mance and sufficient gain at operating frequencies have beenobtained. Therefore, the proposed antenna shows a great potentialfor dual-band communication applications.

REFERENCES

1. W.C. Liu, Compact microstrip-line-fed ring monopole antenna withtuning strip for 5 GHz WLAN operation, Electron Lett 41 (2004),831–832.

2. Y.L. Kuo and K.L. Wong, Printed double-T monopole antenna for2.4/5.2 GHz dual-band WLAN operations, IEEE Trans AntennasPropag 51 (2003), 2187–2219.

3. C.Y. Pan, C.H. Huang, and T.S. Horng, A new printed G-shapedmonopole antenna for dualband WLAN applications, Microwave OptTechnol Lett 45 (2005), 295–297.

4. S. Kim, J. Choi, and Y. Kim, CPW-feed broadband G-shaped monopoleantenna for dualband WLAN applications, Microwave Opt Technol Lett48 (2006), 2310–2311.

5. L.N. Zhang, S.S. Zhong, X.L. Liang, and C.H. Li, Compact meanderedmonopole antenna for tri-band WLAN application, Microwave OptTechnol Lett 49 (2007), 986–988.

6. J.Y. Jan and L.C. Tseng, Small planar monopole antenna with a shortedparasitic inverted-L wire for wireless communications in the 2.4-, 5.2-,and 5.8-GHz bands, IEEE Trans Antennas Propag 52 (2004), 1903–1905.

7. J.H. Yoon, Fabrication and measurement of rectangular ring withopened CPW-fed monopole antenna for 2.4/5.2-GHz WLAN operation,Microwave Opt Technol Lett 48 (2006), 1480–1483.

© 2009 Wiley Periodicals, Inc.

A COMPACT ULTRA-WIDEBAND BAND-PASS FILTER WITH DEFECTEDGROUND STRUCTURE

Peng Chen, Feng Wei, Xiaowei Shi, and Qiulin HuangNational Key Laboratory of Antennas and Microwave Technology,Xidian University, Xi’an 710071, People’s Republic of China;Corresponding author: chenpeng9113@163.com

Received 15 August 2008

ABSTRACT: In this article, a compact ultra-wideband (UWB) band-pass filter (BPF) with improved upper-stopband is presented . In thisdesign, a novel multiple-mode resonator (MMR) with two symmetricalloading stubs and three dumbbell-shaped defected ground structures(DGS) on the back of the filter improve the harmonic-suppressioncharacteristic of conventional BPF. Combining these two structures,a new UWB BPF is fabricated and measured. Measured results showthat the proposed BPF has wide bandwidth from 3.0 to 10.7 GHz,and the insertion loss of the BPF is less than 1.0 dB in this band. Awide stop-band bandwidth with 20 dB attenuation from 12 GHz up to20 GHz is achieved. © 2009 Wiley Periodicals, Inc. Microwave OptTechnol Lett 51: 979 –981, 2009; Published online in Wiley Inter-Science (www.interscience.wiley.com). DOI 10.1002/mop.24234

Key words: ultra-wideband; band-pass filter; defected ground structure;multiple-mode resonator

1. INTRODUCTION

Ultra-wideband (UWB) radio technology has received much at-tention since the release of the commercial use of UWB technol-ogy in 2002. A compact UWB band-pass filter (BPF) is a keypassive component in the system. As a UWB BPF, the frequencybandwidth of the filter must cover from 3.1 to 10.6 GHz, asauthorized by the Federal Communications Commission (FCC) for

commercial applications [1]. A number of works have been madeso far to make up a variety of planar UWB BPF, in which, theMMR-based UWB BPF is initiated in [2]. Recently, a series ofMMR-based UWB BPF have been studied [2–4]. This design hasthe advantages of compact structure, small size, and excellentcharacteristic in the pass-band, but the attenuation in the stop-bandis dissatisfactory.

