain unchanged.


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Fig. 20. Configuration 1 of the PRRS and the decomposition of the electric field of the normal incident wave.

Fig. 22. RCS of the two configurations compared with that of the reference with the electric field of the incident wave parallel to the x-axis (a) co- and (b) cross-polarization. Fig. 21. Configuration 2 of the PRRS and the decomposition of the electric field of the normal incident wave.

For the case of Configuration 2, we arrange the unit cells of the PRRS in four orthogonal directions as shown in Fig. 21(a). The reflected electric fields of the different parts of Configuration 2 are depicted in Fig. 21(b). It can be seen that the electric fields reflected from different parts will cancel out with arbitrary polarization of the incident wave. So, no matter what degree θ is, the co-polarization RCS will be significantly reduced, whereas the cross-polarization RCS will remain constant. As a result, the total RCS, which is the vector sum of the co-polarization and cross-polarization RCS, will be significantly reduced. B. Results, Discussion, and Comparisons The simulated RCS of the two configurations compared with that of the reference is given in Figs. 22 and 23 to validate the above analysis. It can be seen from Fig. 22(a) that when the electric field of the normal incident wave is parallel to the x-axis, the co-polarization RCS of both configurations are significantly reduced. However, the cross-polarization RCS of Configuration 1 increases as shown in Fig. 22(b). Note that the cross-polarization RCS of Configuration 2 increases as well. That is because the PRRS is not a perfect PR within the whole frequency band. Thus, not all the energy of the incident wave is reflected with a 90◦ PR. When the angle between the electric field of the incident wave and the x-axis is 45◦ , the comparisons of the RCS are shown in Fig. 23. As shown in Fig. 23(a), the co-polarization RCS of Configuration 1 is not reduced within the whole frequency band except for two frequency

Fig. 23. RCS of the two configurations compared with that of the reference with the electric field of the incident wave 45◦ off the x-axis (a) co- and (b) cross-polarization.

points near 5.5 and 16 GHz, while that of Configuration 2 is significantly reduced. The cross-polarization RCS of the two configurations are increased for the same reason as the curves shown in Fig. 22(b). The co-polarization RCS of Configuration

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Fig. 24. Bistatic RCS at 11 GHz. (a) Configuration 2. (b) Reference. Fig. 26. Comparison between the simulated and the measured RCSs of the two configurations.

Fig. 25. Schematic view of the RCS measurement setup in an anechoic chamber.

1 near 5.5, 16, and 19.5 GHz is smaller than that of the reference. That is because the reflection coefficients of PRRS at these three frequency points are smaller than 1. Therefore, the simulated results agree with the analysis, which illustrate the validity of the analysis. Furthermore, the frequency points where RCS is significantly reduced in Figs. 22 and 23 are consistent with the frequency points where the PCR of the PRRS is relatively high. If we want to reduce the RCS significantly over the whole frequency band, the PRRS with a high PCR presented in Section II can be used. Through comparative analysis of the effects on RCS reduction of the two configurations, we can draw a conclusion that both co- and cross-polarization RCSs can be significantly reduced by arranging the unit cells of the PRRS in four orthogonal directions as shown in Fig. 21 and Configuration 2 is better than Configuration 1 at RCS reduction. As we know, the energy is conserved. For the cases of RCS reduction using lossless materials, the total scattered energy is not reduced. As a result, the reduction in RCS in a certain angular domain inevitably leads to the increase in the RCS in certain other angular domains. In order to investigate the distribution of the scattered energy, we compare the bistatic RCS of Configuration 2 with that of the reference at 11 GHz in Fig. 24. Compared to the bistatic RCS of the reference, it can be seen that the main lobe of the bistatic RCS of Configuration 2 is significantly reduced while four sidelobes arise. This comparison clearly indicates where the energy reduced in the backward direction travels. In order to measure the scattering property of the Configurations 1 and 2, both of them are fabricated and measured in a microwave anechoic chamber. Fig. 25 shows the schematic view of the experimental setup. Four pairs of horn antennas, selected as emitters and receivers, have been utilized

to cover the frequency band from 5 to 18 GHz. These correspond with the following bands: 3.95–5.85, 5.85–8.2, 8.2–12.4, and 12.4–18 GHz. The measurement is performed in the frequency range lower than 18 GHz due to the limitation of the test conditions. Fig. 26 shows the comparison between the simulated and the measured RCSs of the two configurations. It can be seen that the measured RCSs match well with the simulated ones.

V. C ONCLUSION In this work, a novel design for PRRS based on the AMC structure with the square patch and two grounded vias nonsymmetric to the unit cell has been presented. A broadband PRRS with the PCR being higher than 96% over a fractional bandwidth of 49% has been realized. On this basis, an ultrawideband PRRS with a larger PR band has been designed using a quasi-L-shaped patch instead of the square one. The fractional bandwidth of the ultra-wideband PRRS reaches up to 103.6% with the PCR being higher than 50%. The proposed PRRS was then applied to wideband RCS reduction. Two configurations with different arrangements of the unit cells of the PRRS are proposed and their effects on RCS reduction are discussed. Simulation and experimental results illustrate the capability of the PRRS to reduce the RCS over a wide frequency band. It is shown that both the co- and cross-polarization RCSs can be significantly reduced by arranging the unit cells of the PRRS in four orthogonal directions.

