DESIGN OF DUAL-BAND FILTER USINGDEFECTED SPLIT-RING RESONATORCOMBINED WITH INTERDIGITALCAPACITOR

B. Wu, J. W. Fan, L. P. Zhao, and C. H. LiangSchool of Electronic Engineering, Xidian University Xi’an, Shaanxi710071, People’s Republic of China

Received 9 February 2007

ABSTRACT: A novel type of dual-band microstrip structure and itsequivalent circuit are investigated, which is composed of defected split-ring resonator and interdigital capacitor. The influence of design pa-rameters on the frequency characteristics is discussed, then a dual-bandmicrostrip filter, which has two passbands of 1.8–2.5 GHz and 5.5–5.9GHz with a transmission zero, is designed and fabricated. © 2007Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 2104–2106,2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22673

Key words: defected split-ring resonator (DSRR); interdigital capacitor(IC); dual-band filter; equivalent circuit

1. INTRODUCTION

Microwave filters play important roles in many RF applications. Inrecent years, dual-band filters have been proposed and investigatedas a key circuit component in dual-band wireless communicationsystems [1–3]. In addition, Split-ring resonators (SRRs) have beensuccessfully applied to the fabrication of left-handed metamaterial(LHM) and the design of planar circuits. The dependence of theresonant frequency of the periodic array of SRRs on the ringthickness, inner diameter, radial and azimuthal gap, as well as onthe permittivity of the board, and the embedding medium is alsoinvestigated [4]. The complementary circular SRRs have beenapplied to the design of compact narrow microstrip band-passstructures [5]. The authors have investigated the equivalent circuitand transmission characteristic of a defected SRR for lowpass filterdesign [6].

In this article, the defected SRR with interdigital capacitor(IC-DSRR) are studied and applied to the design of dual-band

filter. The equivalent circuit of this structure is deduced. After anumerical analysis of the variations of the transmission character-istic with the IC-DSRR size, a compact dual-band microstrip filterwith a transmission zero is designed effectively.

2. STRUCTURE AND CIRCUIT ANALYSIS OF IC-DSRRCELL

As shown in Figure 1, the presented IC-DSRR cell is obtained byetching two concentric split-ring defective patterns which havedifferent size and inverse split direction under the interdigital gapin the ground plane. The permittivity of the microstrip line is �r �2.65, the height of the dielectric board is h � 1 mm, and width ofthe conductor line is 4 mm. For simplicity, the length of therectangular split-ring is fixed as a � 10 mm, both the width of ringand the distance between two rings kept at a constant of 1 mm.

The defected split-ring forms a parallel resonator, which has anelectrical coupling with the conductor line, considering the distrib-uted inductance and gap capacitance, the total equivalent circuit isillustrated in Figure 2. This structure has a transmission zerobecause of the series-resonant parallel branch, which leads to ahigh suppression at the band-edge. The transmission zero locationof the IC-DSRR cell is determined by the resonant frequency ofthe parallel resonant circuit, that means it is codetermined by L1,C1, and C3. When the impedance of the parallel branch equalszero, we have the resonant frequency as

fS �1

2��L1�C1 � C3�(1)

According to the equivalent circuit, the admittance of paralleltank and the impedance of series tank can be deduced as follows

Y �

� �2C1C2 �C2

L1

j��C1 � C2� �1

j�L1

� j�C2 �1 � ��/�1�

2

1 � ��/�s�2 (2)

Z � 2� 1

j�C2� j�L2� � j2

��/�2�2 � 1

�C2 (3)

with

Figure 1 A three-dimensional view of the proposed IC-DSRR cellmodel on the substrate with dielectric constant �r � 2.65 and thickness h �1 mm (a � L � s � 10 mm, w � 4 mm, g � c � 1 mm). [Color figurecan be viewed in the online issue, which is available at www.interscience.wiley.com]

Figure 2 Equationuivalent circuit of the IC-DSRR cell

2104 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 49, No. 9, September 2007 DOI 10.1002/mop

�1 �1

�L1C1

, �2 �1

�L2C2

, and �s �1

�L1�C1 � C3�.

3. NUMERICAL ANALYSIS OF IC-DSRR

There is a corresponding relationship between the dimension of theIC-DSRR structure and its frequency characteristic. As shown inFigure 3, while the cell number increase from one to three, thepassband of dual-band filter is widen, and the suppression in theout-of-band is improved evidently. But too many cells will in-crease the complexity of the filter, and also widen the passbandarea that sometimes we do not need. For our purpose, two-cellIC-DSRR structures (Fig. 4) with different dimension are analyzedand simulated by EM simulator Ansoft HFSS v9.

As illuminated in Figure 5, there is only one parameter varyingfor each case. The resonant frequency is decreased when side-length b increases; if the periodic length d goes up, the firstpassband is narrowed while the second passband is widen; theinterdigital gap s should be carefully chosen to achieve a smoothpassband and suitable out-of-band suppression at low frequencyrange.

