A 1-bit electronically reconfigurable beam steerable metasurface reflectarray with multiple polarization manipulations

Yan Shi(史琰), Xi-Ya Xu(徐茜雅), Shao-Ze Wang(王少澤), Wen-Yue Wei(魏文岳), and Quan-Wei Wu(武全偉)

School of Electronic Engineering,Xidian University,Xi’an 710071,China

Keywords: electronically controlled,metasurface reflectarray,beam steering,polarization manipulation

1.Introduction

With rapid development of wireless communication technology, high-performance antennas and arrays become essential to ensure good communication quality in a dynamic and complicate environment.The high-gain beam steerable radiation characteristic is highly demanded to reduce multipath fading and environment interference.As an alternative to high-cost phased array antenna,reconfigurable reflectarray antenna (RRA) can achieve adaptive pattern scanning by reconfigurable metasurface surface and thus has attracted much interest in recent years.[1,2]Owing to low cost, easy fabrication and light weight,the RRA becomes a promising solution in the field of 5G/6G wireless communication, satellite communication, etc.In real-life complicated environment, polarization mismatch significantly degrades the quality and reliability of wireless communication links.Thus, the merging of the polarization diversity technology into the RRA design is an effective method to enhance the anti-interference ability and improve the transmission efficiency of the wireless communication.[3]

In order to generate beam agility of the RRA, the phase shift required by the RRA is manipulated.Compared with mechanical control,e.g.,micromotors or actuators,[4,5]it is more convenient and rapid to adjust the phase shift by using tunable materials such as liquid crystal[6]and graphene[7]or active components including varactor diodes,[8,9]PIN diode,[10,11]and micro-electro-mechanical systems (MEMS).[12,13]In the conventional RRA design,it is desirable to realize a reflection phase range of 2πfor perfect phase compensation.By means of continuously tunable materials or components, the reflection phase between 0°and 360°can be obtained.However,the RRA design based on the continuous phase shift is technically complicated.Instead, a discrete phase shift scheme by quantizing 2πphase range into a group of discrete phase state can greatly simplify the RRA design.[14–23]In Ref.[19],a dual-band 1-bit 16×16 RRA composed of a rectangular patch with two PIN diodes was developed.In terms of the quantization phase of 180°, 1-bit phase shifts forx-polarization at 12.5 GHz and fory-polarization at 14.25 GHz are generated,thus realizing dual-band co-polarization beam scanning.A 1-bit 16×16 RRA with circular polarization (CP) characteristic has been developed.[20]By switching two PIN diodes in a circularly shaped patch element between forward and reverse bias and meanwhile rotating the patch by 90°, 1-bit CP element is obtained,thus achieving 2-D co-polarized beam steering under the left-handed circular polarization wave illumination.In addition, a 1-bit 16×16 reflectarray was designed for 2D cross-polarized beam scanning.[23]By switching two PIN diodes placed in tetra-arrow element from ON state to OFF state, the 1-bit phase shift for the cross-polarization reflection can be obtained under the linear polarization wave illumination.To date,existing works on designing the RRA are limited to single polarization characteristic,i.e.,sole linear or circular co-polarization and sole linear cross-polarization.No reports on RRA designs with multiple polarization characteristics have been found.

Fig.1.Beam scanning RRA with multiple polarization characteristics.

In this paper,we present an electronically reconfigurable 12×12 metasurface reflectarray with beam steering and polarization manipulating capabilities,as shown in Fig.1.The proposed metasurface unit cell is composed of a cross patch with two stubs connected to two PIN diodes.By independently adjusting the biasing voltage of each PIN diode, the metasurface unit cell exhibits the isotropic and anisotropic reflection characteristics.Thus two sets of 1-bit phase shifts corresponding to the co-polarization radiation and the cross-polarization radiation, respectively, can be obtained.With a linearly polarized feeding horn excitation,2D co-polarization and crosspolarization beam steering can be achieved by the proposed optimal RRA.When a feeding horn with right-handed circular polarization(RHCP)characteristic is used,the RRA generates the 2D LHCP beam scanning.The simulation and measurement results are given to validate the proposed design.