In this article, a new design of UWB BPF is presented, and twosymmetrical loading stubs are added to the MMR (Figure 1). Thisdesign not only improves the harmonic-suppression characteristicin high-frequency stop-band but also reduces the size of MMR.Combining this structure, three dumbbell-shaped DGS units areetched on the back of the filter. The DGS have good low-passcharacteristics [5–7]. With these designs, the harmonic-suppres-

Figure 1 Configuration of the proposed filter

Figure 2 Configuration and simulated S-parameters of the proposedstructure for various r (radius of the symmetrical loading stubs)

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 4, April 2009 979

sion characteristic in the high-frequency stop-band is excellent.The figure of the proposed UWB filter is as follows.

2. PARAMETRIC ANALYSIS

There are many parameters that need to be addressed in this filterstructure because of the number of the elements and coupling gaps.Through simulation, this article has given insight on how thesecritical parameters affect the characteristic of harmonic-suppres-sion in high-frequency stop-band. These parameters are variedaround the optimized values.

A. Variation of the radius of the symmetrical loading stubs,B. Variation of the radius of the DGS, andC. Variation of the width of the gap between the DGS.

In this article, a section of microstrip with 50-� characteristicimpedance was simulated. The substrate is RT/Duorid 5880 withthe thickness of 1.0 mm and the dielectric constant of 2.2. From thesimulated results, the stop-band characteristics of the symmetricalloading stubs and DGS in high frequency are in evidence as shownin Figures 2–4. The smaller r (the radius of the symmetricalloading stubs and DGS) corresponds to the higher frequencyharmonic-suppression characteristic, and the larger r correspondsto the lower frequency harmonic-suppression characteristic. Andthe width of the gap between the DGS has effects on the harmonic-suppression characteristic. A wide band-stop with better perfor-mance can be obtained by adjusting the radiuses and gap precisely.

3. PROPOSED UWB FILTER

In this article, the proposed UWB BPF has two innovations incontrast to the conventional MMR BPF structure filter. The firstone is two symmetrical loading stubs on the MMR, and the secondis three dumbbell-shaped DGS units etched on the back of thefilter. The advantages of these structures are that by adjusting theradiuses of the dumbbell-shaped, including the stub and the DGS,the characteristics of the pass-band and stop-band can be con-trolled easily. Each dumbbell-shaped structure only has two vari-ables (the radius and the width of gap between the DGS), andadjusting the variables of these two structures can obtain excellentharmonic-suppression characteristic of the filter. Remarkably, withthe symmetrical loading stubs to the MMR, the size of the MMRcan be reduced greatly when compared with the conventionalMMR BPF (�1/2 wavelength).

The symmetrical loading stubs and DGS have simple equiva-lent circuit models and low-pass property with wide stop-band.Many research activities have been performed to apply it to theLPF designs [6–9]. The equivalent circuit models of the DGS andsymmetrical loading stubs are shown in Figure 5. From the equiv-alent circuit models, it is noted that both of them provided atten-uation poles within the high-frequency stop-band of the filter. Byadjusting the resonant frequencies properly, the proposed structure

Figure 3 Configuration and simulated S-parameters of the proposedstructure for various r (radius of the DGS)

Figure 4 Simulated S-parameters of the proposed structure for various g(width of gap)

Figure 5 Equivalent circuit models of the symmetrical loading stubs andDGS

980 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 4, April 2009 DOI 10.1002/mop

is shown to be able to suppress the harmonic responses efficientlyand provide the deep and wide high-frequency stop-band.

The optimum values of the design parameters are as follows:Lmmr � 10 mm, Wmmr � 3 mm, W1 � 1 mm, W2 � 0.8 mm, Lr

� 8.6 mm, g1 � 0.4 mm, g2 � 0.2 mm, g � 0.2 mm, r1 � 0.8 mm,r2 � 1.2 mm, r3 � 1.4 mm, W50 � 3 mm, the substrate isRT/Duroid 5880 (�r � 2.2) with thickness of 1.0 mm. The pho-tograph of filter is as Figure 6. The simulated and measured returnloss and insertion loss are shown in Figure 7. Measured resultsshow that the proposed BPF has wide bandwidth from 3.0 to 10.7GHz, and the insertion loss of the BPF is less than 1.0 dB in thisband. A wide stop-band bandwidth with 20 dB attenuation from 12GHz up to 20 GHz is achieved.

