For WiMAX application, the optimal parameters of the pro-posed antenna are numerically obtained, as shown in Figure 1, Themiddle branch has a length 26.7 mm (L1) and a width 11 mm (W1),The left branch has a length 19.8 mm (L2) and a width 2.5 mm(W2), The dimension of right branch is the same as that of leftbranch, which is for symmetry radiation pattern.

The measured and simulated radiation patterns of the proposedantenna at some typical operation frequencies are also investi-gated. Figure 3(a) plots the measured and simulated radiationpatterns at 2.5 GHz; the results at 3.5 GHz are shown in Figure3(b); the results at 5.5 GHz are also shown in Figure 3(c).

At 3.1 GHz suppress frequency, the radiation patterns of theproposed antenna degenerates, which caused by adding two nar-row slot strips in the proposed antenna. Figure 4 shows themeasured peak antenna gain. It is observed that the strong gainreduction over the rejected band is obviously shown in Figure 4.Therefore, the proposed design can significantly reduce the effectof couple and interference between the low band and the middleband.

3. CONCLUSION

A novel CPW-fed WiMAX monopole antenna with the triple-bandperformance has been experimentally discussed. This article hasfocused on the band-rejected design to fit WiMAX operation. Theband-rejected function, by etched a pair of slits on printed dual-band monopole antenna, to rejected frequency from 2.9 to 3.2GHz. Especially, the low band approaching the middle band pro-duced the large effect of couple or interference, which needs theuse of band-reject function to suppress. This antenna can providelow cost, because, the broadband antenna and the dual-band an-tenna must use filter to suppress dispensable bands lead to increasecost. The low cost, easy manufacture, good impedance match,good radiation patterns of the proposed antenna are presented inthis article.

REFERENCES

1. IEEE 802.16 Working group on broadband wireless access standards,http://grouper.ieee.org/groups/802/16/index.html

2. Worldwide interoperability for microwave access forum or WiMAXforum, http://www.wimaxforum.org

3. W.C. Liu and C.C. Huang, A CPW-fed L-shaped slot planar monopole

antenna for triple band operations, Microwave Opt Technol Lett 44(2005), 510-512.

4. K.L. Wong, W.C. Su, and F.S. Chang, Wideband internal folded planarmonopole antenna for UMTS/WiMAX folder-type mobile phone, Mi-crowave Opt Technol Lett 48 (2006), 324-327.

5. Y.T. Liu and K.L. Wong, A wideband stubby monopole antenna and aGPS antenna for WiMAX mobile phones with E911 function, Micro-wave Opt Technol Lett 46 (2005), 485-487.

© 2007 Wiley Periodicals, Inc.

WIDEBAND MICROSTRIP BANDPASSFILTER BASED ON INTERCOUPLEDSPLIT-RING RESONATOR

Jack Wu,1 S. N. Qiu,2 C. X. Qiu,2 and I. Shih1

1 Department of Electrical and Computer Engineering, McGillUniversity, Montreal, Quebec H3A 2T5 Canada2 CIS Scientific, Montreal, Quebec J4Z 1W6 Canada

Received 18 January 2007

ABSTRACT: A new design of wideband microwave bandpass filterbased on intercoupled split-ring resonator is proposed. The split-ringresonator structure is well-know for its unique resonance feature thatexhibit a negative permeability. By implementing the split-ring resonatordirectly on a microstrip transmission line, a bandpass filter with a largefractional bandwidth has been achieved. The novel periodic intercoupledsplit-ring resonator bandpass filter is compatible with microwave planartechnology and the passband region is controllable. The filter designhas been validated and demonstrated in both simulations and experi-mental measurements. A bandpass filter with a fractional bandwidth of50% at a center frequency of 14 GHz was achieved with a maximuminsertion loss of 0.75 dB in the passband region. Meanwhile, the frac-tional bandwidth has been demonstrated to be controllable and this wasreduced to 37% by sacrificing the periodicity of the intercoupled split-ring resonator structure. Therefore, the flexible characteristics andthe simple implementation method may render the proposed filter tobe very useful in many frequency-selective and oscillation-suppres-sion applications. © 2007 Wiley Periodicals, Inc. Microwave OptTechnol Lett 49: 1809 –1813, 2007; Published online in Wiley Inter-Science (www.interscience.wiley.com). DOI 10.1002/mop.22588