2.Anisotropic metasurface RRA unit cell design

Figure 2 gives the proposed unit cell of the metasurface RRA.The unit cell is composed of the top substrate F4B with relative permittivity of 2.65 and tangent loss of 0.005 and the bottom substrate with the relative permittivity of 4.4 and tangent loss of 0.026.A metallic ground is placed between two substrates.A cross patch is fabricated on the top surface of the F4B and two stubs along?=45°and?=?45°directions loaded by two PIN diodes(SMP 1340-040 LF from Skyworks Solution Inc.), respectively, are connected to the cross patch.At the end of each stub, a metallic via is used to connect the stub with the DC biasing circuit.A metallic via placed in the middle of the cross patch is introduced to connect the cross patch with the ground.The DC biasing circuit for the PIN diode is fabricated on the bottom surface of the FR4.In the biasing circuit,a radial stub is designed to choke high-frequency signal.The biasing line is placed at the location where the weakest fields exist.

Fig.2.Anisotropic metasurface RRA unit cell.Dimensions of the unit cell (unit: mm): P=16, lx =4, l1 =3.55, dp =0.7, l3 =5,t0 =1.5,ly=9.1,lw=1.1,R=3.55,t1=0.5,w=0.3,dv=0.5,and l2=1.5.

Fig.3.Performance of the unit cell in states I and II for TE polarized incidence.(a)Reflection magnitude.(b)PRPCR.(c)Reflection phase.

In order to elaborate the anisotropic characteristic of the proposed element, consider a transverse electric (TE) polarized wave, i.e., an incoming wave withy-polarized electric field, normally incident on the unit cell enclosed by the periodic boundary.When the DC voltage is applied in the biasing circuit, two PIN diodes are switched between OFF and ON states, thus giving rise to four operation states, as shown in Table 1.In the state I, the PIN diode 1 is in ON state and the PIN diode 2 is in OFF state.The resulting co-polarized and cross-polarized components of the reflection coefficient are shown in Fig.3(a).It can be seen that the cross-polarized component of the reflection coefficientRxyis larger than the co-polarized oneRyyin the band 8.7–9.4 GHz.Note that the corresponding equivalent circuits of the PIN diode in ON and OFF states shown in Fig.4 are used for simulation of the reflection coefficient.On the other hand,when the PIN diode 1 is in the OFF state and the PIN diode 2 in the ON state,which is called state II, the obtained reflection coefficient is plotted in Fig.3(a).TheRxyandRyyin the state II are almost same as those in the state I,and theRxyis larger than theRyyin the band of 8.7–9.4 GHz.It is worth noticing that theRyyvaries sharply with the change of the frequency.This is due to the fact that the imperfect performance of PIN diode results in the rapid change of the equivalent impedance of the proposed element.

Fig.4.Equivalent circuits of PIN diode in ON and OFF states.

Table 1.Operations states of the proposed unit cell.

In order to measure the anisotropic performance,i.e.,polarization conversion (PC), the partial reflection polarization conversion rate(PRPCR)is defined as

in whichTxyandTyyare the cross-polarized and co-polarized components of the transmission coefficient,respectively.Owing to the use of the ground, the transmission coefficientsTxyandTyyare zero.It is worth pointing out that the PRPCR,which is different from the conventional polarization conversion rate (PCR), i.e.,|Rxy|2,[24,25]represents the ratio of the cross-polarized reflection wave to the total reflection wave regardless of the loss effect of the unit cell, and thus can better demonstrate anisotropic characteristic.As shown in Fig.3(b),the band of the proposed unit cell for PRPCR ≥0.9 covers 8.88–9.17 GHz in the two states.In addition,Fig.3(c)shows the phase of the cross-polarized component of the reflection coefficient in the states I and II, i.e.,Pxy.It can be found that in the band of 8.5–9.5 GHz,the phase difference of the crosspolarized component between the two states ΔPxyisπ.

Fig.5.Performance of the unit cell in states I and II for TM polarized incidence.(a)Reflection magnitude.(b)PRPCR.(c)Reflection phase.

Similarly,for transverse magnetic(TM)polarized normal incidence, i.e., an incoming wave withx-polarization electric field, the cross-polarized component of the reflection coefficientRyxis larger than the co-polarized oneRxxin the band of 8.5–9.5 GHz in both the states I and II,as shown in Fig.5(a).Moreover,theRyxand theRxxin the state I are equal to those in the state II.According to Fig.5(b),the band of the proposed unit cell for PRPCR ≥0.9 covers 8.93–9.14 GHz in the two states.In addition,the phase difference of the cross-polarized component between the two states ΔPyxisπin the band of 8.5–9.5 GHz,as shown in Fig.5(c).

