A frequency and bandwidth tunable metamaterial absorber in x-band

A frequency and bandwidth tunable metamaterial absorber in x-band

H. Yuan, B. O. Zhu, and Y. Feng

Citation: Journal of Applied Physics 117, 173103 (2015); doi: 10.1063/1.4919753

View online: https://www.360docs.net/doc/7d16616873.html,/10.1063/1.4919753

View Table of Contents: https://www.360docs.net/doc/7d16616873.html,/content/aip/journal/jap/117/17?ver=pdfcov Published by the AIP Publishing

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A frequency and bandwidth tunable metamaterial absorber in x-band

H.Yuan,B.O.Zhu,a)and Y .Feng

School of Electronic Science and Engineering,Nanjing University,Nanjing 210023,China

(Received 16February 2015;accepted 23April 2015;published online 4May 2015)

Smart control is an attracting and important function for modern electromagnetic wave absorber.This paper presents the design,fabrication,and measurement of a frequency and bandwidth tunable metamaterial absorber (MA)in X-band.The unit cell of the MA consists of a microstrip resonator loaded with the varactors.Simulation and measurement results show that by tuning the bias voltage on the varactors,the peak absorption frequency can be tuned by 0.44GHz with the peak absorption greater than 95%.Field and circuit model analysis is conducted to reveal the work-ing mode and predict the absorbing frequency.After that,by specially designing the bias circuit so as to adjust the bias voltage on neighboring unit cells separately,dual resonance and absorption peaks occur,and the overall absorption bandwidth can thus be tuned conveniently by controlling the difference of the two resonance frequencies.The center absorbing frequency can also be tuned.Simulation and experiment results show that the 75%absorption (à6dB re?ection)bandwidth can be tuned from 0.40GHz to 0.74GHz,which is a two-fold tuning range.This work is believed to

improve the state-of-the-art smart metamaterial absorber.V

C 2015AIP Publishing LLC .[https://www.360docs.net/doc/7d16616873.html,/10.1063/1.4919753]

I.INTRODUCTION

Electromagnetic (EM)wave absorber is a functional ma-terial that absorbs the incident EM energy.Conventional EM absorbers are mainly realized by using materials with high electric loss,e.g.,the Salisbury screen,pyramid shaped foam impregnated with the mixture of carbon and iron,or by using materials with high magnetic polarization loss,e.g.,ferrite tile absorber.1–4

The metamaterial absorber (MA)is another new type of EM absorber proposed recently.5It is normally composed of electrically small unit cells arranged periodically in a two dimensional plane.Each unit cell can be regarded as the res-onant circuit.At resonance,MA exhibits purely effective surface resistance that is matched to the incident EM wave impedance for ef?cient wave absorption.The MA enjoys the features of light weight and low pro?le,but suffers from nar-row and ?xed absorbing frequency bandwidth.6–8

Since MA is realized through arti?cial design of the unit cell structure,one unique advantage of MA is that more functions than absorbing waves can be combined into MA,such as absorbing frequency tuning,9–12absorbing intensity tuning,13polarization independent absorbing,14–16multiple band absorbing,17–20wide incident angle absorber.21,22

Tunable absorbers are particularly useful components in wideband wireless communications systems,including the emerging system of cognitive radio networks and the intelli-gent radar system.During the past years,the tunable absorb-ers reported in literature focus mainly on the frequency tuning.However,bandwidth is also an important factor to tune in applications.23

In this paper,a bandwidth tunable MA in X-band is pre-sented.First,we employ microstrip line resonator to design

an absorbing frequency tunable MA.This approach is advan-tageous in X-band MA design.Then,by independently tun-ing the resonant absorbing frequencies of the unit cells in adjacent columns so as to control the resonant frequency difference,the overall absorbing bandwidth of the proposed MA can be tuned dynamically.The resonant mode of the unit cell is analyzed.Simulation and measurement is carried out to characterize its performance.

