as well as from the patients and the results are shown in Figures 1and 2. These results are novel using microwave techniques for thecase of pericardial fluid. Clinical evaluation of the pericardial fluidsamples is also done and the results are tabulated in Table 1. Theyfound it was relatively rich in lactate dehydrogenase, low inprotein, and high in lymphocytes and monocytes. From Figure 1,it is noticed that the effusion samples exhibit a higher dielectricconstant than that of the normal samples. In Figure 2, the variationof conductivity of normal and effusion samples are plotted. It canbe found that distinct variation in the conductivities of normalsamples and the effusion samples. The increase in conductivity ineffusion samples is due to the presence of higher level of proteincontents such as albumin and globulin than the normal pericardialsamples. Thus in the specified band of frequencies, normal peri-cardial and pericardial effusion samples were studied and exhibitdistinct variation of dielectric constant and conductivity with fre-quency.

6. CONCLUSION

The microwave characterization of the pericardial samples isdone using cavity perturbation technique. The cavity perturba-tion technique is quick, simple, and accurate and it requiresvery low volume of sample for measuring the dielectric prop-erties of tissue samples like pericardial samples. It is observedthat a remarkable change in the dielectric properties of pericar-dial effusion samples with the normal healthy samples and thesemeasurements were in good agreement with clinical analysis.This measurement technique is simple and the extraction ofpericardial fluid from persons is least painful and nonsurgical innature. These results prove an alternative in-vitro method ofdiagnosing onset pericardial effusion abnormalities using mi-crowaves without surgical procedure.

REFERENCES

1. S. Ben Horin, A. Shinfeld, E. Kachel, A. Chetrit, and A. Livneh, Thecomposition of normal pericardial fluid and its implications for diag-nosing pericardial effusions, Am J Med 118 (2005), 636–640.

2. Special issues, IEEE Trans Microwave Theory Tech 50 (2002).3. A. Von Hippel, Dielectric and waves, Artech House, Norwood, MA,

1995.4. S. Gabriel, R.W. Lau, and C. Gabriel, The dielectric properties of

biological tissues. II. Measurements on the frequency range 10Hz to20GHz”Literature survey, Phys Med Biol 41 (1996), 2251–2269.

5. H.F. Cook, Dielectric behavior of human blood at microwave frequen-cies, Nature 168 (1951), 247–248.

6. H.F. Cook, The dielectric behavior of some types of human tissues atmicrowave frequencies, Br J Appl Phys 2 (1951), 295–300.

7. D.K. Ghodgaonkar, V.V. Varadan, and V.K. Varadan, Free spacemeasurement of complex permittivity and complex permeability ofmagnetic materials at microwave frequencies, IEEE Trans InstrumMeasurement 19 (1990), 387–394.

8. D.K. Ghodgaonkar, V.V. Varadan, and V.K. Varadan, A free spacemethod for measurement of dielectric constant and loss tangents atmicrowave frequencies, IEEE Trans Instrum Measurement 38 (1989),789–793.

9. W. Barry, A broadband, automated, stripline technique for the simul-taneous measurement of complex permittivity and complex permeabil-ity, IEEE Trans Microwave Theory Tech 34 (1986), 80–84.

10. Z. Abbas, R.D. Pollard, and R.W. Kelsall, A rectangular dielectricwaveguide technique for determination of permittivity of materials atW-band, IEEE Trans Microwave Theory Tech 46 (1998), 2011–2015.

11. K.T. Mathew, Encyclopedia of RF and microwave engineering, Vol. 4,Wiley-Interscience, USA, 2005, pp. 3725–3735.

© 2008 Wiley Periodicals, Inc.

A COMPACT QUASI-ELLIPTICBANDPASS FILTER USING COUPLEDDUAL QUASI-LUMPED LCRESONATORS

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

Received 14 May 2008

ABSTRACT: This letter proposes a compact microstrip bandpass filterusing coupled dual quasi-lumped LC resonators. With the proposed to-pology, two transmission zeros are generated at both low and high stop-band near the passband to create quasi-elliptic response. A filter sample isdesigned and fabricated to provide an experimental verification. © 2008Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 158–160, 2009;Published online in Wiley InterScience (www.interscience.wiley.com).DOI 10.1002/mop.24009

