R E S E A R CH AR T I C L E

High-conductivity graphene-assembled film-based bandpassfilter for 5G applications

Huang Bangqi1 | Siting Li1 | Rongguo Song2 | Zhanyong Hou2 |

Chengguo Liu2 | Daping He2

1School of Science, Wuhan University of Technology, Wuhan, Hubei, China2Hubei Engineering Research Center of RF-Microwave Technology and Application, Wuhan University of Technology, Wuhan, Hubei, China

CorrespondenceDaping He, Hubei Engineering ResearchCenter of RF-Microwave Technology andApplication, Wuhan University ofTechnology, Wuhan 430070, Hubei,China.Email: hedaping@whut.edu.cn

Funding informationFundamental Research Funds for theCentral Universities (No. 2020-YB-032,205209016); 2018 National Key R&DProgram of China 257; Equipment Pre-Research Joint Fund of EDD and MOE(No. 614A02022262); Foundation ofNational Key Laboratory onElectromagnetic Environment Effects(No.614220504030617); FundamentalResearch Funds for the CentralUniversities, Grant/Award Number:WUT&xxxFF1A;2020IB005; NationalNatural Science Foundation of China(No. 51701146, 51572205 and 51672204)

Abstract

This article presents a compact 5G dual-wideband bandpass filter (DBPF)

based on high-conductivity graphene assembled films (GAFs), which can meet

the requirements of the sub-6 GHz bands, high integration and high informa-

tion transmission rate for 5G wireless communication. The GAF DBPF consists

of two folded open-loop stepped-impedance resonators with a compact size of

21.9 mm × 22.5 mm × 0.53 mm. The measured results show that the presented

GAF DBPF has two passbands with 3 dB fractional bandwidths of 15.95% and

12.72% at center frequencies of 2.14 and 3.5 GHz, respectively. In addition, the

filter exhibits an attenuation level greater than 20 dB up to 7.52 GHz.

KEYWORD S

5G, bandpass filter, dual-wideband, folded open-loop-stepped-impedance resonator, graphene-

assembled film

1 | INTRODUCTION

The demand for high performance dual-band ormulti-band filters is increasing in modern wireless com-munication system, since with the help of filters, signalsof required frequencies transmitted from antennas can beremained while the unwanted frequencies and noises canbe suppressed. In 1980, Makimoto M. and YamashitaS. put forward the concept of stepped-impedance resona-tor (SIR), and proposed that parasitic response and inser-tion loss could be controlled by changing the structure ofSIR.1 Since then, SIR has been extensively investigated inthe design of dual-band filters.2-5 Designing a dual-bandfilter using SIR is relatively convenient and easy to imple-ment, which has the advantages of adjustable passband,

easily accessed transmission zero (TZ) with excellenttransmission performance.6,7 However, with the rapiddevelopment of wireless communication technology,especially the emergence of 5G wireless systems, thecommunication systems need to have the characteristicsof high integration, stable performance, low cost andenvironmental benignancy. As a key component of wire-less communication systems, dual-band microstrip filtersare supposed to share the same merits. Conventionalmetal-based filters fail to fully satisfy these requirementsbecause of their low reserves, high chemical reactivityand poor mechanical properties. Meanwhile, graphene,as the thinnest and lightest 2D material, exhibits highelectrical and thermal conductivity along with excellentflexibility and corrosion resistance,8-10 which make it a

Received: 29 January 2021 Accepted: 2 February 2021

DOI: 10.1002/mmce.22602

Int J RF Microw Comput Aided Eng. 2021;e22602. wileyonlinelibrary.com/journal/mmce © 2021 Wiley Periodicals LLC 1 of 8

https://doi.org/10.1002/mmce.22602

promising material for the development of new genera-tion filters in 5G wireless systems.

