nology Conference, Budapest, Hungary, May 21–23, 2001, pp. 1897–1902.

2. L. Chiu, T.Y. Yum, C.H.K. Chin, Q. Xue, and C.H. Chan, High-efficiency class-B push-pull amplifying array for microwave transmit-ting front end, IEE Proc-Microwave Antennas Propag 153 (2006),25–28.

3. C.H.K. Chin, Q. Xue, H. Wong, and X.Y. Zhang, Broadband patchantenna with low cross-polarisation, Electron Lett 43 (2007), 137–138.

4. A. Ziroff, M. Nalezinski, and W. Menzel, A 40 GHz LTCC receivermodule using a novel submerged balancing filter structure, Proc RadWireless Conf 2003, pp. 151-154.

5. Y.-S. Lin and C.H. Chen, Novel balanced microstrip coupled-linebandpass filters, In Proceedings of URSI International ElectromagneticTheory Symposium 2004, pp. 567-569.

6. C.-H. Wu, C.-H. Wang, and C.H. Chen, Novel balanced coupled-linebandpass filters with common-mode noise suppression, IEEE TransMicrowave Theory Tech 55 (2007), 287–295.

7. C.-H. Wu, C.-H. Wang, and C.H. Chen, Stopband-extended balancedbandpass filter using coupled stepped-impedance resonators, IEEE Mi-crowave Wireless Compon Lett 17 (2007), 507–509.

8. J.-X. Chen, C.-H.K. Chin, and Q. Xue, Double-sided parallel-strip linewith an Inserted conductor plane and its applications, IEEE TransMicrowave Theory Tech 55 (2007), 1899–1904.

9. L. Zhu, B.C. Tan, and S.J. Quek, Miniaturized dual-mode bandpassfilter using inductively loaded cross-slotted patch resonator, IEEE Mi-crowave Wireless Compon Lett 15 (2005).

© 2008 Wiley Periodicals, Inc.

AN ULTRA-WIDE BANDPASS FILTERWITH GOOD OUT-OF-BANDPERFORMANCE

Zhong Li, Guang-Ming Wang, Chen-Xin Zhang, andGe-Nong LongRadar Engineering Department, Missile Institute of Air ForceEngineering University, Sanyuan, Shanxi 713800, China;Corresponding author: lihejun_5@sina.com

Received 19 November 2007

ABSTRACT: An ultra-wideband microstrip filter with good out-of-bandperformance is designed and demonstrated. The filter design is based ona circuit model for an optimum short-circuited stub transmission-linefilter whose unit elements or connecting lines are nonredundant. Substi-tuting the connecting lines by the lowpass filter using stepped impedancehairpin resonator, the undesired spurious bands can be efficiently sup-pressed, and the designed filter length is reduced. According to the dif-ferent requirements, the broadband bandpass and lowpass filter can beindependently designed and modified, which makes the filter more con-venient and easily implemented. © 2008 Wiley Periodicals, Inc.Microwave Opt Technol Lett 50: 1735–1737, 2008; Published online inWiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23517

Key words: ultra-wideband; microstrip filter; bandpass filter; steppedimpedance hairpin resonator

1. INTRODUCTION

Owing to the fast development of broadband wireless communi-cation systems, the design of wide and ultra-wide bandpass filtersdraws much attention. Many researchers have been investigatingvarious kinds of ultra-wideband (UWB) bandpass filters using themicrostrip structure, which is suitable for the printed circuitboards. The multiple-mode resonators (MMR) is used firstly todesign UWB band pass filters by Zhu et al., the designed filter can

make up a 110% passband covering the whole UWB band [1].Coupled lines using backside aperture on ground plane for enhanc-ing coupling [2] and interdigital coupled lines [3] with multimoderesonator are also used for realizing ultra-wide pass band. Thehybrid microstrip and coplanar waveguide structure with tightenedcoupling degree are proposed to make ultra-wide band filters in[4]. For flatter group delay in a wideband bandpass filter, the shuntshort-circuited stubs filter are employed in [5]; the optimum UWBfilter bandwidth is 10.65 GHz at a midband frequency of 8.9 GHzfor a factional bandwidth of about 120%. The shunt two-sectionopen-circuited stubs are employed to produce transmission zeros,the filter has sharp attenuations and high selectivity [6].

