IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 61, NO. 1, JANUARY 2013 125

Single-Layered SIW Post-Loaded ElectricCoupling-Enhanced Structureand Its Filter Applications

Chang Jiang You, Student Member, IEEE, Zhi Ning Chen, Fellow, IEEE,Xiao Wei Zhu, Member, IEEE, and Ke Gong, Student Member, IEEE

Abstract—A broadband post-loaded electric coupling structurein single-layered substrate integrated waveguide with enhancedcoupling is proposed and investigated. The proposed structure canachieve the maximum electric coupling coefficient of more than0.11. The effects of dimensional parameters on the electric cou-pling are studied by simulation. The proposed structure is veri-fied by designing two bandpass filters, namely, a wideband fourth-order quasi-elliptic filter and a narrowband fourth-order quasi-el-liptic filter. The measured results show that the designs are ca-pable of providing the 12% bandwidth for the wideband fourth-order quasi-elliptic filter and the 3% bandwidth for the narrow-band fourth-order quasi-elliptic filter at 5.8-GHz WiMax bands.

Index Terms—Bandpass filter, coupling matrix, electric cou-pling, substrate integrated waveguide (SIW), WiMax.

I. INTRODUCTION

S UBSTRATE integrated waveguide (SIW) has advantagesin the design of planar circuits, such as high quality factor

and low loss [1]–[10]. Compared to a multilayered SIW, thesingle-layered SIW has the advantages of low cost and easy fab-rication so that it is frequently used to design filters, antennas,and other devices [11]. On the other hand, with the demand fornarrower channels, quasi-elliptic filters with high out-of-bandrejection are required. Both electric coupling and magnetic cou-pling are usually used in the designs of quasi-elliptic filters withfinite transmission zeros. It is more convenient to introduce themagnetic coupling in the single-layered SIW by employing amagnetic post-wall window than the electric coupling.

Manuscript received May 11, 2012; revised November 07, 2012; acceptedNovember 09, 2012. Date of publication December 13, 2012; date of currentversion January 17, 2013. This work was supported in part by the Natural Sci-ence Foundation of China (NSFC) under Grant 60921063 and the NationalHigh-Tech Project under Grant 2011AA01A105.C. J. You was with the State Key Laboratory of Millimeter Waves, South-

east University, Nanjing 210096, China. He is now with the Greating-UESTCJoint Experiment Engineering Center, School of Communication and Informa-tion Engineering, University of Electronic Science and Technology of China,Chengdu 611731, China (e-mail: cjyou@uestc.edu.cn).Z. N. Chen is with the Department of Electrical and Computer Engineering,

National University of Singapore, Singapore, and also with the Institute for In-focomm Research, Agency for Science, Technology and Research (A*STAR),Singapore (e-mail: eleczn@nus.edu.sg; chenzn@i2r.a-star.edu.sg).X. W. Zhu and K. Gong are with the State Key Laboratory of Mil-

limeter Waves, Southeast University, Nanjing 210096, China (e-mail:xwzhu@seu.edu.cn; kegong@emfield.org).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/TMTT.2012.2228667

Fig. 1. Proposed electric coupling structure in the single-layered SIW.

The electric coupling structures in the SIW have been re-ported [12]–[15]. However, the electric coupling structure inthe single-layered SIW with the coupling coefficient of morethan 0.075 has not been found in any open literatures, and themaximum 3-dB bandwidth of quasi-elliptic filters based on theelectric coupling structures in the single-layered SIW has beenreached 10.5% [16]–[21]. Such electric coupling structures inthe single-layered SIW with strong electric coupling cannotmeet the requirement of a 3-dB bandwidth of more than 11%of quasi-elliptic filters.In this paper, we propose an electric coupling structure in a

single-layered SIW to achieve a high coupling coefficient and awide coupling strength range. As an example, the structure withthe strong electric coupling and wide electric coupling strengthrange is applied in the designs of a wideband fourth-order quasi-elliptic filter and a narrowband fourth-order quasi-elliptic filter.