However, graphene suffers from relatively large inser-tion loss due to coupling excitation in the microwaveband, making it unsuitable for microwave devices andmost reports are currently limited to theoretical studiesin the terahertz band.11,12 Extensive works have beendone to improve the mediocre conductivity of grapheneand related materials so as to reduce the loss in themicrowave band.13 In 2018, Sinar used a customizedprinter to print graphene ink onto microstrip circuits tomake low-pass filters, proving that the printed graphenederivative circuits can be applied to low-frequencydevices, but are limited for flexible electronics that do notrequire high precision.14,15

In 2018, our group prepared a flexible graphene-assembled film (GAF) with a conductivity of 1.1 × 106 S/m.GAFs are stacks of multiple layers of graphene sheet withgood flexibility and excellent mechanical and chemical sta-bility. In addition to the common merits of graphene,GAFs also show distinct advantages in conductivity andsurface resistance making it a potential candidate for thelarge-scale manufacture of radio frequency (RF) devicesfor 5G wireless systems.16 In our previous work, GAFshave been applied to the fabrication of RF devices such asantennas and low-pass filters.17 Microstrip filters based ongraphene films not only exhibit good performances com-parable to traditional copper-based filters, but also showsuperior temperature and corrosion resistance that tradi-tional copper-based filters cannot compete. Moreover, fil-ters based on graphene films have better stability againstrepetitive bending deformations owing to the flexiblenature of graphene materials.18 It is also worth mentioningthat, taking advantages of the laser engraving method,

microstrip filters based on GAFs with higher precision areachieved.19 The aforementioned merits indicate that GAFswith high conductivity have the potential to be applied indual-band filters.20,21 Although many microwave devicesand microstrip filters based on GAFs have beenimplemented, so far, studies on GAF-based dual bandmicrostrip filters are still lacking, especially for dual-bandmicrostrip filters in 5G wireless systems.22

This article presents a 5G dual-wideband bandpass fil-ter (DBPF) consisting of two folded open-loop stepped-impedance resonators (FOLSIRs) based on GAFs. TheGAF DBPF is designed to operate in the sub-6 GHz bandof 5G with the two passbands center at 2.14 and 3.5 GHz,respectively. More importantly, for the second spuriousresonance frequencies of the two passbands normallycaused by distortion of power amplifier or distortion ofhigh-power inputs, the fabricated GAF DBPF shows anattenuation level of 21 and 24.18 dB, respectively. TheDBPF is fabricated by the laser engraving technique,which can provide a high machining accuracy. The fabri-cated GAF DBPF shows a good filtering effect owing tothe increased isolation between the passbands and thelevel of out-of-band rejection.

2 | CHARACTERIZATION OF GAFS

Figure 1A presents the digital photograph of GAF foldedinto an origami crane, demonstrating its excellent flexi-bility. The GAF has a thickness of ~28 μm as can be seenfrom the cross-sectional scanning electron microscope(SEM) image (Figure 1B). The cross-sectional image alsoshows that the GAF is a layered structure composed ofgraphene nanosheets, which renders GAF good

FIGURE 1 A, Digital photograph of origami crane made by graphene assembled films (GAF); B, Cross-sectional scanning electron

microscope (SEM) image of GAF

2 of 8 BANGQI ET AL.

flexibility. In addition, the density of the GAF is1.6 g/cm3, which is much smaller than that of copper(8.8 g/cm3).

To further verify the flexibility and mechanical stabil-ity of the GAF, a bending test is carried out as shown inFigure 2. The GAF is being bent repetitively with a bend-ing diameter of 2 cm for 2000 times. The relative resis-tance of the GAF as a function of the bending times isplotted in Figure 2 which shows a constant value, dem-onstrating that the GAF with superb mechanical stabilitycan sustain repetitive bending without affecting its elec-trical properties.