Key words: bandpass filter; quasi-lumped LC resonator; transmissionzero; quasi-elliptic response; even and odd mode analysis

1. INTRODUCTION

Bandpass filters [1] of high quality and compact size are alwaysdesirable in model wireless communication system with operatingfunctions of in-band transmission and out-of-band rejection. Theperformance of bandpass filter can benefit a lot from transmissionzero point technique. Generating transmission zero points andproperly locating them at either side of the passband can developsharp and deep rejection outside the passband without sacrificingthe performance of the passband. Thus, a low-order BPF with thehelp of extra transmission zero points can meet the stopbandrequirements that are usually achieved by high-order filters. Toachieve better selectivity, many improvements on creating suitabletransmission zeros were reported recently [2, 3]. The cross-cou-pled configuration is widely used to create transmission zeros, andthus the filter exhibits elliptic function response [4, 5]. And source-load coupling is also well known to be capable of producing nearpassband transmission zeros [6, 7].

In this letter, we present a novel topology of two-pole micro-wave BPFs consisting of only dual quasi-lumped LC resonator asshown in Figure 1. Employing quasi-lumped resonator can con-tribute a lot to size reduction. Further, it will be demonstratedusing even and odd mode analysis that two transmission zeros canbe generated at both low and high stopbands to create a quasi-elliptic response, which enhances the filter’s performance remark-ably.

2. THEORY

The proposed resonator in Figure 1 is a quasi-lumped LC resonatorincluding an interdigital capacitance. The larger patch at its bottomis also a capacitor to compose the resonator and connect to the I/Oport. When two resonators are put side by side as seen in Figure 1,they will couple to one another. Figure 2(a) has given the distrib-uted equivalent circuit. Because this topology is symmetric, themethod of even and odd mode analysis can be applied. Theequivalent even and odd mode circuits are given in Figures 2(b)and 2(c), obtained from the replacement of the reference planewith the magnetic and electric walls, respectively.

Network theory [2] relates the transmission coefficient of thefilter to the odd and even mode input admittances by the followingformula:

158 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 1, January 2009 DOI 10.1002/mop

S21 �Yodd � Yeven

�1 � Yodd��1 � Yeven�(1)

where

Yodd � jY1ootan�11 � Y1oo

Y10 � jY1ootan�12

Y1oo � jY10tan�12(2)

Yeven � jY1oetan�11 � Y1oe

Y1e � jY1oetan�12

Y1oe � jY1etan�12(3)

and Ylo � j�2�Ct

2 � Cg2� � Y2oo

2 � 2�Y2oo2 �Ctcsc�2 � Cgcot�2�

�Ct � Y2oocot�2

(4)

Yle � j�2�Ct

2 � Cg2� � Y2oe

2 � 2�Y2oe�Ctcsc�2 � Cgcot�2�

�Ct � Y2oecot�2(5)

Ct � Cg � Cp (6)

Obviously, the transmission zeros can be obtained by settingYeven � Yodd, which can be solved graphically. Figure 3 depicts thecurves of Yeven (solid line) and Yodd (dashed lines) vs. frequencyfor an exemplary case. The correlative parameters are shown asbelow: w1 � 4.1 mm, l11 � 4.5 mm, l12 � 0.4 mm, w2 � 0.4mm, l2 � 6 mm, wc � 0.3 mm, wc2 � 0.3 mm, gc � 0.3 mm, lc �5.7 mm, gap � 0.3 mm; and the interdigital structure has threepairs of fingers. The substrate has a dielectric constant of 2.65 anda thickness of 0.5 mm. As observed in Figure 3, the intersectionpoints of two lines between the solid line and dashed lines, i.e., thetwo transmission zeros, located at 2.15 and 4.75 GHz, can beobserved, which can create sharp transitions at both sides of thepassband.

3. FILTER IMPLEMENTATION AND MEASUREMENTRESULTS

Based on the above analysis, a microstrip bandpass filter is de-signed and fabricated. The filter has the same dimensions as the

exemplary case in the above section. The filter is fabricated on a0.5-mm-thick dielectric substrate with a relative dielectric constantof 2.65 and a loss tangent of 0.0035. Figure 4 shows a photographof the fabricated bandpass filter.

