polation of the MoM-PO impedance matrix. The overall approachoffers spectacular saving in CPU time required for the wide-bandperformance evaluation of antennas radiating in the presence ofelectrically large conducting objects.

ACKNOWLEDGMENTS

Artur Noga gratefully acknowledges support of this work by thePolish State Committee for Scientific Research under Grant3T11D00829.

REFERENCES

1. R.F. Harrington, Field computation by moment methods, Macmillan,New York, 1968.

2. U. Jakobus and F.M. Landstorfer, Improved PO-MoM hybrid formula-tion for scattering from three-dimensional perfectly conducting bodiesof arbitrary shape, IEEE Trans Antennas Propag AP-43 (1995), 162–169.

3. E.H. Newman, Generation of wide-band data from the method ofmoments by interpolating the impedance matrix, IEEE Trans AntennasPropag AP-36 (1988), 1820–1824.

4. R.E. Hodges and Y. Rahmat-Samii, An iterative current-based hybridmethod for complex structures, IEEE Trans Antennas Propag AP-45(1997), 265–276.

5. S.M. Rao, D.R. Wilton, and A.W. Glisson, Electromagnetic scatteringby surfaces of arbitrary shape, IEEE Trans Antennas Propag AP-30(1982), 409–418.

6. A. Karwowski and A. Noga, Fast MM-PO-based numerical modellingtechnique for wide-band analysis of antennas near conducting objects,Electron Lett 43 (2007), 486–487.

© 2008 Wiley Periodicals, Inc.

A NOVEL CROSSED SLOTTED PATCHDUAL-MODE BANDPASS FILTER WITHTWO TRANSMISSION ZEROS

Sean Wu,1 Min-Hang Weng,2 Shih-Bin Jhong,3 andMaw-Shung Lee3

1 Department of Electronics Engineering and Computer Science,Tung-Fang Institute of Technology, Kaohsiung, Taiwan;Corresponding author: wusean@giga.net.tw2 National Nano Device Laboratories, Taiwan3 Department and Institute of Electronics Engineering, NationalKaohsiung University of Applied Sciences, Kaohsiung, Taiwan

Received 13 August 2007

ABSTRACT: This article presents a miniaturized dual-mode band-pass filter (BPF) with two transmission zeros and low insertion-lossusing crossed slotted patch resonator. The BPF is mainly formed bya �/2 patch resonator which is a pair of crossed slots with unequalwidths is embedded in the center. By tuning the perturbation ele-ment, which resulted from the unequal slot widths along the diago-nal, two resonant modes are excited and a pair of transmission zerosare generated to improve the selectivity of the BPF performance. Theproposed filter at center frequency (f0) of 1.55 GHz has very goodmeasured characteristics including the bandwidth of 1.49 –1.62 GHz(3-dB fractional bandwidth of 8.4%), low insertion loss of 1.74 �0.35 dB. The measured filter performance is in good agreement withthe simulated response. © 2008 Wiley Periodicals, Inc. MicrowaveOpt Technol Lett 50: 741–744, 2008; Published online in Wiley Inter-Science (www.interscience.wiley.com). DOI 10.1002/mop.23219

Key words: dual mode; crossed slot; patch resonator; transmission ze-ros; bandpass filter

1. INTRODUCTION

Compact planar bandpass filters (BPFs) by using dual-mode ringor patch resonators are attractive and have been studied for recentyears since their excellent performances of narrow bandwidth, twotransmission zeros, size reduction and high-selectivity [1–5]. Thedual-mode BPFs primarily comprise a physical coupling of twodegenerate modes excited by a perturbation element, which islocated along the orthogonal plane of resonator in a symmetricalstructure of the ring or patch resonator. The open patch resonatorhas been widely used as a building block for antenna design [6]

Figure 2 Variation with frequency of three typical elements in MoM-POimpedance matrix of the structure of Figure 1