4. CONCLUSION

In this article, a novel MMR BPF with DGS has been presented. Byadding two symmetrical loading stubs to the MMR and etching threedumbbell-shaped DGS units on the back of the filter, the BPF not onlyhas the excellent characteristic of harmonic-suppression in high fre-quency but also reduce the size of the filter. Both the symmetricalloading stubs and DGS are semicircle, and this design makes the

variable reduce to only two. Excellent agreement between the simu-lated and measured results have been obtained, these structures aresimple and flexible for the purpose of harmonic-suppression.

REFERENCES

1. Federal Communications Commission: Revision of part 15 of the com-mission’s rules regarding ultra-wideband transmission systems, FirstReport and Order, FCC 02 (48), 2002.

2. L. Zhu, S. Sun, and W. Menzel, Ultra-wideband (UWB) bandpass filtersusing multiple-mode resonator, IEEE Microwave Wireless ComponLett 15 (2005), 796–798.

3. J.-S. Hong and H. Shaman, An optimum ultra-wideband microstripfilter, Microwave Opt Technol Lett 47 (2005), 230–233.

4. C.-L. Hsu, F.-C. Hsu, and J.-T. Kuo, Microstrip bandpass filters forultra-wideband (UWB) wireless communications, IEEE MTT-S Int Dig12 (2005), 679–682.

5. C. Kim, J. Park, D. Ahn, and J. Lim, A novel 1-D periodic defectedground structure for planar circuits, IEEE Microwave Guide Wave Lett10 (2000), 131–133.

6. D. Ahn, J. Park, C. Kim, J. Kim, Y. Qian, and T. Itoh, A design of thelow-pass filter using the novel microstrip defected ground structure,IEEE Trans Microwave Theory Tech 49 (2001), 86–92.

7. J. Lim, C. Kim, D. Ahn, Y. Jeong, and S. Nam, Design of low-passfilters using defected ground structure, IEEE Trans Microwave TheoryTech 53 (2005), 2539–2545.

8. A.B.A.-Rahman, A.K. Verma, A. Boutejdar, and A.S. Omar, Control ofbandstop response of hi-lo microstrip low-pass filter using slot inground plane, IEEE Trans Microwave Theory Tech 52 (2004), 1008–1013.

9. H.-J. Chen, T.-H. Huang, C.-S. Chang, L.-S. Chen, N.-F. Wang, Y.-H.Wang, and M.-P. Houng, A novel cross-shaped DGS applied to designultra-wide stopband low-pass filters, IEEE Microwave Wireless Com-pon Lett 16 (2006), 252–254.

© 2009 Wiley Periodicals, Inc.

COMPACT MICROSTRIP HARMONIC-SUPPRESSED QUADRATURE HYBRIDS

Li-Yen Wang, Kewen Hsu, and Wen-Hua TuDepartment of Electrical Engineering, National Central University,Jhongli, Taoyuan 32001, Taiwan; Corresponding author:whtu@ee.ncu.edu.tw

Received 23 July 2008

ABSTRACT: In this article, compact 3-dB branch-line hybrids withharmonic suppression are presented. In comparison to the conventionalhybrids, the proposed ones achieve two features: compactness and har-monic suppression. Two hybrids, one with third harmonic suppressionand the other with third and fifth harmonics suppression, are designed,fabricated, and measured. T-shaped or cross-shaped equivalent lines areused to replace the quarter-wavelength microstrip lines of the conven-tional hybrids. The equivalent lines achieve the same fundamental char-acteristic and act like bandstop filters at the particular harmonics toimprove suppression. Moreover, the proposed hybrid with third har-monic suppression achieves 29.5% size reduction as compared to theconventional one. © 2009 Wiley Periodicals, Inc. Microwave OptTechnol Lett 51: 981–985, 2009; Published online in Wiley InterScience(www.interscience.wiley.com). DOI 10.1002/mop.24221

Key words: harmonic suppression; quadrature hybrid; microstriphybrid

1. INTRODUCTION

Quadrature hybrid is widely used in RF/microwave circuits, suchas balanced amplifiers and array antenna feeding networks. Since

Figure 6 Photograph of the proposed filter

Figure 7 Simulated and measured S-parameters of the proposed UWBBPF

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 4, April 2009 981