Key words: bandpass filter; microstrip transmission line; split-ring res-onator

1. INTRODUCTION

Bandpass filters are one of the most common and yet essentialbuilding blocks in today’s communication system. Many micro-wave components and devices depend on filters to achieve thefrequency-selective or harmonic-suppression applications [1–3].Conventionally, the bandpass filters are constructed with parallel-coupled microstrip lines, which utilize half-wavelength line reso-nators [4, 5]. However, recent development in commercial com-munication systems makes broad bandwidth transmission at higherfrequencies desirable. Therefore, the drawback of narrow band-width of parallel-coupled filters has to be addressed. The structureswith improved fractional bandwidth have been proposed but thesestructures utilize short circuit stub as inductors and shunt metalpads as capacitors, which require via and increase the overalldevice area [6, 7].

Split-ring resonator structure was first proposed by Pendry et al.[8] to achieve a large imaginary component in effective perme-ability due to its unique resonance nature. Smith and coworkers [9,10] further investigated the split-ring resonator structure together

Figure 4 Measured antenna gain for proposed antenna (band-rejected).Antenna parameters are the same as Figures 2(b) and 2(c)

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 49, No. 8, August 2007 1809

with metallic wires placed in proximity. Such combined structureexhibits bandpass characteristics with backward wave propagationand it is known as the left-handed medium. This realizes thestructure proposed by Veselago in 1968 [11], which has a negativepermeability and a negative permittivity simultaneously. Soonafter, Split-ring resonator and complementary split-ring resonatorstructures have been exploited in many different applications in-cluding microwave filters [12–17]. However, most of these struc-tures have the resonators implemented in the ground plane orbeside the parallel coupled microstrip to enhance the filteringperformance. Planar edge-coupled split-ring resonator was pro-posed as bandpass filter but the fractional bandwidth still remainssmall [18].

In this work, intercoupled split-ring resonator structure (Fig. 1)is utilized and it is implemented directly on the microstrip line torealize a bandpass filter at microwave frequencies. The proposedstructure not only exhibits a special property of having a widepassband width (fractional bandwidth � 50%) but also it is verysimple to design and implement. Therefore, the proposed structurecan be a suitable candidate for microwave filters employed inmicrostrip planar technology. First of all, the design methodologyfor the intercoupled split-ring resonator structure and its equivalentcircuit are presented. Experimental and simulation results of twoproposed structures with different passband frequencies are then

presented, in order to verify the design. At the end, the findings ofthis work are summarized.

2. DESIGN METHODOLOGY

The starting point of designing a microwave filter is to select alowpass prototype network with normalized element values. Then,by performing frequency and element transformation, the lowpassprototype can be realized with lumped network, which has thedesired frequency band performance. A lowpass prototype with aladder network structure and its transformation to a bandpass filterutilizing the impedance inverters (K-inverters) are shown inFigure 2.

The K-inverter is a two-port network that has a phase shift of90° and it can be realized with a quarter-wavelength microstriptransmission line [19]. Thus, one way to comprehend this inpractical microwave filter design is to utilize the intercoupledsplit-ring resonator structures.

The proposed bandpass filter is consisted of half-wavelengthresonators that are intercoupled together to form cascaded split-ring structures. Therefore, the input and the output ports can bealigned, which is an advantage when compared with the conven-tional parallel-coupled microstrip filters where there is an offsetbetween the ports. Figure 3 shows a single intercoupled split-ringresonator and its equivalent circuit. The half-wavelength resonatoris represented by the black region. The dashed line regions are partof the adjacent resonators.