Figure 3 Input impedance versus frequency of monopole antenna shownin Figure 1

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 50, No. 3, March 2008 741

because of its high radiation efficiency. However, it suffers fromhigh radiation loss as well as large overall size when comparedwith the ring resonator. Dual-mode patch resonators provide anattractive combination of high miniaturization because of the useof a single patch resonator to implement two poles, and a goodpower-handling capability due to the current spread over thestructure for the filter. To improve the filter design, some researchefforts have been invested to reduce the radiation loss and overallsize by introducing a pair of crossed slot [7] or using high tem-perature superconductor substrate [8–10]. In [7], a square patchresonator etched by a pair of crossed slots is successfully devel-oped to demonstrate the simultaneous size and loss reduction. Byfurther suitably selecting the unequal lengths of crossed slots alongthe two diagonal planes of the patch resonator, a Chebyshev-typedual-mode patch BPF is realized and shows a low insertion loss.However, no transmission zero is located on either side of thepassband, causing a poor selectivity.

In this article, a novel dual-mode square patch resonator with apair of crossed slots with unequal widths is designed and displaystwo transmission zeros near the passband edges to improve thefilter performance. The design procedure is presented and theproposed filter centered at 1.55 GHz is fabricated and tested. Theexperimental results of the fabricated filter show a good agreementwith the simulation results.

2. ANALYSIS OF CROSS SLOTTED PATCH DUAL MODEBPF

Figure 1 shows the structure of the proposed crossed slotted patchdual mode BPF, which the open patch resonator is etched by a pairof unequal crossed slots oriented along its two diagonal planesA-A� and B-B�, respectively. This filter is designed on the sub-strate of RT/Duroid 6010 substrate with thickness of 1.27 mm andrelative permittivity �r of 10.2. 50-� input/output ports are spa-tially separated at 90° and have strong coupling interface with thepatch resonator. In [7], the �/2 open patch resonator had a single-mode operating when lengths of a pair of crossed slots were equal.However, in this study, the crossed slots have the same crossed slotlengths but the two different crossed slot widths. Under the fixedlength of crossed slots, the crossed slot width (Wb) along thediagonal plane B-B� are adjusted slightly wider than the slot width(Wa) along A-A�. As such, this geometrical difference in width canbe considered as an alternative perturbation element, similar to theconventional corner cut, to determine or control the couplingbetween the two degenerate modes.

In the design procedure, the main passband is determined first.The fundamental resonant frequency is determined by the crossedslot length and rapidly shifts downward when the crossed slotlength increases, while the insertion loss tends to go up slightly andthe frequency bandwidth becomes gradually narrow, as claimed in[7]. To achieve a BPF having the central frequency of 1.55 GHz,the crossed slots length of Ls � 20.4 mm are first used in thesquare patch resonator with side length of Lp � 20 mm, as shownin Figure 1.

Figure 2(a) shows simulated frequency response of the pro-posed BPF under different slot widths and equal slot lengths byusing the full-wave EM simulator [11]. The frequency responseactually depends on the widths of crossed slots. It is proven againthat the open patch resonator shows a single-mode operating whenlengths and widths of a pair of crossed slots were all equal.Additionally, only a stop-band response is obtained when thewidths of crossed slot are equal. However, it is clearly observedthat the crossed slots with different widths can be the perturbationelement to generate two transmission zeros located on either sideof the passband. Meanwhile, Figure 2(b) shows the return lossperformance, indicating the crossed slots with different widths can

Figure 1 Practical layout of the designed CSP-DMF designed on a1.27-mm-thick substrate with a dielectric constant of 10.2

Figure 2 Simulated frequency response of the proposed BPF underdifferent slot widths. (a) insertion loss (b) returned loss

742 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 50, No. 3, March 2008 DOI 10.1002/mop

generate two modes or two attenuation poles located in the middleof passband.