Although these filters can achieve the UWB bandpass charac-teristic, they usually introduce some spurious bands in the com-munication system. These undesired bands become an importantdrawback for ultra-wide bandpass filters performance due to theirproximity to the pass band of interest. To gain good stopbandrejection, the traditional design approach is cascading the broad-band bandpass and bandstop filters. Obviously, the method makesthe filter to have greater size and more loss. In this article, a novelmethod to suppress the spurious response of microstrip bandpassfilter is proposed. By embedding the lowpass filter using steppedimpedance hairpin resonator (SIHR) into the shunt-stub UWBbandpass filter, the stopband rejection bandwidth of the bandpassfilter can be significantly improved, and the total filter structure ismore compact.

2. THEORY ANALYSIS AND DESIGN

2.1. Shunt-Stub UWB Bandpass FilterThe configuration of the optimum short-circuited stub bandpassfilters consists of a cascade of shunt stubs of equal length alter-nating with uniform transmission lines, each of them twice the stubelectrical length. Although the filter consists of only n stubs, it hasan insertion function of degree 2n 1 in frequency so that itspassband response has 2n 1 ripple. Therefore, this kind filterwill have a fast rate of cutoff compared with n ripples for an n-stubbandpass filter.

The designed UWB bandpass filter has a pass band of 1.4–5.8GHz, the central frequency is about 3.6 GHz. To determine theimpedance values of the short circuit stubs and line elements, thetabulated element values supplied in [7] for optimum distributedhighpass filters are used. The characteristic impedances of theconnecting lines are all close to the terminal impedance of 50 .The filter implemented on a substrate with dielectric constant r 4.1, thickness h 1.0 mm. Considering the impedance consis-tency with the embedded lowpass filter, through the simulation andoptimization by the commercial software Ensemble 8.0 based onmethod of moment (MOM), all the connecting lines can be mod-ified to have a 50 characteristic impedance, and the three stubsimpedance is decreased to best allowing the fabrication tolerances.The resulting filter size values are shown in Figure 1(a), it has avertical symmetry structure and the three stubs have the samelength 11.5 mm. The simulation results depicted in Figure 1(b),five reflection zeros can be observed in the pass bands for thethree-degree UWB filter.

2.2. SIHR Lowpass FilterThe compact elliptic-function lowpass filter use a single microstripSIHR with direct-connected feed lines, as shown in Figure 2(a).The SIHR consists of the single transmission line l2 (the charac-teristic impedance is Z2) and coupled lines with a length of l1 (theeven- and odd-mode impedance are Zoe and Zoo, respectively). Byselecting Z2 [mt] ZooZoe , the size of the SIHR is smaller than that

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 50, No. 7, July 2008 1735

of the conventional hairpin resonator. This lowpass filter canprovide the advantages of compact size, sharp cutoff, and widestop band frequency response [8].

According to the equivalent circuit in [9], the elliptic-functionlow pass filter is synthesized that the 3-dB cutoff frequency is [mt]5.8 GHz and the attenuation pole is about 11.4 GHz, which is thecenter frequency of the first spurious pass band in Figure 1.Through the electromagnetic simulation and optimization, the di-mensions of the SIHR are as follows: w1 2.5 mm, l1 2.3 mm,w2 0.8 mm, l2 4.3 mm, and gap size s 0.2 mm. To gainwideband attenuation, two resonators are linked directly to con-struct the cascaded SIHR, and the layout is shown in Figure 2(b).The parallel-coupled lines length l1 can be turned to adjust trans-mission pole locations so as to improve the passband characteris-tics and change the location of the attenuation pole. These differentlayout and size are simulated, and the result is shown Figure 2(c).From the Figure 2(c), we find that the cascaded SIHR structureprovides a much sharper cutoff frequency response and deeperrejection band when compared with the results of using the singleSIHR, and the attenuation pole will tend to drop with the l1increasing.

3. EXPERIMENTAL RESULTS

The proposed filter structure is shown in Figure 3(a), substitutingthe connecting lines by the cascaded SIHR lowpass filter. To gainbroad stop band, two different couple line length (l1 2.3 mm, 2.7mm, respectively) cascaded SIHR are employed. Something isimportant that the electrical length between the adjacent shuntstubs must be constant after substituting.