In the state III, both the PIN diodes 1 and 2 are in ON state, and the corresponding reflection coefficient is given in Fig.6.It can be observed that theRyyin TE polarization and theRxxin TM polarization are approximately equal to 1, and are far larger than theRxyin TE polarization and theRyxin TM polarization in the band of 8.5–9.5 GHz.In the state IV with both the PIN diodes 1 and 2 in OFF states, the co-polarized and cross-polarized components of the reflection coefficient are approximately equal to those in the state III.Moreover,the co-polarized component of the reflection coefficient is far larger than the cross-polarized one in the band of 8.5–9.5 GHz for both TE and TM polarizations.In addition,the phase difference of the co-polarized component between the state III and the state IV for TE and TM polarizations is 180°±30°in the band of 8.97–9.2 GHz and 8.76–9.02 GHz, respectively.Note that the phase difference between the state III and the state IV is not exactly 180°.This is due to the different variation trends of the reflective phasePyyin two states, which is mainly caused by the parasitic effect of the PIN diodes.It is worthwhile pointing out that the phase difference of 180°is not a necessary requirement.On the one hand,the 1-bit quantized phase method is indeed an approximation to the accurate phase distribution required by the RRA.On the other hand,the optimization procedure based on the reference phase can be employed to determine the RRA layout for the optimal radiation performance.The details about the 1-bit quantized phase method and the reference phase based optimization are given in the following section.

Fig.6.Reflection magnitudes and phases in states III and IV.(a)Magnitude for TE polarized incidence.(b)Phase for TE polarized incidence.(c)Magnitude for TM polarized incidence.(d)Phase for TM polarized incidence.

In order to further elaborate the anisotropic and isotropic responses of the proposed element in different operation states,the surface current distributions on the element at 8.9 GHz in the four states are plotted in Fig.7 undery-polarized wave incidence.In the state I,the PIN 1 is in the ON state,and thus the microstrip line in the direction of?=225°is connected to the cross patch.They-polarized incident electric field induces the current flowing along?=225°.On the other hand, with the PIN 2 in the OFF state, the microstrip line in the direction of?=135°is disconnected to the cross patch,thus resulting in the equivalent capacitance between them.The current flowing along?=135°is induced by they-polarized incident electric field.Therefore,the induced current flows from the microstrip line in the direction of?=225°to the microstrip line in the direction of?=135°via the cross patch.In this scenario,the superposition of surface currents gives rise to the total current flowing along?xaxis direction,achieving the PC.In the state II, the PIN 1 is in the OFF state and the PIN 2 is in ON state.Similar to the state I, the current flows from the microstrip line in the direction of?=135°to the microstrip line in the direction of?=225°via the cross patch.Therefore,the total current obtained by the superposition of surface currents flows alongxdirection,exhibiting the PC characteristic.Since the total current in the state II is opposite to that in the state I,the phase difference of the cross-polarized components of the reflection coefficient between the state I and the state II isπ.

Fig.7.Surface current distributions on the RRA element at 8.9 GHz in four states.

In the state III, both the PIN 1 and the PIN 2 are in the ON state, and thus the microstrip lines in the directions of?=225°and?=135°are connected to the cross patch simultaneously.In this scenario,the induced current flows from two microstrip lines to the cross patch, and thus the total current by superposing the surface currents flows alongydirection.In the state IV with both the PIN 1 and PIN 2 in OFF state, the microstrip lines in the directions of?=225°and?=135°are disconnected to the cross patch simultaneously, and the induced current flows from the cross patch to two microstrip lines owing to the equivalent capacitances between the cross patch and the microstrip lines.The total current obtained by the superposition of surface currents flows along?ydirection.In this scenario,the phase difference of the co-polarized components of the reflection coefficient between the state III and the state IV isπ.

3.Metasurface RRA design for beam scanning

According to the above discussions, we can know that the proposed metasurface unit cell is of both anisotropic and isotropic reflection characteristics in different operation states.The reflection phase of 180°can be achieved in either between the state I and the state II or between the state III and the state IV.With the unit cell, we can build up a 1-bit isotropic and anisotropic metasurface RRA.

Fig.8.The RRA diagram.

3.1.Co-polarized and cross-polarized 1-bit RRA

As shown in Fig.8,consider an RRA composed ofM×Nunit cells inxoyplane.The feeding horn with the radiation patternF(r)and the phase center atrfilluminates the reflectarray.The reradiated field from the reflectarray along ?udirection can be calculated by[11]

in whichrmnis the location of themnth unit cell,i.e.,the unit cell inmth row andnth column of the reflectarray,A(r)is the element pattern,and ?u0is the direction of the desirable beam.The required compensation phase of themnth unit cell is obtained as

Note that all unit cells are considered to have the same pattern and the coupling between them is neglected in Eq.(2).