The measured results for the proposed filter, together with theEM simulation results are displayed in Figure 5. It can be observedthat the simulated and measured results display a good agreement.The filter has a center frequency of 2.74 GHz with a bandwidth of1.67% (from 2.52 to 2.98 GHz). The passband from 2.52 to 2.98GHz has a return loss better than �24 dB. The maximum insertionloss in the passband is 1.83 dB, whereas the simulated one is lowerthan 0.16 dB. This insertion loss would be mainly due to the effectof the input/output subminiature A (SMA) connectors and to thedielectric loss of the substrate as well as the resistance loss of themetal strip.

In addition, two stopbands exhibit a rejection level lower than�23 dB within 1–2.2 and 3.5–5 GHz (for measurement results,lower than �25 dB). Significantly, two transmission zeros at 1.96and 4 GHz are observed as expected. The zero at the low-skirt sideFigure 1 Configuration of the proposed filter

Figure 2 (a) The distributive equivalent circuits for two coupled reso-nator. (b) The odd mode equivalent circuits for two coupled resonator. (c)The even mode equivalent circuits for two coupled resonator

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 1, January 2009 159

on the measured result can provided suppressions of �48.7 dB at1.96 GHz, and the zero at the high-skirt side on the measured resultcan provided suppressions of �47.3 dB at 4 GHz. And the sup-pression during 3.8–8 GHz was higher than �30 dB.

4. CONCLUSIONS

In this study, a bandpass filter schema based on quasi-lumpedresonator has been proposed and investigated in detail. In theproposed configuration, our work demonstrated that two transmis-sion zeros are generated at both low and high rejection bands,which can provide sharp transitions at both sides of the passband.

A bandpass filter with a center frequency of at 2.74 GHz weredesigned and demonstrated as verified. EM simulation and exper-imental results were thoroughly described. The measurement re-sults were found to agree well with the EM simulation results. Theproposed bandpass filter has better selectivity, good stopbandrejection, wider stopband, and compact size. With these features,the proposed filter is suitable for future microwave communicationsystems.

REFERENCES

1. G.L. Matthaei, L. Young, and E.M.T. Jones, Microwave filters, imped-ance-matching networks, and coupling structures, Artech House, Ded-ham, MA, 1964.

2. J.-S. Hong and M.J. Lancaster, Microstrip filter for RF/microwaveapplication, Wiley, New York, 2001.

3. K. Wada, Y. Noguchi, H. Fujumoto, and J. Ishii, A tapped-linecoplanar waveguide bandpass filter with finite attenuation poles, In:Proceedings of the Asia-Pacific Microwave Conference, 1994, pp.763–766.

4. B.-C. Min, Y.H. Choi, S.K. Kim, and B. Oh, Cross-coupled bandpassfilter using HTS microstrip resonators, applied superconductivity, IEEETrans 11 (2001), 485–488.

5. C.-F. Chen, T.-Y. Huang, and R.-B. Wu, Compact microstrip cross-coupled bandpass filters using miniaturized stepped impedance resona-tors, In: Proceedings of the Microwave Conference Proceedings, APMC2005 (Asia-Pacific Conference Proceedings) 1 (2005), p. 4.

6. C.-K. Liao and C.-Y. Chang, Design of microstrip quadruplet filterswith source-load coupling, IEEE Trans Microwave Theory Tech 53(2005), 2302–2308.

7. S. Amari, Direct synthesis of folded symmetric resonator filters withsource-load coupling, Microwave Wireless Compon Lett 11 (2001),264–266.

© 2008 Wiley Periodicals, Inc.

TUNABLE BANDSTOP FILTER-BASEDDEFECTED GROUND STRUCTUREUSING A METAL PLATE

Yongje SungDepartment of Radio Sciences Engineering, Korea University, Seoul136-701, Korea; Corresponding author: yjsung@korea.ac.kr

Received 21 May 2008

ABSTRACT: To obtain the electromagnetic bandgap structure with awide stopband, this article presents a novel approach for realizing atunable bandstop filter using defected ground structure (DGS) section.To change the rejection band of DGS section, a movable conductingplate is applied in parallel over an H-shaped defect on the groundplane. The measured results show that the proposed bandstop filter withone cell etched hole has a 30 dB rejection bandwidth of 27.96%,

Figure 3 Yeven and Yodd versus frequency

Figure 4 Fabricated bandpass filter

Figure 5 Measured and simulated results

160 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 1, January 2009 DOI 10.1002/mop