The UWB filter is fabricated on a substrate with dielectricconstant of 4.1, thickness of 1.0 mm, measured by the vector

network analysis (VNA) Agilent 8720ET. The measured S-parameters in Figure 3(b) show that the UWB bandpass filter3-dB bandwidth is 4.4 GHz (1.4 –5.8 GHz), the huge stopbandis obtained up to 20 GHz, the in-band insert loss is lower than1.5 dB, and return losses are better than 10 dB. The insert losstended to increase toward higher frequencies, which wouldaccount for the frequency-dependent losses of the two SMAconnectors and the dielectric material. The measured groupdelay in Figure 3(c) varies between 0.4 and 0.6 ns in the passband. This means that the developed UWB filter has a goodlinearity as well. A comparison of the topologies of the filter inFigures 1(a) and 3(a), the proposed filter obtains 23% lengthreduction.

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1736 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 50, No. 7, July 2008 DOI 10.1002/mop

4. CONCLUSION

A novel UWB bandpass filter topology with broad stopband re-jection is proposed. The experiment filter has 122% 3-dB band-width, less than 1.5 dB in-band insert losses, the variation of groupdelay less than 0.2 ns, and a wide stopband bandwidth with 17 dBattenuation up to 20 GHz. The design method is that the broadbandstopband filter is internally embedded within the section betweentwo short-circuited stubs of the broadband bandpass filter, which isnot increase the size. With the lowpass and bandpass filter de-signed separately, the method is more convenient and easily im-plemented. This compact and high performance lowpass filtershould be useful for many broad system applications.

REFERENCES

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

2. L. Zhu, H. Bu, and K. Wu, Aperture compensation technique forinnovative design of ultra-broadband and microstrip bandpass filter,IEEE MTT-S Int Microwave Symp Dig 1 (2000), 315-318.

3. S. Sun and L. Zhu, Capacitive-ended interdigital coupled lines for UWBbandpass filters with improved out-of-band performances, IEEE Micro-wave Wireless Compon Lett 16 (2006), 440-442.

4. N. Thomson and J.S. Hong, Compact ultra-wideband microstrip/copla-nar waveguide bandpass filter, IEEE Microwave Wireless Compon Lett17 (2007), 184-186.

5. J.S. Hong and H. Shaman, An optimum ultra-wideband microstrip filter,Microwave Opt Technol Lett 47 (2005), 230-233.

6. P. Cai, Z.W. Ma, X.H. Guan, T. Anada, and G. Hagiwara, Synthesis andrealization of ultra-wideband bandpass filters using the z-transformtechnique, Microwave Opt Technol Lett 48 (2006), 1398-1401.

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

8. L.H. Hsieh and K. Chang, Compact lowpass filter using stepped im-pedance hairpin resonator, Electron Lett 37 (2001), 899-900.

9. L.H. Hsieh and K. Chang, Compact elliptic-function low-pass filtersusing microstrip stepped-impedance hairpin resonators, IEEE TransMicrowave Theory Tech 51 (2003), 193-199.

© 2008 Wiley Periodicals, Inc.

RESONANT MICROSTRIP MEANDERLINE ANTENNA ELEMENT FOR WIDESCAN ANGLE ACTIVE PHASED ARRAYANTENNAS

K. S. Beenamole,1 Prem N. S. Kutiyal,1 U. K. Revankar,1 andV. M. Pandharipande2

1 Electronics and Radar Development Establishment, Bangalore,India; Corresponding author: ksbeena@yahoo.com2 Department of ECE, Osmania University, Hyderabad, India

Received 19 November 2007

ABSTRACT: A compact, wide bandwidth, wide beamwidth resonantmicrostrip meander line antenna element is reported for active phasedarray applications. The antenna element offers a return loss better than10 dB over a bandwidth of 12% in S-band with a beamwidth of 130°in E-plane. The element has been tested in an array for its wide-angle scanperformance. Simulated and measured results are presented. © 2008 WileyPeriodicals, Inc. Microwave Opt Technol Lett 50: 1737–1740, 2008;Published online in Wiley InterScience (www.interscience.wiley.com).DOI 10.1002/mop.23531

Key words: meander line antenna; wide band wide beam patch an-tenna; phased array element

1. INTRODUCTION

Active phased arrays have now become a practical proposition formodern day radar systems, by overcoming the major problems oflow reliability and low efficiency inherent in the passive phasedarray configurations. Active phased arrays require wide band, widebeam antenna elements with low cross polarization levels forobtaining a wide array scan zone over a broad bandwidth. Manydifferent types of radiating elements have been used in phasedarray radars operating in different frequency bands. In this work, anew type of microstrip antenna element has been studied theoret-ically and experimentally, to be employed in an active phased

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DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 50, No. 7, July 2008 1737