To quantify this coupling between the two split modes, theresponse of various relative width offset from the two slots aresimulated. For design purposes, the center frequency and crossedslots length (Ls) are usually the prespecified parameters. Figure3(a) shows two simulated split mode frequencies of 1.48 and 1.61GHz as Wb � 1.6 mm. Figure 3(b) shows two split resonantfrequencies and coupling coefficient for different crossed slotswidths. The coupling coefficient (K) can be calculated as follows[12]:

K �f 2

2 � f 12

f 22 � f 1

2 (1)

where f2 and f1 are the resonant frequency of the odd and evenmodes, respectively. The different width of the crossed slots (per-turbation element) along B-B� slightly contracts the current flow-ing route of odd mode and slightly extends the current flowingroute of even mode in connection with the two modes symmetry

over diagonal plane, leading to the increase in f2 and to reductionin f1, respectively [6].

Figure 4 shows the current magnitude profile for the BPF withdifferent crossed slot widths operating at center frequency 1.55GHz. The simulated current density shows the signal path iselectrically extended by going around from the one side to theother through the artificially created two channels along upper sideand lower side [7]. The geometrical arrangement of the slots yieldsthe current flow rerouting on either side of the diagonal planesA-A�. Two modes can be excited with a nearly identical level andthe incident wave from the input line can be transmitted effectivelyonto the output line such that a two-pole filtering behavior is wellrealized.

3. EXPERIMENTAL RESULTS

The designed filter has been manufactured in microstrip technol-ogy. A photograph of the resulting prototype is shown in Figure5(a). The fabricated BPF is designed at the central frequency of1.55 GHz with the following physical parameters: Lp � 20 mm, Ls

� 20.4 mm, Wa � 0.4 mm, Wb � 1.0 mm, g � 0.1 mm. Theoverall size of fabricated BPF is about 20 mm � 20 mm, Theoverall size of fabricated BPF is about 20 mm �20 mm, indicatinga more compact size than that without cross slot structure at thesame center frequency. The measurement is characterized in anAgilent 8753E network analyzer. The measured results have returnloss of 12.4 � 5.4 dB, insertion loss of 1.74 � 0.35 dB in thepassband from 1.49 to 1.62 GHz, FBW (defined as 3dB bandwidthover the center frequency) of 8.4% at 1.55 GHz, and two trans-mission zeros are clearly observed at 1.36 GHz with �56 dBattenuation and 1.8 GHz with �50 dB attenuation, as shown inFigure 5(b). The two transmission zeros on the both sides of thepassband actually improve the sharp rejection rate and good se-lectivity, compared with the BPF with unequal lengths of thecrossed slots. The low insertion loss is verified again and the resultis that an effective signal rerouting leads to the drop off of theresonant frequency while the two oppositely oriented current den-sities contribute to the reduction of radiation loss generated by thepatch resonator. Although the slight differences between the sim-ulations and measurements might be due to fabrication tolerance or

Figure 3 (a) Simulated two split mode frequencies as Wb � 1.6 mm and(b) coupling coefficient and the two resonant frequencies (odd and evenmodes) as functions of the differences of the two unequal crossed slotwidths

Figure 4 Simulated current magnitude profile of the dual-mode patchsurface at the center frequency in the presence of the crossed slots. (Ls �20.4 mm, Wa � 0.4 mm, Wb � 1.0 mm)

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 50, No. 3, March 2008 743

the variation of material properties, the performances of the pass-band are still closely matched between the measured results andthe simulated results. The superior features indicate that the dual-mode BPF has potential to be utilized in communication applica-tions.

4. CONCLUSIONS

A novel dual mode BPF using a cross slotted patch resonator hasbeen proposed. The lengths of the crossed slots are used to deter-mine the main passband frequency and the unequal widths of thecrossed slots are used to generate two transmission zeros as well asdual modes located on either side of the passband. The proposedfilters have sharp rejection, good selectivity and two transmissionzeros. The compactness in circuit size makes the proposed designattractive for further developments and applications in modernmobile radio systems.

