insertion losses are �2.08 (2.41 GHz) and �2.25 dB (5.23 GHz),respectively. The insertion loss difference between simulated andmeasured results is thought to be the circuit loss (the sum ofconductor loss, dielectric loss, and radiation loss), fabricationdefect, and unperfected SMA feeding.

4. CONCLUSION

In this study, the low environmental pollution process of printmethod is used to fabricate a miniature microstrip square-ringdual-band bandpass filter. The properties of coupling coefficienthave been listed and the proposed filter has been designed andfabricated on the 1-mm thickness Al2O3 ceramic substrate byprinting method. The fabricated filter has 3.73% (90 MHz) band-width at 2.41 GHz and 4.02% (210 MHz) bandwidth at 5.23 GHz.The measured minimum insertion losses are �2.08 and �2.25 dB,respectively. The proposed filter has the advantages of small size,simple structure, tiny insertion loss, adequate bandwidth, accurateoperating frequencies, good out-of-band rejection, and low envi-ronmental pollution. It can be used for the 2.4/5.2 GHz WLAN(IEEE-802.11 a/b/g).

ACKNOWLEDGMENTS

The authors acknowledge the financial support of the NationalScience Council of the Republic of China (NSC 96-2262-E-390-005-CC3).

REFERENCES

1. C.L. Huang and C.L. Pan, Dual-band multilayer ceramic microwavebandpass filter for applications in wireless communication, MicrowaveOpt Technol Lett 32 (2002), 327–329.

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

3. J. Lee, M.S. Uhm, and I.B. Yom, A dual-passband filter of canonicalstructure for satellite applications, IEEE Microwave Wireless ComponLett 14 (2004), 271–273.

4. Y.X. Guo, L.C. Ong, M.Y.W. Chia, and B. Luo, Dual-band bandpassfilter in LTCC, IEEE MTT-S Int Microwave Symp Dig, Long Beach,CA (2005).

5. 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.

6. J.X. Chen, T.Y. Yum, J.L. Li, and Q. Xue, Dual-mode dual-band

bandpass filter using stacked-loop structure, IEEE Microwave WirelessCompon Lett 16 (2006), 502–504.

7. J.S. Hong and M.J. Lancaster, Microstrip filters for RF/microwaveapplications, Wiley, New York, 2001.

8. K.C. Gupta, R. Garg, I. Bahl, and P. Bhartis, Microstrip lines andslotlines, 2nd ed., Artech House, Boston, 1996.

© 2008 Wiley Periodicals, Inc.

A COMPACT WIDEBAND MICROSTRIPBANDPASS FILTER USING PARALLEL-COUPLED-LINES WITH PERIODICALGROOVES

Lin Li and Zheng-Fan LiDepartment of Electronic Engineering, Shanghai Jiao Tong University,No. 800 Dongchuan Road, Shanghai 200240, China; Correspondingauthor: lilin_door@hotmail.com

Received 15 June 2008

ABSTRACT: A wideband bandpass filter using parallel-coupled-lineswith periodical grooves is presented in this article. Owing to the slowwave effect brought by periodical grooves, the proposed structure’s res-onant frequency can be lowered effectively. This effect has been used inthis article to realize wideband filter with compact size. Based on thistheory, a two-pole compact wideband bandpass filter using proposedstructures is designed and fabricated as a sample. The simulated andmeasured results show the validity of the proposed structure. © 2008 WileyPeriodicals, Inc. Microwave Opt Technol Lett 51: 518–520, 2009;Published online in Wiley InterScience (www.interscience.wiley.com).DOI 10.1002/mop.24077

Key words: wideband bandpass filter; parallel-coupled-lines; periodicalgrooves; slow wave effect

1. INTRODUCTION

The development of ultra-wideband (UWB) technologies for com-mercial communication applications has created a demand ofwideband filters [1]. The microstrip parallel-coupled-line filter [2,3] has been widely used in microwave communication systems.And due to the microstrip parallel-coupled-line filter’s simplesynthesis procedure, good repetition, and a wider realizable band-width, it has also gained much attention in wideband filters design[4]. However, the whole length of this type of filter is too long,especially when it is applied in the low frequency and the order ofa filter becomes high, which has limited seriously the applicationof microstrip parallel-coupled-line filters in wideband filter design.