3 | FILTER CONFIGURATION ANDDESIGN

The schematic diagram of the proposed GAF DBPF isshown in Figure 3A with detailed structural parameterdefinitions. The DBPF consists three layers, including theGAF conductor strip, substrate, and ground. The feasibil-ity of the DBPF structure has been verified in one of ourprevious work.18 The DBPF consisting of two FOLSIRswith two 50-Ohm feed lines are directly connected to theresonator via tap feed. FOLSIR is obtained by trans-forming the structure of SIR, miniaturization and TZ areachieved by bending transformation. The structure con-sists two high-impedance lines with a line width of W1,two low-impedance lines with a line width of W2, and animpedance conversion part with a line width of W3 inbetween the two low-impedance parts (LIPs) to neutral-ize the transmission line characteristic changes caused bythe structural conversion from SIR to FOLSIR. The high-

impedance parts (HIPs) of the two FOLSIRs are designedto be close enough to each other to facilitate coupling.The equivalent circuit diagram of FOLSIR is shown inFigure 3B. Compared with SIR, the equivalent circuit ofFOLSIR increases the terminal coupling capacitor C andthe characteristic impedance ZS corresponding to theseries impedance conversion part with width of W3. Theseries impedance transformation part with width W3 notonly eliminates the influence of the terminal couplingcapacitor C on the resonance frequency of the resonator,but also realizes a novel passband adjustment. FOLSIRthus has a more compact structure, an adjustable reso-nance frequency, and a controllable TZ.

For λg/2 SIR,1 the relationship between the electricallength θ and impedance ratio K is described as:

FIGURE 2 The flexibility and mechanical stability test of

graphene assembled films (GAF) with bending radius of 2 cm

FIGURE 3 A, Schematic diagram of the 5G graphene

assembled films (GAF) dual-wideband band-pass filter (DBPF); B,

The equivalent circuit of the folded open-loop stepped-impedance

resonators (FOLSIR)

BANGQI ET AL. 3 of 8

θ=arctanffiffiffiffiK

p, ð1Þ

where K is the impedance ratio of HIP and LIP definedas K = Z2/Z1. The impedance ratio is further related tothe resonant frequencies as follow:

f s1f 0

=π

2arctanffiffiffiffiK

p , ð2Þ

where f0 and fs1 are the fundamental resonant frequencyand the first spurious resonant frequency of the SIR,respectively.

Taking microstrip as the basic structure, the charac-teristic impedance of each part is determined by the fol-lowing formula:

Z0 =120πffiffiffiffiffiεre

p � 1W=h+1:393+ 0:667ln W=h+1:444ð Þ , ð3Þ

εre =εr +12

+εr−12

1+12hW

� �−12

W=h≥ 1, ð4Þ

where W is width of the conductor, h is the thickness of thesubstrate, εr and εre are the relative dielectric constant andthe effective dielectric constant of the substrate, respectively.

The GAFDBPF is fabricated on Rogers 5880 with a rela-tive dielectric constant of 2.2 and thickness of 0.787 mm.Taking the frequency values of f0 = 2.14 GHz andfs1 = 3.5 GHz, the initial physical size of FOLSIR is calcu-lated by Equations (1)-(4) to be with W0 = 0.627 mm,W2 = W3 = 2.415 mm, L1 + L2 = 16.177 mm, andL3 + L4 + L5 = 31.33 mm. It is also worth mentioning that

by merely changing the size parameters of the filter, it canbe readily adapted to frequency range 2 (24.25-52.60 GHz)of 5G applications according to Equations (1)-(4).

The attenuation caused by conductor loss of micro-strip line (αc) can be expressed by the following formula(unit: 1/m):

FIGURE 4 Conductor loss αc vs conductor conductivity σFIGURE 5 The simulated |S21| responses of filters with

different part width W3, A; gap distance g, B; and slit width S1, C

4 of 8 BANGQI ET AL.

αc =Rs

Z0W, ð5Þ

Rs =1σδ

=

ffiffiffiffiffiffiffiffiωμ02σ

r, ð6Þ

where R0 is the characteristic impedance of the micro-strip line with width W, and Rs is the surface resistanceof the conductive strip and the ground plate, σ is the elec-trical conductivity, μ0 is the magnetic permeability in freespace, ω is the angular frequency, and δ is the skin depth.

Figure 4 plots the relation between the conductor lossof the microstrip line (αc) and the conductivity of the con-ductor material (σ). When the conductivity of the con-ductor material reaches 1 × 106 S/m, the curve isapproaching its plateau region. Therefore, although theconductivity of graphene films is an order of magnitudelower than that of copper, the difference in conductorlosses are almost negligible.