II.THE X-BAND FREQENCY TUNABLE MA EMLOYING MICROSTRIP RESONATOR A.Unit cell structure

The proposed MA is a two dimensional periodic struc-ture.The unit cell of the frequency tunable absorber is com-posed of a microstrip line resonator loaded with varactor at the center,as shown in Fig.1(a).The yellow part represents the metal and the grey part represents the dielectric substrate.The horizontal metal line at the upper and lower edge of the unit cell is to supply the bias voltage on the varactor.A full metal is on the bottom layer of the

substrate.

FIG.1.The frequency tunable MA.(a)The unit cell and (b)the fabricated MA sample.

a)

Author to whom correspondence should be addressed.Electronic mail:bzhu@https://www.360docs.net/doc/7d16616873.html, 0021-8979/2015/117(17)/173103/6/$30.00V

C 2015AIP Publishing LLC 117,173103-1

JOURNAL OF APPLIED PHYSICS 117,173103

(2015)

Since the MA is a periodic structure,and the unit cell is symmetric with respect to both X and Y axes,the re?ection property for normal wave incidence can be studied by ana-lyzing the single unit cell enclosed by the perfect electric conductor(PEC)-perfect magnetic conductor(PMC)boun-dary condition.The PEC-PMC boundary condition actually constitutes a PEC-PMC waveguide which supports the plane wave as the fundamental mode with zero cut off frequency. The unit cell can be considered as the microstrip resonator loaded at the termination of the waveguide,and the wave-guide works as the feeding line for the resonator.When the unit cell resonates,it exhibits pure resistance to the feeding line,and its total input reactance is zero.By adjusting the size of the microstrip line and the thickness of the substrate, the resonance frequency can be designed.In order to achieve high absorption,the unit cell periodicity should be optimized in order to match the line impedance of the PEC-PMC wave-guide to the circuit impedance of the unit cell.Through tun-ing the bias voltage on the varactor at microstrip resonator center,the junction capacitance of the varactor is changed so that the resonance frequency,i.e.,the absorbing frequency, can be shifted.

B.Working mechanism

Fig.2illustrates the working mechanism.The unit cell can be regarded as two segments of microstrip line with short circuit termination due to the PEC boundary condition.The two segments are coupled through the varactor.Under the excitation of the fundamental mode of the PEC-PMC wave-guide,i.e.,plane wave excitation,the two segments of micro-strip line resonate out of phase,as shown in the Fig.2.Hence, the z component of the electric?eld is zero at YZ plane,so that there is a virtual PEC at that plane.As a result,each seg-ment can be modeled as the resonant circuit illustrated in Fig.

2.At lower frequencies,the varactor is an ordinary variable capacitor as shown in Fig.2(a).In the X-band,the varactor’s parasitic inductance dominates so that the varactor shall be regarded as a tunable inductor as shown in Fig.2(b).On the other hand,short-terminated microstrip line exhibits capaci-tance(inductance)to the varactor when it is longer(shorter) than its quarter guided operation wavelength,Hence,the pro-posed absorber can resonate at both higher and lower frequen-cies.When C varactor?0.9pF,simulation result shows that the ?rst resonance mode is at

3.16GHz,and the second one at 8.58GHz.Fig.2compares the electric?eld distribution of these two modes.As can be seen from the amplitude pro?le of the electric?eld distribution,the microstrip line in the half unit cell is shorter than quarter wavelength at3.16GHz,and longer than quarter wavelength at8.58GHz.The second reso-nant mode is important for high frequency MA design because it allows the moderate size of the unit cell rather than the one too small to be fabricated at high frequencies.

The current distribution on the top and ground metal also demonstrates the difference of the two working modes.A cur-rent strength null is observed near the unit cell center in the second resonant mode shown in Fig.2(b),while such a mini-mum point does not exist in the?rst mode shown in Fig.2(a), verifying that the microstrip line is shorter or longer than the quarter wavelength in the?rst or second mode,respectively. The current directions on the varactor are also different in these two modes relative to the current on the strip,due to the electric length variation of the microstrip resonator.