LR and CR are the series inductance and capacitance of thehalf-wavelength resonator. Cps are the parasitic capacitances to theground. Since the width of the split-ring structure is quite small,the effect of the parasitic is small and hence they are neglected inthe following analysis. If w of the resonator shown in Figure 3 isselected to be the same as the microstrip line width, then thecharacteristic impedance of the line will remain near 50 �. There-fore, the bandpass filter network shown in Figure 2 can be approx-imated with the intercoupled split-ring resonators and the values ofLs and Cs can be calculated with the basic element transformationequations given by

Figure 1 Intercoupled split-ring resonator (SRR) structure implementedon microstrip line to realize a bandpass filter

Figure 2 (a) A lowpass prototype with ladder network structure and (b)its generalized bandpass transformation [19]

Figure 3 (a) An intercoupled split-ring resonator and (b) its equivalentcircuit

TABLE 1 Substrate and Microstrip Line Specifications

Substrate Corning 1059 (SiO2)

Dielectric constant (�r) 3.9Substrate thickness (h) 0.37 mmMicrostrip line width (w) 0.80 mm

1810 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 49, No. 8, August 2007 DOI 10.1002/mop

LR � Ls �Z0

X � �0gi; CR � Cs �

1

�02Ls

(1)

The fractional bandwidth (X) of the bandpass filter is given by

X ��2 � �1

�0; �0 � ��1�2 (2)

where �1 and �2 indicate the passband-edge angular frequencyand Z0 is the characteristic impedance of 50 �. Therefore, toobtain the element values (gi) for the periodically intercoupledsplit-ring resonators, an order-five bandpass filter prototype withinsertion loss of 3.01 dB at the cutoff frequency of 1 rad/s ischosen. Also, the element values are forced to be the same sincethe resonators are periodically identical. Therefore, g is obtained tobe 0.995. Then, the equivalent passband inductor and capacitorvalues can be calculated to provide the required filter response.The dimensions of intercoupled split-ring resonators are simulatedwith Agilent Advanced Design System’s Momentum full-wavesimulation to obtain the matching inductor and capacitor values.Thus, a microwave bandpass filter with a desired fractional band-width at cutoff frequencies of �1 and �2 can be designed andimplemented easily. In the next section, two cases of proposedstructure are designed and fabricated. The first case demonstratesthe control of passband frequency with periodically identical in-tercoupled split-ring resonators. The second case shows the controlof passband width by sacrificing the periodicity of the resonators.The simulation results are compared against the experimentallymeasured results to validate the proposed design methodology.

3. SIMULATION AND MEASUREMENTS RESULTS

Table 1 shows the specifications of the substrate and microstripline utilized to simulate and experimentally implement the inter-coupled split-ring resonators.

3.1. Case 1: The Control of Passband FrequencySince strong couplings are desirable, every half-wavelength reso-nators are intercoupled with adjacent resonators to form split-ringstructures along half of their length. Therefore, the quarter-wave-length coupling length (a) that consisted of split-ring resonatordetermines the center frequency (f0) of the passband region.

a ��g

4; �g �

c

f0��eff

(3)

where �g is the guided wavelength, c is the speed of light, and �eff

is the effective permittivity of the medium. To demonstrate thecontrollability of the center frequency of the passband region, threeintercoupled split-ring resonators with a different coupling lengthare simulated. Meanwhile, the target fractional bandwidth (X) isfixed to be 65%. Their corresponding simulation values and di-mensions are shown in Table II.

Figure 4 shows the S21 of the simulated intercoupled split-ringresonator structures.

Therefore, it can be seen from Figure 4 that the passband regionof the intercoupled split-ring resonator structure is very easy tocontrol. The fractional bandwidths are 64, 65, and 66% for aequals to 3.1, 3.6, and 4.33 mm, respectively. Also, the insertionlosses in the passband region are around 1.2 dB. Stopband atten-uations are all higher than 30 dB. Figure 5 shows the correspond-ing S11 responses.