ACKNOWLEDGMENT The authors would like to thank T.-J. Cui and Q. Cheng from the Southeast University, Nanjing, China, as well as D. Liu from the IBM T. J. Watson Research Center, Yorktown Heights, NY 10598 USA for their valuable comments.

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Yongtao Jia was born in Tangshan, Hebei province, China, in 1989. He received the B.Eng.Degree in electrical engineering from Xidian University, Xi’an, China, in 2010, where he is currently pursuing the Ph.D. degree. His research interests include antenna RCS reduction, metasurface, and wideband phased arrays.

Ying Liu (M’07) received the M.S. and Ph.D. degrees in electromagnetics from Xidian University, Xi’an, China, in 2001 and 2004, respectively. From 2006 to 2007, she was a Postdoctoral Researcher with Hanyang University, Seoul, South Korea. Currently, she is a Full Professor with the National Key Laboratory of Science and Technology on Antennas and Microwaves, Xidian University. Her research interests include antenna theory and technology, prediction, and control of antenna RCS. She has authored or coauthored over 100 refereed journal papers. She is the author of Prediction and Reduction of Antenna Radar Cross Section (Xi’an: Xidian Univ. Press, 2010) and Antennas for Mobile Communication Systems (Beijing: Electronics Industry Press, 2011). Dr. Liu is a Senior Member of Chinese Institute of Electronics (CIE). She is the Reviewer of several international journals and serves as TPC member or session chair for several IEEE flagship conferences. She was the recipient of “New Century Excellent Talents in University” of the Ministry of Education for China in 2011.

Y. Jay Guo (F’14) received the B.S. and M.S. degrees from Xidian University, Xi’an, China, in 1982 and 1984, respectively, and the Ph.D. degree from Xian Jiaotong University, Xi’an, China, in 1987, all in electromagnetics. Currently, he is a Distinguished Professor and the Director of the Global Big Data Technologies Centre, University of Technology Sydney (UTS), Sydney, N.S.W., Australia. Prior to this appointment in 2014, he served as a Research Director in CSIRO for over 9 years, managing a number of ICT research portfolios. Before joining CSIRO, he held various Senior Leadership positions with Fujitsu, Siemens, and NEC, London, U.K. His research interests include reconfigurable antennas, conformal and wideband arrays, metamaterial, millimeter-wave and THz communications, and sensing systems. Dr. Jay Guo is a Fellow of the Australian Academy of Engineering and Technology and IET, and a Member of the College of Experts of Australian Research Council (ARC). He has chaired numerous international conferences. He is the International Advisory Committee Chair of IEEE VTC2017, General Chair of ISAP2015, iWAT2014, and WPMC’2014, and TPC Chair of 2010 IEEE WCNC, and 2012 and 2007 IEEE ISCIT. He serves as Guest Editor of special issues on “Antennas for Satellite Communications” and “Antennas and Propagation Aspects of 60–90 GHz Wireless Communications,” both in the IEEE T RANSACTIONS ON A NTENNAS AND P ROPAGATION, and Special Issue on “Communications Challenges and Dynamics for Unmanned Autonomous Vehicles,” IEEE Journal on Selected Areas in Communications (JSAC). He was the recipient of a number of prestigious Australian national awards, and was named one of the most influential engineers in Australia in 2014 and 2015.

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Kun Li was born in Chengdu, Sichuan Province, China, in 1989. He received the B.S. degree in microwave- and millimeter-wave system and device technology from Nanjing University of Aeronautics and Astronautics, Nanjing, China, in 2013. He is currently pursuing the M.S. degree from Xidian University, Xi’an, China. His research interests include RCS reduction of antennas, polarization conversion metasurfaces, and circularly polarized antenna as well as the Fabry– Perot cavity resonant antenna.

Shu-Xi Gong received the B.S. and M.S. degrees from Xidian University, Xi’an, China, in 1982 and 1984, respectively, and the Ph.D. degree from Xi’an Jiao Tong University, Xi’an, China, in 1988. Currently, he is a Full Professor and Director of the National Key Laboratory of Science and Technology on Antennas and Microwaves, Xidian University. He has authored or coauthored over 200 refereed journal papers. He is the author of Principles of Generalized Eigenfunction Expansions in Electromagnetic Theory (Xi’an: Xidian Univ. Press, 2010), Prediction and Reduction of Antenna Radar Cross Section (Xi’an: Xidian Univ. Press, 2010), and Antennas for Mobile Communication Systems (Beijing: Electronics Industry Press, 2011). His research interests include antenna theory and technology, prediction and control of antenna RCS, and RCS calculation of complex targets.

Dr. Gong is a Senior Member of Chinese Institute of Electronics (CIE) and Vice Chairman of the Antenna Society of CIE. He is an Editorial Board Member of Chinese Journal of Xidian University and Journal of Microwaves. He was the recipient of the Science and Technology Advancement Award of Shaanxi Province, the Science and Technology Advancement Award of the Ministry of Information Industry of China, the Yilida Best Paper Award of Chinese Journal of Radio Sicence, the Outstanding Young Scholar Award of the Ministry of Mechanical Electronics of China, and the Excellent Young Backbone Teacher Award of the National Education Committee of China.