II. ANALYSIS OF COUPLING STRUCTURE

The proposed single-layered SIW electric coupling structureis implemented by etching an H-shaped slot on the top surfaceof two adjacent rectangular cavities and introducing two postsas loading, as shown in Fig. 1. The two cavities are formed bycentrally separating a bigger SIW cavity by a row of via-holes.The H-shaped slot is symmetrically cut between two cavitiesand the loading posts symmetrically located in the H-shapedportion.The parameters of the structure are shown in Fig. 1. The

and denote the width and length of the two same horizontalslots, respectively, while the and represent the widthand length of the vertical slot, respectively. The and standfor the diameter of the loading post and the horizontal gap of

0018-9480/$31.00 © 2012 IEEE

126 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 61, NO. 1, JANUARY 2013

Fig. 2. (a) -field of the first resonant mode in the proposed structure.(b) -field of the first resonant mode in a conventional magnetic couplingstructure with a magnetic post-wall window.

the loading post from the center, respectively. The gap betweenthe window post and the horizontal slot is 0.2 mm. The is thediameter of via-hole used to form the electric wall of the SIWand fixed as 0.5 mm. The and denote thehorizontal and vertical dimensions of the two identical rectan-gular SIW cavities, respectively.The -field of the first resonant mode in the proposed electric

coupling structure is 180 out-phase for the two adjacent cavi-ties while the -field of the first resonantmode in a conventionalmagnetic coupling structure with a magnetic post-wall windowis in phase in the two adjacent cavities, as shown in Fig. 2. The-field of the second resonant mode in the proposed electric

coupling structure is in phase in the two adjacent cavities whilethe -field of the second resonant mode in a conventional mag-netic coupling structure with a magnetic post-wall window is180 out-phase for the two adjacent cavities, as shown in Fig. 3.Therefore, the proposed structure is capable of offering the elec-tric coupling.As described in [22], the coupling coefficient can be calcu-

lated using the following formula:

(1)

where denotes the resonant frequency when the symmetricalplane, namely, the A–A plane in Fig. 2, is set as a magneticboundary and is the resonant frequency when the symmet-rical plane is set as an electric boundary. Figs. 2(a) and 3(a),respectively, show the -field of the resonant modes when thesymmetrical plane of proposed structure is an electric and amagnetic boundary, respectively. The resonant frequency of the

Fig. 3. (a) -field of the second resonant mode in the proposed structure.(b) -field of the second resonant mode in a conventional magnetic couplingstructure with a magnetic post-wall window.

former is lower than that of the latter. The conditions ofand , respectively, stand for the electric coupling and mag-netic coupling.All the parameters in the proposed electric coupling structure,

as shown in Fig. 1, have the influence on the strength of elec-tric coupling. For instance, the proposed electric coupling struc-ture fabricated on the Rogers 4350 substrate with a thickness of

mm, loss tangent of 0.004, and a relative dielectricconstant of is designed and investigated by eigen-mode simulation. The simulation is completed by using AnsoftHFSS. The initial values of parameters in the simulation are asfollows: , , , ,

, , , and (allin millimeters).Fig. 4 shows the electric coupling coefficient of two rectan-

gular SIW cavities with the proposed electric coupling struc-ture against and . It has been shown that the electriccoupling coefficient decreases rapidly as the gradient reduces ifthe increases from 0.1 to 0.5 mm. It is also found that theelectric coupling coefficient increases slowly as the gradient be-comes small if the increases from 0.1 to 0.5 mm.Fig. 5 illustrates the electric coupling coefficient of two rect-

angular SIW cavities with the proposed electric coupling struc-ture against and . It has been observed that the electriccoupling coefficient increases rapidly as the gradient decreasesif variable increases from 3.0 to 13.0mm. It is also seen thatthe electric coupling coefficient increases slightly as the gra-dient becomes small if increases from 3.0 to 13.0 mm.Fig. 6 shows the electric coupling coefficient of two rectan-

gular SIW cavities with the proposed electric coupling structure

YOU et al.: SINGLE-LAYERED SIW POST-LOADED ELECTRIC COUPLING-ENHANCED STRUCTURE 127

Fig. 4. Electric coupling coefficient of two rectangular single-layered SIW cav-ities with the proposed electric coupling structure against and .