In this article, a topology of parallel-coupled-lines with peri-odical grooves is proposed to realize wideband filters. Though thisstructure has been used in [5, 6] to suppress harmonics in narrow-band filters design, however, to our knowledge, it has not beenadopted in wideband filters design. And it will be demonstrated inthis article that this structure can help to reduce the filter’s sizeowing to the periodical grooves’ slow wave effect. To verify itseffectiveness, an experimental filter has been optimally designed,fabricated, and tested. The measured results show good agreementwith the simulated ones.

2. FILTER THEORY

Figures 1(a) and 1(b) present the topologies of a conventionalmicrostrip line and a microstrip line with periodical grooves,respectively. For a microstrip line with periodical grooves, accord-ing to the theory of periodical structures, the phase velocity will be

1 2 3 4 5 6

-80

-60

-40

-20

0

SimulatedMeasured

Frequency (GHz)

S2

1 M

ag

nit

ud

e (d

B)

Figure 6 The simulated and measured results of the proposed square-ring filter

518 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 2, February 2009 DOI 10.1002/mop

lowered. Owing to this slow wave effect brought by periodicalgrooves, the resonant frequency of the microstrip line periodicalgrooves will be reduced in comparison to the conventional micros-trip lines. This can be explained by equivalent circuit analysis. Theequivalent circuit of the microstrip line with periodical grooves isconsidered as a cascade network composed of multiple uniformelementary sections demonstrated in Figure2. The section consistsof a microstrip line with a capacitor tapped at the centre, where thecapacitor C1 represents the earth capacitance brought by thegroove. The ABCD matrix of the section can be got as below [7]:

�A BC D� � � cos

�

2jZ01sin

�

2

jY01sin�

2cos

�

2�� 1 0

j�C1 1�

� � cos�

2jZ01sin

�

2

jY01sin�

2cos

�

2�

� � cos� �Z01�C1

2sin� j�Z01sin� � Z01

2�C1

2�cos� � 1��

j�Y01sin� ��C1

2�cos� � 1�� cos� �

Z01�C1

2sin� �

(1)

Then the section’s equivalent electrical length can be written as:

�unit � arccos�cos� �Z01�C1

2sin�� (2)

It can be concluded from the above relation that �unit � � whencos� � 0.5Z01�C1sin�. In other words, when the periodicalgrooves are introduced, the microstrip line’s resonant frequency isreduced. This is a clear indication of the slow-wave phenomenon.And from (2), we deduce that the unit’s equivalent electrical lengthis reduced when C1 increase. The resonant frequcency of theproposed microstrip line versus the length of the tooth tc is dem-onstrated in Figure 3 to illustrates this phenomenon because tcexerts a significant influence on the value of C1. The line’s dimen-sions are shown as below: W1 � 0.5 mm, Wg � 0.75 mm, Wd �0.35 mm. All the related data in the figure are achieved from thesimulation results using Agilent ADS2005. As shown in Figure 3,a longer tooth results in a lower resonant frequency, and thus leadsto a more compact filter. And when tc comes to 2 mm, the resonantfrequency even drops to 54% with respect to the resonance fre-quency of the conventional coupled lines. Obviously, when theproposed parallel-coupled-lines with periodical grooves are usedto replace the conventional coupled lines, the filter’s size will bereduced effectively.

3. RESULTS AND DISCUSSION

Based on the above analysis, a two-pole microstrip widebandbandpass filter is designed and fabricated. The filter is fabricatedon a 1.5-mm-thick dielectric substrate with a relative dielectricconstant of 2.65. The filter’s topology is shown in Figure 4. It iscomposed of three identical parallel-coupled-line sections shown

Figure 1 The topologies of a conventional microstrip line (a) and amicrostrip line with periodical grooves (b)

Figure 2 A elementary section

Figure 3 The resonant frequency of the proposed coupled-line versus theteeth length

Figure 4 The two-pole wideband bandpass filter’s topology

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 2, February 2009 519

in Figure 5. The correlative parameters of all the three parallel-coupled-line sections’ correlative parameters are arranged as below:W1 � 0.45 mm, Wg � 0.75 mm, Wd � 0.35 mm, tc � 0.65 mm, g11

� 0.17 mm, g11 � 0.2 mm. Such a parallel-coupled-line structureresonates at around 3.4 GHz, whereas the conventional parallel-coupled-line with the same length resonates at about 4.15 GHz. Thistwo simulated results verify the existence of slow wave effect.