According to the phase distribution of the reflectarray solved by Eq.(2), the phase behavior can be digitally quantized as

where PV(φ)represents a principle value operation transforming the phaseφinto the interval[0,360°],andφrefdenotes the reference phase.It can be seen that the introduction of theφrefin Eq.(3)has no effect on magnitude of the electric field solved by Eq.(2).Based on the quantized phase distribution given by Eq.(4),the proposed 1-bit unit cells can be arranged into a reflectarray.

In order to realize the co-polarized radiation,the unit cells operating in the states III and IV are designed as the unit cell 0 and unit cell 1, respectively, with the reflection phases of 0°and 180°.By changing theφref, the reflectarray layout is adjusted,and thus the optimal radiation performance of the RRA can be achieved.[26]Figure 9 gives the compensation phase distributions withM=N=12 and their digitally quantized layouts without and with theφreffor three radiation beam directions (θ=0°,?=0°), (θ=50°,?=0°), and (θ=40°,?=90°).It can be seen that the quantized layouts have the similar distributions to the compensation phase.Compared to the quantized layout without theφref,the quantized layouts are slightly adjusted by using theφref.

For the cross-polarized radiation,the unit cells operating in states I and II are designed as the unit cell 0 with the reflection phase of 0°and the unit cell 1 with the reflection phase of 180°,respectively.Similarly,the optimal quantized layouts are solved from Eq.(4)by changing theφreffor different radiation beam directions.Figure 10 shows the required compensation phase distributions and the quantized layouts without and with theφreffor three radiation beam directions(θ=0°,?=0°),(θ=50°,?=0°),and(θ=50°,?=90°).Similar distributions can be observed from the quantized layouts and the compensation phase distributions.

3.2.Circularly polarized 1-bit RRA

In order to achieve the CP radiation, consider an RHCP wave generated by a feeding horn illuminates the reflectarray.Without loss of generality, an incident wave along?zaxis is assumed.In this scenario, the incident electric field can be expressed as

in which|Ei|ej?iis thex-component of the incident electric field.When the unit cell operates in the state I, the reflective electric field can be obtained as follows:

With Eq.(7),the reflective wave along+zdirection in the state I is LHCP wave.

On the other hand,for the unit cell operating in the state II,the reflective electric field can be obtained as

Note that in deriving Eq.(8),the conditions of|,andhave been used.Therefore,the reflective wave in the state II is also the LHCP wave.The phase difference of the reflective electric fields between the state I and the state II isπ.Thus,the unit cells in the states I and II are designed as the unit cell 0 and the unit cell 1,respectively.The reflectarray with the LHCP radiation characteristic can be achieved by arranging the unit cells 0 and 1.It is worthwhile noticing that the quantized layout for the CP radiation is same as that for the cross-polarized radiation owing to the use of the unit cells in the states I and II.In addition, the RHCP wave can be radiated by the proposed reflectarray if the horn antenna with LHCP characteristic excites the reflectarray.

Fig.9.Phase distributions and the digitally quantized layouts without and with the reference phase for co-polarized beam steering:(a)(0°,0°)beam,(b)(50°,0°)beam,(c)(40°,90°)beam.

It is worthwhile pointing out that with the unit cells in the states III and IV,the reflectarray cannot radiate good RHCP wave for the RHCP wave illumination.This is due to the fact that in the either state III or state IV, the phase difference of the co-polarized components of the reflection coefficients between the TM polarization and the TE polarization is not approximatelyπ,as shown in Figs.6(b)and(d).Thus the CP radiation performance becomes deteriorated.The main reason is attributed to effect of the PIN diode on performance of the unit cell.It can be noticed that in the state III with two PIN diodes in ON state or the state IV with two PIN diodes in OFF state,the whole structure is only symmetry aboutyaxis.The responses of the corresponding structures fory-polarized incidence are distinctly different from those forx-polarized incidence.By contrast,the behavior of the unit cell in the state I is similar to that in the state II except 180°out-of-phase reflection.

Fig.10.Phase distributions and the digitally quantized layouts without and with the reference phase for cross-polarized beam scanning:(a)(0°,0°)beam,(b)(50°,0°)beam,(c)(50°,90°)beam.