The intercoupled split-ring resonator with a equals to 3.1 mmwas experimentally fabricated and the S-parameters were mea-

TABLE 2 Simulation and Design Parameters for Case 1

f0 (GHz) a (mm) Ls (nH) Cs (pF) X g

10 4.33 1.22 0.2165% 0.99512 3.60 1.02 0.17

14 3.10 0.87 0.15

Figure 4 S21 of intercoupled split-ring resonator structures with differentpassband regions

Figure 5 S11 of intercoupled split-ring resonator structures with differentpassband regions

Figure 6 Comparison between the simulated (S) and measured (M)S-parameters for the case with a � 3.1 mm

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 49, No. 8, August 2007 1811

sured with Anritsu 37347A Vector Network Analyzer to compareagainst the simulated ones. Figure 6 illustrates the comparisonbetween the simulated and measured S-parameters.

It is observed from Figure 6 that the f0 of the measured sampleis located at 13.9 GHz, which is very close to the simulation result.In addition, the measured sample exhibits a passband region from10.52 to 17.31 GHz, which corresponds to a fractional bandwidthof 50.3%. The maximum in-band insertion loss is 0.75 dB for themeasured sample. Therefore, this experiment confirms the feasi-bility of utilizing the intercoupled split-ring resonator structure inmicrowave bandpass filter applications.

3.2. Case 2: The Control of Passband WidthTo control the fractional bandwidth of the intercoupled split-ringresonator structure, transfer functions such as Butterworth, Che-byshev, or elliptic responses used for lowpass prototype filters canbe considered. In this section, an order-five Butterworth prototypewith a fractional bandwidth (X) of 40% centered at 12 GHz is usedto demonstrate the implementation of bandwidth-controlled inter-coupled split-ring resonator structure. Table III summarizes thedesign parameters used to realize the modified resonator structure.

Figure 7 illustrates the topology of the microstrip line contain-ing the intercoupled split-ring resonators, which has nonperiodiccells with the spacing (s) and resonator width (w) that correspondto an order-five Butterworth prototype.

Figure 8 shows the S-parameter results obtained from thesimulation and the experimental measurement.

It is seen from Figure 8 that f0 for both simulated and measuredresults lie very close to each other at 12 GHz. Also from themeasured S21, the 3 dB band-edge is from 9.85 to 14.30 GHz,which corresponds to a fractional bandwidth of 37%. The maxi-mum in-band insertion loss for the simulated and measured S21 is0.45 and 0.83 dB, respectively. The attenuations in the stopbandsare better than 20 dB for the measured case. It is noted that theinsertion loss and stopband attenuation can be reduced in twoaspects. The first aspect is the design technique. Higher orderprototype or different transfer function can be used to improve thefilter performance. The second aspect is concerning the fabricationmethod and the equipment calibration technique. With a moreprecise control on etching and higher quality calibration standards,the measured insertion loss can be further improved.

4. CONCLUSION

A novel intercoupled split-ring resonator structure has been de-signed and analyzed in this article. Together with microstrip trans-mission line, the proposed structure has unique characteristics suchas a wide passband region with low insertion loss and a highattenuation in stopband regions. Thus, it can be utilized in appli-cations such as microwave bandpass filter. Unlike the traditionalparallel-coupled structures, the intercoupled split-ring resonatorsoccupy less space in the lateral direction and are compatible withmicrowave planar technology. It has been demonstrated in bothsimulation and experiments that the proposed structure can exhibita fractional bandwidth of over 50%. The passband frequency isalso relatively simple to design. In addition, common filter proto-type response can be applied easily to control the passband width.Therefore, the intercoupled split-ring resonator structure incorpo-rated with microstrip line has been successfully demonstrated withpromising results for them to be applied as microwave filters.

REFERENCES

1. M. Le Roy and A. Perennec, Novel circuit models of arbitrary-shapeline: Application to parallel coupled microstrip filters with suppressionof multi-harmonic responses, Eur Microwave Conf 2 (2005), 4–6.

2. A.G. Lamperez and M.S. Palma, High selectivity X-band planar di-plexer with symmetrical box-section filters, Eur Microwave Conf 1(2005), 4–6.