Fig. 5. Electric coupling coefficient of two rectangular single-layered SIW cav-ities with the proposed electric coupling structure against and .

Fig. 6. Electric coupling coefficient of two rectangular single-layered SIW cav-ities with the proposed electric coupling structure against and .

against and . It shows that the electric coupling coefficientfirst increases rapidly and then decreases as increases from 2to 9 mm. The inflexion point is located in the range of 5–6 mm.As increases from 0.2 to 1.2 mm, the electric coupling coef-ficient increases slightly.Therefore, it is concluded that the and greatly

affect the electric coupling coefficient.

Fig. 7. Topology of the fourth-order wideband quasi-elliptic bandpass filter.Numbers 1–4 denote four resonators, while S and L represent source and load,respectively.

III. WIDEBAND FILTER

To apply the strong electric coupling of proposed electriccoupling structure in filter design, a fourth-order quasi-ellipticbandpass filter working at 5-GHz WiMax bands with a 3-dBbandwidth of 700 MHz and a 1-dB bandwidth of 600 MHz isdesigned. The topology of the filter is shown in Fig. 7. The1-dB passband is designed as 5.2–5.8 GHz and two transmis-sion zeros are designed at 5.0 and 6.0 GHz, respectively. Themaximum return loss is assumed to be 15 dB. According to [23],the initial values of coupling coefficients and the external factorfor the filter are as follows:

(2)

where the positive sign represents the electric coupling and thenegative sign the magnetic coupling. Due to the 1-dB relativebandwidth more than 10.5%, the calculated coupling coeffi-cients are approximate initial values and further optimizationfor filter is necessary. The magnetic coupling is implemented byemploying a magnetic post-wall window and its coupling coef-ficient is also calculated by using (1), but with a negative sign.The configuration of the fourth-order wideband quasi-el-

liptic bandpass filter is based on the Rogers 4350 sub-strate with a thickness of mm and a relativedielectric constant of , as shown in Fig. 8.The dimensions of the optimized filter are as follows:

, ,,

(all in millimeters).The fourth-order wideband quasi-elliptic bandpass filter was

fabricated. The simulated and measured -parameters of thefilter are shown in Fig. 9. The filter was measured when itwas enclosed in a metal box, which was specially designedaccording to the simulation model. The measured results showthat the 3-dB passband ranges from 5.142 to 5.803 GHz, ora bandwidth of 12.1%, and the 1-dB passband from 5.206 to5.732 GHz, or a bandwidth of 9.63%. The measured minimuminsertion loss in the passband is 1.68 dB, which is 0.44 dBhigher than the simulated one. The measured transmission zeroin the lower edge is located at 4.995 GHz and the measuredtransmission zero in the upper edge is located at 6.162 GHz.The comparisons with existing quasi-elliptic bandpass filtersemploying the single-layered SIW electric coupling structuresare shown in Table I. It is found that the designed filter has a

128 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 61, NO. 1, JANUARY 2013

Fig. 8. Configuration of the fourth-order wideband quasi-elliptic bandpassfilter.

Fig. 9. Measured and simulated -parameters of the fourth-order widebandquasi-elliptic filter.

TABLE ICOMPARISONS WITH REPORTED FILTERS EMPLOYING THE

SINGLE-LAYERED SIW ELECTRIC COUPLING STRUCTURES

wider 3-dB bandwidth and electric coupling coefficient of 0.08,which is stronger than existing solutions.

IV. NARROWBAND FILTER

A fourth-order narrowband quasi-elliptic bandpass filterworking at the 5-GHz WiMax bands with a 3-dB bandwidth of

Fig. 10. Topology of the fourth-order narrowband quasi-elliptic bandpass filter.Numbers 1–4 denote four resonators, while S and L represent source and load,respectively.

Fig. 11. Configuration of the fourth-order narrowband quasi-elliptic bandpassfilter.