4. MEASUREMENT RESULTS

To validate the numerical simulation results, a dual-band filtermade up of IC-DSRR structure is fabricated by etching patterns on

Figure 3 Transmission curves with different cell number, the resonantfrequencies fS, f1, and f2 can be determined by the parameter extractionmethod as introduced in Ref. 7

Figure 4 Structure of two-cell IC-DSRR, the other parameters have thesame values as the proposed IC-DSRR cell model

Figure 6 Photographs of the fabricated IC-DSRR dual-band filter.(a)Top view and (b) bottom view. [Color figure can be viewed in the onlineissue, which is available at www.interscience.wiley.com]

Figure 5 Transmission curves with different (a) side-length b, (b) peri-odic length d, (c) interdigital gap s

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 49, No. 9, September 2007 2105

RF printed circuit board. The substrate has a permittivity of 2.65and a height of 1 mm, the other parameters are chosen as follows:a � 10 mm, b � 17 mm, s � 0.6 mm, and periodic length d � 20mm. Figure 6 shows the photographs of the fabricated IC-DSRRdual-band filter. As depicted in Figure 7, this filter has two pass-bands of 1.8–2.5 GHz and 5.5–5.9 GHz. The insertion loss is nomore than 2 dB at the first passband and about 3.5 dB at the secondpassband. A high suppression of �45 dB is obtained at thetransmission zero located around 1.5 GHz, and the attenuation inthe gap between the two passbands is nearly �20 dB, which hassatisfied our purpose, one can get high suppression by increasingcell number.

5. CONCLUSION

A novel dual-band filter composed of defected SRRs and inter-digital capacitors is presented in this article. Based on the equiv-alent circuit analysis of the cell model, the dependence of thetransmission characteristic on the IC-DSRR size is investigated bynumerical simulation technology, then a compact dual-band mi-crostrip filter with a transmission zero around 1.5 GHz is opti-mized and fabricated efficiently.

ACKNOWLEDGMENT

This work is supported by the NSFC under project no. 60501023.

REFERENCES

1. L.C. Tsai and C.W. Hsue, Dual-band bandpass filters using equal-lengthcoupled-serial-shunted lines and Z-transform techniques, IEEE TransMicrowave Theory Tech 52 (2004), 1111–1117.

2. S.F. Chang, Y.H. Jeng, and J.L. Chen, Dual-band step-impedancebandpass filter for multimode wireless LANs, Electron Lett 40 (2004),38–39.

3. J.T. Kuo and H.S. Cheng, Design of quasielliptic function filters with adual-passband response, IEEE Microwave Compon Lett 14 (2004),472–474.

4. P. Markos, C.M. Soukoulis, Numerical studies of left-handed materialsand arrays of split ring resonators, Phys Rev E 65 (2002), 036622–1–036622–8.

5. J. Bonache, F. Martin, F. Falcone, et al, Application of complementarysplit-ring resonators to the design of compact narrow band-pass struc-tures in microstrip technology, Microwave Opt Technol Lett 46 (2005),508–512.

6. B. Wu, B. Li, and C.-H. Liang, Design of lowpass filter using a novel

split-ring resonator defected ground structure, Microwave Opt TechnolLett 49 (2007).

7. B. Wu, T. Su, B. Li, and C.-H. Liang, Design of tubular filter based oncurve-fitting method, J Electromagn Waves Appl 2 (2006), 1071–1080.

© 2007 Wiley Periodicals, Inc.

PRINTABLE YAGI ANTENNA WITHCLOSELY SPACED ELEMENTS

Sungkyun Lim1 and Hao Ling2

1 Hawaii Center for Advanced Communications, College ofEngineering, University of Hawaii at Manoa, Honolulu, HI 968222 Department of Electrical and Computer Engineering, The Universityof Texas at Austin, Austin, TX 78712

Received 16 February 2007

ABSTRACT: A two-element, planar Yagi antenna with closely spacedelements is reported. Multiple folding in the driver is used to boost upthe low radiation resistance due to the close spacing between the driverand the director. The antenna dimensions are first optimized in wireform and then transferred into a printed version on PET film. A proto-type planar monopole Yagi is built and measured at 1 GHz. The maxi-mum realized gain of the antenna on 50-�m PET film is measured to be9.56 dB. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett49: 2106–2109, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22660

Key words: planar antennas; folded antennas; Yagi antennas

1. INTRODUCTION

The traditional Yagi antenna is an elegant design that achievesgood gain in a very simple structure. The antenna comprises adriven element and parasitic elements including a reflector and oneor more directors. The spacing between the elements is typicallyon the order of 0.15–0.4 � [1, 2]. In some applications, it isdesirable to decrease the inter-element spacing in order to reducethe overall size of the structure. However, while the directivity ofthe antenna can be maintained when the element spacing is de-creased, the radiation resistance of the antenna drops precipitously,leading to low antenna efficiency and poor matching. In a previousstudy, we introduced a Yagi antenna with closely spaced elementsfor HF skywave applications [3]. The optimized design has goodrealized gain performance despite an inter-element spacing of only0.02 �. There have also been several related works recently on thesubject of arrays with closely spaced driven elements [4, 5].

In this study, we investigate a planar realization of the closelyspaced Yagi antenna. The concept of multiple folding on the driverelement is used to step up the radiation resistance. The antennadimensions are first optimized using a genetic algorithm (GA) inconjunction with the numerical electromagnetics code (NEC)based on a wire implementation of the antenna. The wire versionof the optimized antenna is then transferred into a printed versionon PET film. Prototype structures are constructed and measured.The resulting structure is very simple, and is potentially useful asa high-gain printed antenna in such application as RFID.

2. ANETNNA DESIGN

A planar, wire version of a two-element, closely spaced, dipoleYagi antenna is first investigated. Since the radiation resistancedrops drastically when the driven element and the parasitic ele-ment are located close to one another, multiple folding is used inthe driver to step up the radiation resistance to 50 �. Here two

Figure 7 Comparison between the simulation and measurement results,the slight difference is because of fabricated precision

2106 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 49, No. 9, September 2007 DOI 10.1002/mop