REFERENCES

1. J.S. Hong and M.J. Lancaster, Bandpass characteristics of new dual-mode microstrip square loop resonators, IEE Electron Lett 31 (1995),891–892.

2. M. Matsuo, H. Yabuki, and M. Makimoto, Dual-mode stepped-imped-

ance ring resonator for band-pass filter applications, IEEE TransMicrowave Theory Tech 49 (2001), 1235–1240.

3. B.T. Tan, S.T. Chew, M.S. Leong, and B.L. Ooi, A modified micro-strip circular patch resonator filter, IEEE Microwave Wireless Com-pon Lett 12 (2002), 252–254.

4. J.S. Hong and M.J. Lancaster, Theory and experiment of dual-modemicrostrip triangular patch resonators and filters, IEEE Trans Micro-wave Theory Tech 52 (2004), 1237–1243.

5. M.-H. Weng, Novel dual-mode circular patch bandpass filter withenhanced stopband performance, IEICE Trans Electron E88-C (2005),1872–1879.

6. J.R. James and P.S. Hall, Handbook of microstrip antennas (IEEElectromagnetic Waves Series 28). Stevenage, UK, Peregrinus, 1989.

7. L. Zhu, P.M. Wecowski, and K. Wu, New planar dual-mode band-passfilter using crossed slotted patch resonator for simultaneous size andloss reduction, IEEE Trans Microwave Theory Tech 47 (1999), 650–654.

8. A. Cassinese, F. Palomba, G. Pica, and A. Andreone, Dual mode crossslotted filters realized with superconducting films, Appl Phys Lett 77(2000), 4407–4409.

9. A. Cassinese, A. Andreone, M. Barra, C. Granata, P. Orgiani, F.Palomba, G. Panariello, G. Pica, and F. Schettino, Dual mode super-conducting planar filters based on slotted square resonators, IEEETrans Appl Supercond 11 (2001), 473–476.

10. S.J. Fiedziuszko, J.A. Curtis, S.C. Holme, and R.S. Kwok, Low lossmultiplexers with planar dual mode HTS resonators, IEEE TransMicrowave Theory Tech 44 (1996), 1248–1257.

11. IE3D Simulator, Zeland Software, Fremont, CA, 1997.12. J.S. Hong and M.J. Lancaster, Microstrip filters for RF/microwave

application, Wiley, Reading, 2001.

© 2008 Wiley Periodicals, Inc.

SINGLE AND TRIPLE OUTPUTCOHERENCE-FREE MICROWAVEPHOTONIC FILTER USING A 3 � 3COLLINEAR COUPLER IN A SAGNACFIBER LOOP

G. Ning, P. Shum, and J. ZhouNetwork Technology Research Centre, Nanyang TechnologicalUniversity, Research TechnoPlaza XFrontier, Block 4th Storey 50Nanyang Drive, Singapore 637553; Corresponding author:ngxstone@pmail.ntu.edu.sg

Received 13 August 2007

ABSTRACT: New structures that can realize single and triple outputcoherence-free microwave photonic filter are presented. They arebased on a Sagnac loop interferometer containing a single drive in-tensity modulator and a 3 � 3 collinear coupler, and no non-recip-rocal bias unit is required. Notch responses are obtained by modu-lating the clockwise and counterclockwise propagating waves insidethe Sagnac loop at different times. Theoretical analysis is obtainedfor both DFB laser source and broad-band source. Measured resultsverify the theoretical expressions and demonstrate robust notch filterresponses. © 2008 Wiley Periodicals, Inc. Microwave Opt TechnolLett 50: 744 –747, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23218

Key words: microwave photonic filter; Sagnac loop; intensity modula-tor; optical signal processing

1. INTRODUCTION

All-optical processing of microwave and millimeter-wave signalsprovides advantages such as large time-bandwidth products, in-

Figure 5 The proposed cross slotted patch dual-mode bandpass filter. (a)photograph (b) predicted and measured frequency response

744 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 50, No. 3, March 2008 DOI 10.1002/mop