The resonant and absorbing frequency is predicted by using a little more accurate circuit model including the fringe inductance and capacitance at the gap discontinuity of the microstrip line where the varactor is soldered,as shown in Fig.3(a).The microstrip line exhibits capacitive reactance in X-band,and the varactor has an inductive reactance.L repre-sents the fringe inductance,and C represents the fringe ca-pacitance on the gap.The model’s total impedance Z is calculated as

Z?Z striptiwLt

1

iwC

Z varactor

1

iwC

tZ varactor

(1)

where Z strip is the input impedance of the short-terminated microstrip line and Z varactor is the varactor’s impedance.The resonant absorbing frequency can be obtained by letting the reactance of the total impedance Z equal to0.L,and C are determined by using the simulated resonant frequencies under two different varactor capacitance.

In Figs.3(b)and3(c),we use Eq.(1)to calculate the reso-nant frequencies for various varactor capacitance C varactor or microstrip resonator length n.When C varactor or n is increased, the resonance absorbing frequency decreases monotonously. Good agreement between the calculated results and simulated ones is achieved,which validates Eq.(1).

C.Experiment and simulation results

To verify the performance of the proposed tunable MA, we designed,fabricated,and measured a sample MA board. The metal is copper with the electric conductivity 5.8?107S/m.The optimized geometry dimensions of the unit cell are:m?16mm,n?10mm,w?8mm,h?0.8mm, gap?0.4mm,and k?0.2mm.The thickness of the copper ?lm is0.017mm.The dielectric substrate is FR4with the rel-ative permittivity4.1and loss tangent0.025.The varactor was modeled as the resistor,inductor,and capacitor in series in EM simulation.The varactor capacitance range is from 2.350pF at0V to0.466pF at15V.The varactor parasitic re-sistance is2.5X,and the parasitic inductance is1.1nH.A 210?244mm2sample board consisting of20?14unit cells was fabricated using printing circuit board technology,as shown in Fig.1(b).The SMV1231–011varactor is used in the board.

The experiment was performed in the microwave anechoic chambers.A vector network analyzer(Agilent E8363C)and two horn antennas were used to transmit EM waves onto the sample board and receive the re?ected sig-nals as sketched in Fig.4(a).The distance of the antenna to the sample is around3m.The incident angle is5degree to the normal of the sample.Due to the metallic ground so that EM transmission is zero,we only measured the re?ection coef?cient S11of the sample.The energy absorption can be further calculated as1àj S11j2.

FIG.2.The equivalent circuit model,

the E-?eld distribution at XZ plane,

and the surface current distribution(a)

for the?rst resonance mode at

3.16GHz and(b)for the second reso-

nance mode at8.58GHz.

with different(b)varactor capacitances and(c)microstrip resonator lengths.

When we increase the DC bias voltage from 0.0V to 15.0V,the varactor capacitance becomes smaller,leading to the rise of the absorbing frequency,as shown in Fig.5(a).The maximum re?ection attenuation is above 12dB during the absorbing frequency tuning.When the DC bias voltage is 0.0V,the measured absorption is 98.3%at 8.48GHz with the full width at half magnitude (FWHM)of 5%.As increas-ing the bias voltage,the absorption peak shifts to higher fre-quencies continuously.At 15.0V,the frequency of the peak absorption is 8.92GHz with the absorption rate of 95.2%and the FWHM of 5%.The tuning range of the absorbing fre-quency is 0.44GHz.