3. R. Phromloungsri, S. Patisang, K. Srisathit, and M. Chongcheawch-amnan, A harmonic-suppression microwave bandpass filter based onan inductively compensated micro strip coupler, Asia Pac MicrowaveConf Proc, Suzhou, China (2005), 4–7.

4. J.T. Kuo and M. Jiang, Enhanced microstrip filter design with auniform dielectric overlay for suppressing the second harmonic re-sponse, IEEE Microwave Wireless Comp Lett 14 (2004), 419–421.

5. R.J. Wenzel, Exact design of TEM microwave networks using quarter-wave lines, IEEE Trans Microwave Theory Tech 12 (1964), 94–111.

6. C.L. Hsu, F.C. Hsu, and J.K. Kuo, Microstrip bandpass filters forultra-wideband (UWB) wireless communications, Long Beach, CA,IEEE MTT-S Int (2005), 12–17.

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

8. J.B. Pendry, A.J. Holden, D.J. Robbins, and W.J. Stewart, Magnetismfrom conductors and enhanced nonlinear phenomena, IEEE TransMicrowave Theory Tech 47 (1999), 2075–2084.

9. D.R. Smith, W.J. Padilla, D.C. Vier, S.C. Nemat-Nasser, and S.Schultz, Composite medium with simultaneously negative permeabil-ity and permittivity, Phys Rev Lett 84 (2000), 4184–4187.

10. R.A. Shelby, D.R. Smith, and S. Schultz, Experimental verification ofa negative index of refraction, Science 292 (2001) 77–79.

TABLE 3 Simulation and Design Parameters for Case 2

Elements gi Ls (nH) Cs (pF) s (mm)

n � 1, 5 0.618 1.02 0.17 0.05n � 2, 4 1.618 2.68 0.066 0.11n � 3 2 3.32 0.053 0.16

f0 (GHz) � 12, X � 40%, a (mm) � 3.60.

Figure 7 Layout of the intercoupled split-ring resonators (SRR) thatcorresponds to an order-five Butterworth prototype

Figure 8 Comparisons between the simulated and measured S-parame-ters for the order-five intercoupled split-ring resonator structure with theButterworth transfer function

1812 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 49, No. 8, August 2007 DOI 10.1002/mop

11. V.G. Veselago, The electrodynamics of substances with simulta-neously negative values of � and �, Sov Phys Usp 10 (1968), 509–514.

12. J. Bonache, F. Martin, J. Garcia-Garcia, I. Gil, R. Marques, and M.Sorolla, Ultra wide band pass filters (UWBPF) based on complemen-tary split rings resonators, Microwave Opt Technol Lett 46 (2005),283–286.

13. J. Garcia-Garcia, F. Martin, F. Falcone, J. Bonache, I. Gil, T. Lopetegi,M.A.G. Laso, M. Sorolla, and R. Marques, Spurious passband sup-pression in microstrip coupled line band pass filters by means of splitring resonators, IEEE Microwave Wireless Compon Lett 14 (2004),416–418.

14. S.N. Burokur, M. Latrach, and S. Toutain, Study of the effect ofdielectric split-ring resonators on microstrip-line transmission, Micro-wave Opt Technol Lett 44 (2005), 445–448.

15. J. Garcia-Garcia, J. Bonache, I. Gil, F. Martin, R. Marques, F. Falcone,T. Lopetegi, M.A.G. Laso, and M. Sorolla, Comparison of electro-magnetic band gap and split-ring resonator microstrip lines as stopband structures, Microwave Opt Technol Lett 44 (2005), 376–379.

16. H.W. Wu, Y.K. Su, M.H. Weng, and C.Y. Hung, A compact narrow-band microstrip bandpass filter with a complementary split-ring reso-nator, Microwave Opt Technol Lett 48 (2006), 2103–2106.

17. J. Bonache, I. Gil, J. Garcia-Garcia, and F. Martin, Novel microstripbandpass filters based on complementary split-ring resonators, IEEETrans Microwave Theory Tech, 54 (2006), 265–271.