3.3.Beam scanning performance

A commercial feeding horn (HD-100SGAN15N from Hengda Microwave Inc.) with the operation frequency of 8.2–12.4 GHz is used to excite the reflectarray.The horn with the phase center at (?56.6 mm,?56.5 mm, 226 mm) is placed in a distance of 240 mm from the reflectarray.The focal diameter ratio(F/D)is chosen as 1.25 to ensure good spillover efficiency and illumination efficiency.[27]

Fig.11.Co-polarization beam steering performance at 8.9 GHz: (a)xoz plane,(b)yoz plane.

Figure 11 shows the co-polarization beam steering performance at 8.9 GHz.It can be seen from Fig.11(a)that inxozplane,the peak gain of 19.42 dBi occurs at the radiation beam alongθ=?20°direction,with aperture efficiency of 21.43%and sidelobe level (SLL) of?12.4 dB.The gain drops from 19.42 dBi to 14.68 dBi when the radiation beam scans from 0°to 50°.The scanning range for the gain difference ≤3 dB covers?40°to 40°,with half power beamwidth(HPBW)less than 10°and SLL better than?8.23 dB.Inyozplane,the scanning range for the gain drop ≤3 dB is from?40°to 40°,where the peak gain of 17.96 dBi and the aperture efficiency of 15.31%and the SLL better than?8.21 dB are obtained according to Fig.11(b).Furthermore, the radiation pattern inxozplane at 8.9 GHz for the radiation beam along broadside direction is shown in Fig.12.The co-polarized component is 23 dB larger than the cross-polarized one,ensuring good polarization isolation.

Fig.12.Pattern at 8.9 GHz for the co-polarized beam along θ =0°direction.

Fig.13.Cross-polarization beam steering performance at 8.9 GHz: (a)xoz plane,(b)yoz plane.

Fig.14.Pattern at 8.9 GHz for the cross-polarized beam along θ =0°direction.

Figure 13 shows the cross-polarized beam scanning performance at 8.9 GHz.Inxozplane,the scanning range for gain difference ≤3 dB is±60°,with the peak gain of 17.41 dBi and the aperture efficiency of 13.49%at the radiation beam alongθ=?20°direction.For different beam directions, the SLL is below?11.8 dB and the HPBW is less than 19°.Inyozplane, the scanning range for gain drop ≤3 dB is±50°.For the radiation beam alongθ=20°direction, the peak gain is 17.02 dBi with the aperture efficient of 12.33%.When the radiation beam is in the direction ofθ=60°,the gain decreases by 4.15 dB.In the beam scanning range, the SLL better than?10 dB and the HPBW less than 14°are obtained.Note that the peak gains inxozandyozplanes for the cross-polarized radiation are less than those of the co-polarized radiation.The main reason is due to the loss of the substrate and the PIN diodes.In addition, it can be seen from Fig.14 that the copolarized component is 20 dB less than the cross-polarized one.

Figure 15 gives the LHCP beam scanning performance at 9 GHz under the RHCP wave illumination.It can be observed that when the radiation beam steers from 0°to 50°, the gain drop is 2.52 dBic inxozplane and 2.86 dBic inyozplane.The peak gain of 16.98 dBic and the aperture efficiency of 12.22%inxozplane and the peak gain of 16.96 dBic and the aperture efficiency of 12.16% inyozplane, respectively, are obtained.The SLL better than?10 dB and the HPBW less than?15°are achieved in the beam scanning range.In addition, the LHCP gain is 10 dB larger than the RHCP one,as shown in Fig.16.

4.Fabrication and measurement of metasurface RRA

The prototype of the proposed metasurface RRA is fabricated,as shown in Fig.17(a).The whole structure consisting of a reflectarray, a feeding horn, two control circuit boards.Figure 17(b)shows the 12×12 reflectarray.There are 24 sockets that are placed at the two edges of the bottom surface of the reflectarray, as shown in Fig.17(c).Each socket can provide 12 independent DC channels for 6 elements.There are 24 flat cables used to connect two control boards and 24 sockets.In each control board,144 light-emitting diodes(LED)are used to indicate the operation states of the PIN didoes.The LED is lightened when the corresponding PIN diode is in ON state,as shown in Fig.17(d).The control program corresponding to each digitally quantized layout is written into a 32-bit microcontroller unit(MCU)in the control board,thus realizing the beam steering with multiple polarization manipulations.Due to our measurement limitations, the used feeding horn only radiates ay-polarized electric field.

Fig.15.LHCP beam steering performance at 9 GHz: (a)xoz plane,(b)yoz plane.