The peak absorption rate changes slightly as the peak absorbing frequency varies because we optimized the unit cell for the best absorption rate only at one particular capaci-tance value.The dashed lines in Fig.5(a)show the simulated re?ection at 0.0V and 15.0V for comparison.They differ from the measurement results a little bit.This is mainly due to two reasons.First,the varactor parameters used in simula-tion are measured at the frequency far lower than the X-band,so that the parasitic impedance effect of the diode at high frequency such as X-band is not taken into account.Second,the varactor soldering position may also affect the resonance frequency.However,the agreements are good overall between the experimental results and the simulation ones.The intermediate cases are also simulated,as shown in Fig.5(b).The peak absorption frequency shifts as the bias voltage is adjusted.The smallest capacitance corresponds to the highest bias voltage,and vice versa.

III.THE BANDWIDTH TUNABLE META-ABSORBER A.Unit cell configuration

Based on the frequency tunable MA presented above,we propose a bandwidth tuning mechanism by controlling

the bias voltages on different unit cells separately such that the whole MA will have tunable dual resonances and the overall absorption bandwidth can thus be adjusted and broadened.In addition,the center absorbing frequency can still be tuned if the bias voltages are tuned together.

The unit cell of the bandwidth tunable absorber is shown in Fig.6(a).In this design,bias voltages on the two varactors in adjacent microstrip resonators shown in Fig.6(b)are tuned independently.To do so,a bias network layer is designed as shown in Fig.6(c)and fabricated below the ground plane.Via holes are drilled to electrically connect the metal strips on the top layer and the bias voltage circuit layer.They are insulated from the ground plane.Under normal incidence,there is vir-tual PEC plane penetrating the upper and lower edges of the unit cell,hence placing via holes there does not in?uence the working performance of the absorber.

B.Experiment and simulation results

The optimized geometric dimensions of the bandwidth tunable MA are as follows:a ?16mm,b ?20mm,L 1?6mm,w 1?4.7mm,w 2?0.4mm,w 3?0.2mm,w 4?5.3mm,h 1?h 2?0.8mm.r 1?0.2mm is the radius of the via hole,and r 2?0.4mm is the radius of the hole on the ground where the via penetrates.Copper and FR4dielectric materials were used as the metal and dielectric substrate parts.

The full wave EM simulation based on the ?nite integra-tion technique was performed at the design and optimization stage.The unit cell boundary condition was applied to analyze the single unit cell.The incident wave’s electric ?eld is per-pendicular to the microstrip line gap since the design is polar-ization sensitive.The simulated re?ections are depicted as the dashed curves in Fig.7.When the two varactors are 0.6pF equally in one unit cell,the MA has only one absorbing peak with the narrowest bandwidth.As the right varactor is tuned to 2.35pF (0V)and the left one to 0.466pF (15V),the absorbing peak splits and bandwidth becomes the widest.From the results,we clearly see the bandwidth changing with different varactor capacitance.For the 75%absorption rate (à6dB re?ection),the bandwidth changes from 0.43GHz (10.76–11.19GHz)to 0.72GHz (10.55–11.28GHz),nearly doubling the bandwidth.

When the bandwidth is the widest,it is clear enough to observe the two absorption peaks at 10.78GHz and 11.10GHz in Fig.7,respectively.Fig.8shows the current distribution at these two frequencies.At 10.78GHz,

most

FIG.4.Measurement setup for (a)the frequency tunable MA and (b)the bandwidth tunable

MA.

FIG.5.(a)Measured (solid)and simu-lated (dash),and (b)simulated absorp-tion of the tunable MA at different bias voltage or varactor capacitance for the normal incident EM waves.

currents are distributed in the right resonator with the varac-tor capacitance of 2.35pF.At 11.10GHz,currents are mainly distributed in the left one with the varactor capaci-tance of 0.466pF.

The fabricated bandwidth tunable MA is shown in Figs.6(e)and 6(f).The sample board is 210mm ?202mm,consist-ing of 10?12unit cells.The experiment was performed using the focusing lens antennas measurement system,which is shown in Fig.4(b).The sample was placed at the focus of the two lens antennas,and the free space Transmission-Re?ection-Line calibration was applied to move the reference

plane to the MA’s surface.Two ports S parameters were measured with this setup,but only S 11is meaningful.