18. S.H. Jang and J.C. Lee, Design of novel cross-coupling elliptic func-tion filters with the miniaturized edge-coupled split ring resonators,Microwave Opt Technol Lett 45 (2005), 495–499.

19. J.S. Hong and M.J. Lancaster, Filters for RF/microwave applications,Wiley, USA, 2001, Chapter 3.

© 2007 Wiley Periodicals, Inc.

NOVEL SUSPENDED-LINE MICROSTRIPCOUPLER USING BCB ASSUPPORTING LAYER

A. Corona-Chavez,1 I. Llamas-Garro,1 Jung-Mu Kim,2 andYong-Kweon Kim2

1 Instituto Nacional de Astrofisica, Optica y Electronica, Luis EnriqueErro 1, Tonanz., Puebla 72840, Mexico2 School of EECS, Seoul National University, Korea

Received 23 January 2007

ABSTRACT: In this letter a novel �/4 micromachined directional cou-pler is presented with a 10 �m benzocyclobutene (BCB) layer used tosuspend one transmission line over another one in order to achieve a3-dB coupling. The coupler is centered at a frequency of 24 GHz. © 2007Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 1813–1814, 2007;Published online in Wiley InterScience (www.interscience.wiley.com).DOI 10.1002/mop.22574

Key words: directional coupler; BCB; coupled lines; millimeter wavecouplers

1. INTRODUCTION

Directional couplers are essential components in every millimeterwave transceiver system. One of the main advantages of coupled-line couplers over branch-line couplers is their wider bandwidthresponse of about 50% for a 0.4-dB coupling unbalance, comparedwith 15% for the branch-line couplers [1].

One of the main disadvantages of coupled-lines is the difficultyof achieving tight couplings such as 3 dB. For this reason it hasbeen suggested to use couplers in tandem [2]. In [3] two 8.3-dBcouplers centered at 60 GHz are connected in tandem to achieve a

3-dB coupling. Nevertheless, this configuration is larger than asingle coupler as more than one coupler is needed.

In [4] a CPW suspended air-bridge coupler is presented at acenter frequency of 30 GHz. However, if large microstrips aresuspended over an air bridge, residual stress will limit the size ofthe structure. In this letter we propose a microstrip-suspended linecoupler where benzocyclobutene (BCB) is used to support the topcoupled-line, giving good mechanical strength. The main advan-tages of using BCB as the supportive structure are that relativelythick layers can be manufactured [5], it has low loss tangent(0.008), and it requires simple manufacturing process [6].

2. DESIGN

The coupler was designed on a 125-mm-thick quartz substrate witha permitivity er � 3.8. The loss tangent is about tan � � 0.00033.The top layer was suspended over a 10-�m-thick BCB layer (er �2.6) as shown in Figure 1. To achieve the correct coupling the topline overlaps L � 20 �m the bottom line as shown in Figure 1(larger overlappings correspond to tighter couplings). At the centerof the structure the top line is crossed to the opposite side to realizea codirectional coupler. The meandered transmission lines wereoptimized to 56% miter for best performance (see Fig. 2).

3. FABRICATION

First a Cr/Au seed layer is thermally evaporated on a 125-�m-thick quartz substrate. A photoresistive mold is then formed overthe seed layer to pattern the coupler bottom layer which is formedby electroplating techniques. Then the mold and seed layer isremoved and a 10-�m BCB layer formed by UV lithography at acuring temperature of 150°C. The top layer of the coupler isformed by the same process starting with the seed layers, mold,and electroplating described at the beginning of this paragraph. Allmetal layers are 3-�m thick. Finally, the coupler is placed inside atest housing, where the coupler is fixed to the gold-coated brass

Figure 1 (a) Top view of proposed suspended line directional coupler.(b) Side view showing the quartz substrate, the gold top, and bottomtransmission lines and the BCB supportive layer. All dimensions are inmicrometers

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 49, No. 8, August 2007 1813