150 MHz and a 1-dB bandwidth of 100 MHz is designed usingthe wide electric coupling range of the proposed electric cou-pling structure in the single-layered SIW, as shown in Fig. 10.The 1-dB passband is set as 5.75–5.85 GHz and two trans-mission zeros are designed at 5.6 and 6.0 GHz, respectively.The maximum return loss in band is assumed to be 18 dB.According to [23], the initial values of coupling coefficientsand the external factor for the filter are as follows:

(3)

where, again, the positive sign denotes the electric coupling andnegative sign the magnetic coupling. The magnetic coupling isimplemented by employing the magnetic post-wall window andthe electric coupling with coupling coefficient of less than 0.001is achieved by using the proposed electric coupling structure.The configuration of the fourth-order narrowband

quasi-elliptic bandpass filter is fabricated on the Rogers4350 substrate with a thickness mm andrelative dielectric constant , as shown inFig. 11. The dimensions of the optimized filter are:

,,

, and(all in millimeters).

YOU et al.: SINGLE-LAYERED SIW POST-LOADED ELECTRIC COUPLING-ENHANCED STRUCTURE 129

Fig. 12. Measured and simulated -parameters of the fourth-order narrowbandquasi-elliptic filter.

The simulated and measured -parameters of the filter areshown in Fig. 12. The filter was measured when it was enclosedin a specific metal box, which was designed according to thesimulation model. The measured 3-dB passband ranges from5.733 to 5.879 GHz, or a bandwidth of 2.51% and the mea-sured 1-dB passband from 5.764 to 5.853 GHz, or a bandwidthof 1.53%. The measured minimum insertion loss in the bandis 6.35 dB, 1.55 dB higher than the simulated one. The inser-tion-loss discrepancies between the measurement and simula-tion may be due to the permittivity fluctuation and the fabrica-tion tolerance, both of which affect the coupling coefficient andresonant frequency. Moreover, in both lower and upper stop-band, suppression differences between the measurement andsimulation could be caused by the electric coupling strength de-viation generated by fabrication tolerance.

V. CONCLUSION

An electric coupling structure in the single-layered SIW withenhanced electric coupling has been presented. A parametricstudy has been carried out. The structure has been implementedin the single-layered SIW and shown the advantages of low-costand easy fabrication. With the achieved wider electric couplingrange, the wideband quasi-elliptic bandpass filter with a 3-dBbandwidth of more than 12% and the narrowband quasi-ellipticbandpass filter with a 3-dB bandwidth of less than 3% have beenimplemented and measured.

ACKNOWLEDGMENT

The authors wish to thank Dr. J. Shi, Institute for InfocommResearch, Agency for Science, Technology and Research(A*STAR), Singapore, for his valuable comments. The authorsalso wish to thank J. D. Wang, Department of Electronic En-gineering and Information Science, University of Science andTechnology of China, Hefei, China, for her enthusiastic help.

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130 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 61, NO. 1, JANUARY 2013

Chang Jiang You (S’09) was born in SichuanProvince, China, in 1983. He received the B.S.degree from the University of Electronic Scienceand Technology of China (UESTC), Chengdu,China, in 2006, and the Ph.D. degree from SoutheastUniversity, Nanjing, China, in 2012.From April 2011 to October 2011, he was a Re-

searcher with the Institute for Infocomm Research,Agency for Science, Technology and Research(A*STAR), Singapore. He is currently a Lecturerwith the Greating–UESTC Joint Experiment En-

gineering Center, School of Communication and Information Engineering,UESTC. His research interests include RF/microwave passive circuits, RF/mi-crowave systems, and millimeter-wave circuits and systems.

Zhi Ning Chen (M’99–SM’05–F’08) received theB.Eng., M.Eng., and Ph.D. degrees in electrical en-gineering from the Institute of Communications En-gineering (ICE), China, and the Ph.D. degree fromthe University of Tsukuba, Ibaraki, Japan.From 1988 to 1995, he was with ICE, as a Lec-

turer, and later an Associate Professor, as well aswith Southeast University, Nanjing, China, as a Post-doctoral Fellow and then as an Associate Professor.From 1995 to 1997, he was with the City Universityof Hong Kong, as a Research Assistant and then as a