The measured re?ection of the bandwidth tunable MA is exhibited in Fig.7.V 1and V 2are the DC bias voltage of the left and right resonators in the unit cell,respectively.Because the left resonator is identical to the right one,we only change the left bias voltage from 0V to 7.5V,and the right one from 15V to 7.5V.When V 1?0V and V 2?15V,the bandwidth of the 75%absorption rate is 0.74GHz (from 10.72GHz to 11.50GHz),which is the widest bandwidth.As V 1increases to 7.5V and V 2decreases to 7.5V,the band-width of the 75%absorption rate narrows to the smallest value 0.34GHz (from 11.03GHz to 11.43GHz).The varia-tion is about two folded.In the measurement,Time Gate Technique is used to reduce multiple interference so that the measured curves in Fig.7are smooth.

The simulated and measured results observed in Fig.7are summarized in Table I .The small difference is due to the para-sitic impedance effect of the diode at high frequency and the varactor soldering position.However,the agreements are good overall between the experiment results and the simulation ones.

IV.SUMMARY

A frequency tunable X-band MA employing microstrip resonator is presented.It works under the second order mode where the varactor behaves as a tunable inductor,and the short terminated microstrip line as a lumped capacitor.This mode allows tunable MA design in high frequency band such as X-band where the parasitic inductance of the varac-tor would dominate.It is shown that the peak absorption frequency can be adjusted by 0.44GHz with the peak absorp-tion greater than 95%.By designing the bias circuit so as to adjust the varactors’capacitance in adjacent columns inde-pendently,a dual resonances MA is achieved.The distance of the two resonance frequencies can be adjusted so that the absorbing bandwidth can be tuned.In addition,the

absorbing

FIG.7.The simulation (dashed)and the experimental (solid)results of the bandwidth tunable MA at different bias voltages for normal incident EM

waves.

FIG.8.The current distributions of the bandwidth tunable MA.(a)At 10.78GHz.(b)At 11.10GHz.

TABLE I.The simulation and experiment results of the 75%absorption bandwidth.

Simulation (GHz)

Experiment (GHz)Narrowest bandwidth 0.430.40The range

10.76–11.19

11.03–11.43

Widest bandwidth 0.730.74The range

10.55–11.28

10.72–11.45

FIG.6.The bandwidth tunable MA.(a)The perspective view.(b)The top view.(c)The bottom view.(d)The side view.(e)The top and (f)the bottom view of the fabricated bandwidth tunable MA board.

frequency tuning is maintained if the bias voltages on differ-ent unit cells are tuned together.Simulation and experiment results show that the bandwidth of the75%absorption rate can be changed from0.40GHz to0.74GHz,which is two-fold https://www.360docs.net/doc/7d16616873.html,pared with the?rst order mode where the varactor exhibits the capacitive reactance,the second order mode is suitable for high frequency MA design where the varactors work as a tunable inductor and the microstrip line part provides the necessary capacitive reactance to meet the resonance condition.However,the frequency tuning range of the second order mode is generally smaller than that of the?rst order mode because of the dispersive effect of both the varactor impedance and microstrip line input impedance. Currently,the proposed design is polarization sensitive.It can be further developed to achieve polarization insensitive EM wave absorbing by using symmetric unit cell structure in the future.21,22Furthermore,the period of the proposed absorber is smaller than the half working wavelength so that the re?ected energy concentrates on the specular direction for oblique incidence.The tunable bandwidth MA can be used in some scienti?c and technological areas,particularly in electronically tunable spatial?lters.This work is believed to improve the state-of-art research of smart metamaterial absorber.

ACKNOWLEDGMENTS

This work was supported by the National Nature Science Foundation of China(Nos.61301017,61371034,60990322, and61101011),and the Key Grant Project of MoE of China (No.313029),and partially supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions(PAPD)and Jiangsu Provincial Key Laboratory of Advanced Manipulating Technique of Electromagnetic Wave.

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