Fig.16.Pattern at 9 GHz for the CP radiation beam along θ =0° direction.

Fig.17.The fabricated RRA: (a) whole structure, (b) top view of reflectarray, (c) bottom view of reflectarray, (d) control board with LED indicators.

Fig.18.Comparison of the co-polarized beam steering performance between the measurement and the simulation at 8.9 GHz: (a)xoz plane,(b)yoz plane.

Figure 18 shows comparison of the co-polarized beam steering performance between the measurement and the simulation at 8.9 GHz.Good agreement between them is observed as the radiation beam steers from 0°to 50°.The measured peak gains inxozandyozplanes are 20.08 dBi and 18.68 dBi,with the aperture efficiencies of 24.95%and 18.07%, respectively.The measured SLL is better than?10 dB and the measured HPBW is less than 14°in the beam scanning range.Figure 19 shows the variation of the gain with the frequency for the broadside radiation.The measured 1 dB gain bandwidth covering 8.6–9 GHz with the fractional bandwidth of 4.55%is slightly smaller than the simulated one of 8.6–9.17 GHz with the fractional bandwidth of 6.42%.The measured 3 dB gain bandwidth is 8.3–9.2 GHz with the fractional bandwidth of 10.29%, which is slightly larger than the simulated one of 8.5–9.35 GHz with the fractional bandwidth of 9.52%.The slight discrepancy is attributed to the fabrication errors, substrate tolerance,and the experiment errors,etc.

Fig.19.Variation of the gain with the frequency for the co-polarized radiation.

The simulated and measured beam steering performance for the cross-polarized radiation is shown in Fig.20, in good agreement.The measured peak gain and the aperture efficiency inxozplane are 17.26 dBi and 13.3%and those inyozplane are 16.62 dBi and 11.23% within the scanning range of±60°.In the beam scanning range, the SLL better than?10 dB and the HPBW less than 17°are achieved.It can be seen from Fig.21 that the measured 1 dB and 3 dB gain bandwidths cover 8.7–8.9 GHz and 8.5–9.1 GHz,respectively,slightly smaller than the simulated ones of 8.74–9.3 GHz and 8.55–9.5 GHz.

For the CP radiation, as shown in Fig.15, the simulated beam scanning range for the gain drop ≤3 dB covers 0°to 50°.In addition, Fig.22 shows variation of the simulated LHCP gain with the frequency.The 1 dB and 3 dB gain bandwidths cover 8.91–9.41 GHz and 8.64–9.5 GHz,respectively.

Table 2 gives performance comparison between the proposed metasurface RRA design and the similarly reported designs.It can be found that the proposed reflectarray achieves the 2D beam steering and the polarization manipulations including linear co-polarized and cross-polarized radiations and circular cross-polarization radiation.Moreover,the wide beam scanning range for the gain drop no more than 3 dB is achieved,with the moderate 3 dB gain bandwidth and the aperture efficiency.

Fig.20.Comparison of the cross-polarized beam steering performance between the measurement and the simulation at 8.9 GHz: (a)xoz plane,(b)yoz plane.

Table 2.Performance comparison between the proposed design and the reported designs.

Fig.21.Variation of the gain with the frequency for the cross-polarized radiation.

Fig.22.Variation of the gain with the frequency for the CP radiation.

5.Conclusion and perspectives

In summary, a 1-bit electronically reconfigurable metasurface reflectarray with beam steering and polarization manipulation capabilities has been developed.A simple topology integrated with two PIN diodes is designed.By switching the PIN diodes between ON state and OFF state,the proposed unit cell operates four states,two of them exhibiting the crosspolarized responses and the others showing the co-polarized characteristics.Therefore, two groups of 1-bit unit cells are achieved for the cross-polarized and co-polarized radiations,respectively.Under the illumination by a linearly polarized wave, the designed metasurface RRA can achieve the 2D copolarization and cross-polarization beam steering.When the RHCP wave is incident on the RRA,the LHCP beam scanning is realized.The 12×12 RRA prototype is fabricated.Measured results are given to demonstrate the wide beam scanning range of about±50°and the good aperture efficiencies of about 25%for co-polarization radiation and about 13%for the cross-polarization radiation,meaning that the proposed metasurface RRA is a good candidate for wireless communication in the complex environment.

Acknowledgments

Project supported by the National Key Research and Development Program of China (Grant No.2021YFA1401001)and the National Natural Science Foundation of China(Grant No.62371355).