The designed filter is then optimized by a 3D electro-magnetic simulation software with respect to part width

W3, gap g and slit width S1 as shown in Figure 5. Thecenter frequency of the second passband shifts lower withincreasing W3 while the center frequency of the first pass-band barely changes as depicted in Figure 5A. The reasonfor this change is that the increase in W3 leads to adecrease in Zs and a decrease in the characteristic imped-ance of the low-impedance part, that is, the impedanceratio of the GAF DBPF increases. According to Equa-tion (2), we can see that the center frequency of the sec-ond passband will decrease, and vice versa.

The center frequency of the second passband can beadjusted by changing W3. Compared with the traditionalmethod of adjusting the resonance frequency of the reso-nator by loading branch lines, we are now offering higherdegree of freedom of adjustment. As the gap distanceg between the two FOLSIRs decreases, the bandwidths oftwo passbands of the GAF DBPF continue to increase(Figure 5B). In the equivalent circuit (Figure 3B), the twoends of the FOLSIRs are connected through a lumpedcapacitor C, which is called an electrical coupling path.By changing the lumped capacitor C, the coupling valueof the coupling path will affect the position of theTZ. The frequency of TZ increases with increasing S1(Figure 5C). When the TZ is at proper frequency, the20 dB upper stopband is widened from 0.71 to 3.82 GHzand the second spurious resonance frequency can besuppressed completely.

4 | RESULTS AND DISCUSSION

In Table 1, we report the optimal layout dimensions ofthe proposed GAF DBPF with an overall size of21.9 mm × 22.5 mm. Figure 6 shows the simulatedelectric field distributions of the GAF DBPF at differentfrequencies of 2.14, 2.68, and 3.50 GHz, respectively.

TABLE 1 Parameters of proposed GAF DBPF

Parameters Value (mm) Parameters Value (mm)

W1 0.627 S2 7.35

L1 10.851 L5 22.5

L2 4.512 S1 0.87

W2 2.3 g 0.49

L3 3.1 w 2.425

L4 3.04 l 7.8

W3 2.3 T 3.84

Abbreviations: DBPF, dual-wideband bandpass filter; GAF, grapheneassembled films.

FIGURE 6 The simulated electric field distributions of the graphene assembled films (GAF) dual-wideband band-pass filter (DBPF) at

2.14, 2.68, and 3.5 GHz from left to right

BANGQI ET AL. 5 of 8

In the passbands at both 2.14 and 3.5 GHz, the signalscan pass through the filter with very little loss indicatedby the strong electric field intensity. In contrast, the sig-nal at the stopband of 2.68 GHz is suppressed in thetransmission line. These results demonstrate significantselection efficiency of signal propagation in the designedstopband and passbands.To verify the filtering efficiencyof the proposed GAF DBPF, we connect the fabricated fil-ter in the size comparable to a coin (Figure 7A) to AgilentN5247A vector network analyzer and obtain the |S11| and|S21|.The measured results (Figure 7B) show that the twopassbands center at 2.17 and 3.52 GHz with 3 dB FBWsof 15.19% and 12.72%. The passbands have measuredinsertion losses of 1.73 and 2.06 dB and return losses of21.82 and 26.19 dB at the center frequency and spuriousfrequency, respectively, which are in good agreementwith the simulation results. Four TZs at 1.68, 2.67, 3.93,and 4.69 GHz can be observed outside the two passbandsthat improve band-to-band isolation and out-of-bandrejection level. Through clever structural design, the TZgenerated by the resonator is used to suppress its ownparasitic resonance, so that the out-of-band suppressionlevel of the designed GAF DBPF is greatly improved. Thedesigned GAF DBPF has 21 and 24.18 dB attenuationlevel for the second spurious resonance frequency of thetwo passband center frequencies, from 3.85 to 7.52 GHz.