Figure 6 shows the measured results for the proposed filter,together with the EM simulation results. The simulated and mea-sured results display a good agreement. The slight frequency shiftis attributed to unexpected tolerance in fabrication and materialparameter. The filter has a center frequency of 3.22 GHz. Mea-sured data show that its passband is from 2.2 to 4.24 GHz, whichindicates its relevant fractional bandwidth is about 63%. Theminimum insertion loss in the passband is 0.64 dB, whereas thesimulated one is lower than 0.04 dB. This insertion loss would bemainly due to the effect of the input/output subminiature A con-nectors, and to the dielectric loss of the substrate as well as theresistance loss of the metal strip.

In addition, two transmission zeros are observed at around 5.04GHz and 5.63 GHz, respectively, which can provide the attenua-tion over 50 dB at all the two zeros. The transmission zeros besidethe right skirt can provide a sharper transition from the passband tothe upper stopband. And the whole length of the filter is about 40mm, which is about 80% with respect to the conventional parallel-coupled-line filters.

4. CONCLUSION

A microstrip wideband filter using parallel-coupled-lines with pe-riodical grooves is presented in this letter. A two-pole widebandbandpass filter with proposed structure is constructed for valida-tion. The simple configuration, compact size, and easy fabrication,

together with sharp rejection responses at the right skirt, make thisfilter useful for many microwave-system applications.

REFERENCES

1. Y.C. Yoon and R. Kohno, Optimum multi-user detection in ultra-wideband (UWB) multiple-access communication systems, Proc IEEEInt Conf Commun 2 (2002), 812–816.

2. C.-Y. Chang and T. Itoh, A modified parallel-coupled filter structurethat improves the upper stopband rejection and response symmetry,IEEE Trans Microwave Theory Tech 39 (1991), 310–314.

3. S.B. Cohn, Parallel-coupled transmission-line-resonator filters, IEEETrans Microwave Theory Tech 6 (1958), 223–231.

4. J.-T. Kuo and E. Shih, Wideband bandpass filter design with three linemicrostrip structures, Proc Inst Elect Eng 149 (2002), 243–247.

5. B.S. Kim, J.W. Lee, and M.S. Song, An implementation of harmonic-suppression microstrip filters with periodic grooves, IEEE MicrowaveWireless Compon Lett 14 (2004), 413–415.

6. J.T. Kuo and M.H. Wu, Corrugated parallel-coupled-line bandpassfilters with multispurious suppression, IET Microwaves AntennasPropag 1 (2007), 718–722.

7. D.M. Pozar, Microwave Engineering, 2nd ed., Wiley, NewYork, 1998.

© 2008 Wiley Periodicals, Inc.

DESIGN AND IMPLEMENTATION OFPLANAR ULTRA-WIDEBAND ANTENNASCHARACTERIZED BY MULTIPLENOTCHED BANDS

Yuan Dan Dong, Wei Hong, Jian Yi Zhou, and Zhen Qi KuaiState Key Labaratory of Millimeter Waves, School of InformationScience and Engineering, Southeast University, Nanjing 210096,People’s Republic of China; Corresponding author:yddong@emfield.org

Received 15 June 2008

ABSTRACT: A new approach for the design of planar ultra-wideband(UWB) antennas having multiple band-notched functions is studied andpresented. In this approach, a planar dual-stopband filter based on adual-mode resonator is first investigated carefully, which employs ahalf-mode substrate-integrated waveguide cavity to improve the quality(Q) factor. Then, this filter is applied to the design of a UWB antennawith dual band-notched characteristics. Finally, two triple band-notchedantennas are implemented by incorporating two different resonators inthe radiation element of the UWB antenna, respectively. Voltage stand-ing wave ratio (VSWR), gain, and radiation pattern are measured. Highand sharp band-rejection characteristic for each of the frequency notchas well as a very wide impedance bandwidth is achieved. © 2008 WileyPeriodicals, Inc. Microwave Opt Technol Lett 51: 520–526, 2009;Published online in Wiley InterScience (www.interscience.wiley.com).DOI 10.1002/mop.24076

Key words: multiple notched bands; half mode substrate-integratedwaveguide; UWB antenna

1. INTRODUCTION

Ultra-wideband (UWB) short-range wireless access technologieshave drawn increased attention in recent years because of theirmerits such as high data rate, small emission power, and low cost.However, the vast broad bandwidth in the operation of UWBsystems leaves them vulnerable to whatever interferences fromnarrowband sources that might exist in their environment, such asWiMAX systems, wideband local area network (WLAN), blue-tooth, etc. In order to avoid the potential interferences, it is

Figure 5 The topology of the proposed parallel-coupled-line section

Figure 6 The measured and EM simulation results for the proposed filter

520 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 2, February 2009 DOI 10.1002/mop