Research Fellow. In 2001 and 2004, he visited the University of Tsukuba, undera JSPS Fellowship Program (senior level). In 2004, he was with the IBM T. J.Watson Research Center, as an Academic Visitor. From 1999 to 2012, he waswith the Institute for Infocomm Research (I2R) (formerly known as the Centrefor Wireless Communications and Institute for Communications Research),as Member of Technical Staff (MTS), Senior MTS, Principal MTS, SeniorScientist, Lead Scientist, and Principal Scientist, as well as Head of the RF andOptical Department. Since 2012, he has been with the Department of Electricaland Computer Engineering, National University of Singapore, Singapore, as aFull Professor. He concurrently holds a joint appointment as Advisor with I2R,as well as being a Visiting/Adjunct/Guest Professor with Southeast University,Nanjing University, Shanghai Jiaotong University, Tsinghua University, TongjiUniversity, University of Science and Technology, Dalian Maritime University,Chiba University, National Taiwan University of Science and Technology,and the City University of Hong Kong. He has authored or coauthored over400 technical papers. He authored/edited Broadband Planar Antennas, UWBWireless Communication, Antennas for Portable Devices, and Antennasfor Base Stations in Wireless Communications. He contributed chapters toUWB Antennas and Propagation for Communications, Radar, and Imaging,Antenna Engineering Handbook, and Microstrip and Printed Antennas. Heholds 27 granted and filed patents with 31 licensed deals with industry. Hiscurrent research interest includes electromagnetic engineering, antennas forcommunication, radar, and imaging and sensing systems.Dr. Chen serves the IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION

as an associate editor. He served the IEEE Antennas and Propagation Society

(IEEE AP-S) as a Distinguished Lecturer (2009–2011). He was the foundinggeneral chair of the International Workshop on Antenna Technology (iWAT),International Symposium on InfoComm and Media Technology in Bio-Medicaland Healthcare Applications (IS 3T-in-3A), International Microwave Forum(IMWF), and the Asia–Pacific Conference on Antennas and Propagation(APCAP). He was the recipient of the International Symposium on Antennasand Propagation Best Paper Award 2010, the CST University PublicationAward 2008, the IEEE AP-S Honorable Mention Student Paper Contest 2008,the IES Prestigious Engineering Achievement Award 2006, the I2R QuarterlyBest Paper Award 2004, and the IEEE iWAT 2005 Best Poster Award.

Xiao-Wei Zhu (S’88–M’95) was born in Nanjing,Jiangsu Province, China, in 1963. He received theB.E., M.E., and Ph.D. degrees in radio engineeringfrom Southeast University, Nanjing, China, in 1984,1996, and 2000, respectively. Since 1984, he has beenwith Southeast University, Nanjing, China.He is currently a Professor with the School of

Information Science and Engineering, SoutheastUniversity. He has authored or coauthored over 80technical publications. He holds ten patents. Hisresearch interests include RF and antenna technolo-

gies for wireless communications, as well as microwave and millimeter-wavetheory and technology. He is also interested in power amplifier nonlinearcharacter and its linearization research with a particular emphasis on widebandand high-efficiency GaN PAs.Dr. Zhu was the recipient of the 1994 First Class Science and Technology

Progress Prize presented by the Ministry of Education of China and the 2003Second Class Science and Technology Progress Prize of Jiangsu Province,China.

Ke Gong (S’09) was born in Henan Province,China, in November, 1977. He received the B.S.degree in physics from Xinyang Normal University,Xinyang, China, in 2000, and the M.S. degree fromthe School of Electronic Science and Engineering,Southeast University, Nanjing, China, in 2005, andis currently working toward the Ph.D. degree atSoutheast University, Nanjing, China.Since 2000, he has been with Xinyang Normal

University. In 2006, he became a Lecturer. He iscurrently with the State Laboratory of Millimeter

Waves, Southeast University. From May to November 2010, he was with theInstitute for Infocomm Research (I2R), Agency for Science, Technology andResearch (A*STAR), as a Research Engineer, and from April to September2011, as a Research Fellow. His current research interests include microwaveand millimeter-wave components, circuits, and systems.

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