5 | CONCLUSIONS

A compact 5G GAF DBPF in the size of a coin consistingof two FOLSIRs is investigated and verified through

simulation and measurement. The experimental resultsshow that the GAF DBPF has a good filtering effect sincethe TZs on both sides of the passband increase not onlythe isolation between the passbands but also the level ofout-of-band rejection. The graphene-assembled films canserve as a 5G DBPF with excellent performance, whichhave the great advantages of small in-band insertion loss,large return loss, and good rectangular coefficient.Besides, graphene owns many unique properties like lowprofile, less weight and low cost, and mechanical stabil-ity. With such advantages, the reported GAF DBPF canbe considered as an excellent candidate for 5G wirelesscommunication systems, especially where low cost, lightweight and easy manufacturing are required.

CONFLICT OF INTERESTThe authors declare no potential conflict of interest.

DATA AVAILABILITY STATEMENTData available on request from the authors.

ORCIDDaping He https://orcid.org/0000-0002-0284-4990

REFERENCES1. Makimoto M, Yamashita S. Bandpass filters using parallel

coupled Stripline stepped impedance resonators. IEEE TransMicrow Theory Tech. 1980;28(12):1413-1417.

2. Sagawa M, Makimoto M, Yamashita S. Geometrical structuresand fundamental characteristics of microwave stepped-impedance resonators. IEEE Trans Microw Theory Tech. 1997;45(7):1078-1085.

FIGURE 7 A, Photograph of the graphene assembled films (GAF) dual-wideband band-pass filter (DBPF); B, Simulated (dotted line)

and measured (solid line) jS11, S21 j responses of the GAF DBPF

6 of 8 BANGQI ET AL.

3. Li X, Zhang Y. Dual band bandpass filter using meander splitloop resonator. Microw Opt Technol Lett. 2017;59(10):2490-2493.

4. Hong JS, Shaman H, Chun YH. Dual-mode microstrip open-loop resonators and filters. IEEE Trans Microw Theory Tech.2007;55(8):1764-1770.

5. Zhang YP, Sun M. Dual-band microstrip bandpass filterusing stepped—impedance resonators with new couplingschemes. IEEE Trans Microw Theory Tech. 2006;54(10):3779-3785.

6. Tsai L-C, Hsue C-W. Dual-band bandpass filters using equal-length-coupled-serial-shunted lines and Z-transform technique.IEEE Trans Microw Theory Tech. 2004;52(4):1111-1117.

7. Ren B, Ma Z, Liu H. Miniature dual-band bandpass filter usingmodified quarter-wavelength SIRs with controllable passbands.IEEE Microw Wirel Compon Lett. 2008;18(2):88-90.

8. Wang J, Guan Y, Yu H. Transparent graphene microstrip fil-ters for wireless communications. J Phys D Appl Phys. 2017;50(34).

9. Maosheng C, Xixi W, Min Z. Electromagnetic response andenergy conversion for functions and devices in low-dimensionalmaterials. Adv Funct Mater. 2019;29(25):1807398.1-1807398.54.

10. Yao Y, Cheng X, Qu SW, Yu J, Chen X. Graphene-metal basedtunable band-pass filters in the terahertz band. IET MicrowAntennas Propag. 2016;10(14):1570-1575.

11. Ghahremani A, Moradi G. Planar tunable graphene based low-pass filter in the terahertz band. Appl Optics. 2018;57(27):7823-7829.

12. Su W, Chen B. Graphene-based tunable terahertz filter withrectangular ring resonator containing double narrow gaps.Pramana-J Phys. 2017;89(3):1-5.

13. Shen B, Zhai W, Zheng W. Ultrathin flexible graphene film: anexcellent thermal conducting material with efficient EMIshielding. Adv Funct Mater. 2014;24(28):4542-4548.

14. Dogan S, Knopf GK, Nikumb S. Cyclic liquid-phase exfoliationof electrically conductive graphene-derivative inks. IEEE TransMicrow Theory Tech. 2018;17(5):1020-1028.

15. Sinar D, Knopf GK. Printed graphene derivative circuits as pas-sive electrical filters. Nanomaterials. 2018;8(2):123.

16. Song R, Wang Q, Mao B. Flexible graphite films with high con-ductivity for radio-frequency antennas. Carbon. 2018;130:164-169.

17. Zhou W, Liu C, Song R. Flexible radiofrequency filters basedon highly conductive graphene assembly films. Appl Phys Lett.2019;114(11):113503.1-113503.5.

18. Zhou W, Liu C, Huang GL. Design and manufacture of lowpassmicrostrip filter with high conductivity graphene films. MicrowOpt Technol Lett. 2019;61(4):972-978.

19. Song R, Huang GL, Liu C. High-conductive graphene filmbased antenna array for 5G mobile communications. Int J RFMicrow Comput Eng. 2019;29(6):1-8.

20. Cai Y, Da Xu K, Guo R, Zhu J, Liu QH. Graphene-basedPlasmonic tunable dual-band bandstop filter in the far-infraredregion. IEEE Photonics J. 2018;10(6):1-1.

21. Goldflam MD, Ruiz I, Howell SW. Tunable dual-bandgraphene-based infrared reflectance filter. Opt Express. 2018;26(7):8532-8541.

22. SaDon SNH, Kamarudin MR, Ahmad F. Graphene arrayantenna for 5G applications. Appl Phy A. 2017;123(10):118-125.

AUTHOR BIOGRAPHIES

Bangqi Huang was born in Hubei,China. She received the Bachelor of Sci-ence degree from Center China NormalUniversity, Wuhan, China in 2017. Cur-rently, she is pursuing the Master of Sci-ence degree in physics at the HubeiEngineering Research Center of RF-

Microwave Technology and Application, School of Sci-ence, Wuhan University of Technology, Wuhan, China.Her research interests include conductive graphene filmsand. Transparent electromagnetic shielding materials.

Siting Li received the B.S. degree inElectronic Information Science andTechnology from Wuhan Universityof Technology in 2018. Currently, sheis pursuing the Master of Sciencedegree in physics at the Hubei Engi-neering Research Center of RF-

Microwave Technology and Application, School ofScience, Wuhan University of Technology, Wuhan,China. Her research interests include conductivegraphene films and near-field communication (NFC).

Rongguo Song was born in Shan-dong, China. He received Master ofScience degree in School of Sciencefrom Wuhan university of Technol-ogy, Wuhan, China in 2018. He iscurrently pursuing PhD degree in theHubei Engineering Research Center

of RF-Microwave Technology and Application,Wuhan University of Technology, Wuhan, China. Hisresearch interests include graphene based materials,RF and microwave devices design.

Zhanyong Hou was born in Henan,China. He received the Bachelor ofScience degree in Electronic Informa-tion Science and Technology andMaster of Science degree from theWuhan University of Technology,Wuhan, China, in 2017. His research

interests include conductive graphene films, RF andmicrowave devices design.

Chengguo Liu was born in Henan,China. He received the Bachelor ofScience degree from Henan Univer-sity, Master of Science degree fromSichuan University and PhD degreefrom Xidian University. He is the

BANGQI ET AL. 7 of 8

deputy director of Hubei EngineeringResearch Technology and Application, Wuhan Uni-versity of Technology, Wuhan, China. His researchinterests include electromagnetic field theory, anten-nas, and radiowave propagation.

Daping He is a full professor atWuhan University of Technology. Heobtained his PhD degree in MaterialsProcessing Engineering from WuhanUniversity of Technology in 2013. Hewas a Postdoctoral Fellow in the Uni-versity of Science and Technology of

China. Then he joined University of Bath as a NewtonInternational Fellow and University of Cambridge as

a Postdoctoral Fellow. His research interest is prepa-ration and application of nano composite materialsinto new energy devices, sensors, and RF microwavesfield. He has published over 70 peer-reviewed papersand 5 Chinese patents.

How to cite this article: Bangqi H, Li S, Song R,Hou Z, Liu C, He D. High-conductivity graphene-assembled film-based bandpass filter for 5Gapplications. Int J RF Microw Comput Aided Eng.2021;e22602. https://doi.org/10.1002/mmce.22602

8 of 8 BANGQI ET AL.