370 MHz or 15.8% (from 2.15 GHz to 2.52 GHz) and 390 MHz or 16.6% (from 2.15 GHz to 2.54 GHz), respectively, for simulated and measured results. The variations of the antenna gain are only 0.2 dBi from 2.1 GHz to 2.4 GHz, and the maximum antenna gain is 3.8 dBi at 2.15 GHz. Figure 6 shows the simulated radiation patterns in two orthogonal planes at 2.4 GHz. Therefore, this figure clearly reveals that our proposed antenna radiates a good LHCP wave in wide elevation angles, including the directions of +z and -z. 4. CONCLUSION A single layer circularly polarized CPW-fed circular slot antenna has been successfully demonstrated. Even fabricated on the inex- pensive FR4 substrate, this antenna still reveals excellent perfor- mance at 2.4 GHz. To sum up, by properly adjusting the length of the protruded signal strip, the proposed antenna can be designed to have an impedance bandwidth of 740 MHz (33.3%) and a 3 dB AR bandwidth of 390 MHz (16.6%), good broadside LHCP radiation patterns over a wide elevation angle range, and the maximum antenna gain of 3.8. To conclude, our proposed antenna could provide good CP radiation for wide bandwidth transmitting and receiving applications. ACKNOWLEDGMENTS This project is supported by the National Science Council under grant NSC 94-3111-466-003-Y21. REFERENCES 1. K.C. Gupta, R. Garg, and I. J. Bahl, Microstrip lines and solt lines, 2nd ed. Artech House, Norwood, MA, 1996. 2. W. Menzel and W. Grabherr, A microstrip patch antenna with coplanar feed line, IEEE Microwave Guided Wave Lett 1 (1991), 340 –342. 3. T.J. Ellis, J.P. Raskin, G.M. Rebeiz, and L.P.B. Katehi, A Wideband CPW-fed microstrip antenna at millimeter-wave frequencies, In Proc IEEE MMT-S Interantional Microwave Symposium Digest, vol. 2, Anaheim, CA, June 1999, pp. 629 – 632. 4. R.L. Li, N.A. Bushyager, J. Laskar, and M.M. Tentzeris, Determination of reactance loading for circularly polarized circular loop antennas with a uniform traveling-wave current distribution, IEEE Trans Antennas Propag 53 (2005), 3920 –3929. 5. R.S. Elliott, Antenna theory and design, IEEE Press, Piscataway, NJ, 2003, pp. 71–73. 6. K.M. Chang, R.J. Lin, I.C. Deng, J.B. Chen, Q.X. Ke, J.R. Chang, A Novel design of a CPW-fed square slot antenna with broadband circular polarization, Microwave Opt Technol Lett 48 (2006), 2456 –2459. 7. I.C. Deng, J.B. Chen, Q.X. Ke, J.R. Chang, W.F. Chang, and Y.T. King, A circular CPW-fed slot antenna for broadband circularly polarized radiation, Microwave Opt Technol Lett, in press © 2008 Wiley Periodicals, Inc. BANDPASS FILTER WITH IMPROVED SPURIOUS PERFORMANCE USING MODIFIED RING DIELECTRIC RESONATOR IN MIC ENVIRONMENT Kumar Vaibhav Srivastava, Vishwa V. Mishra, and Animesh Biswas Department of Electrical Engineering, Indian Institute of Technology, Kanpur, Uttar Pradesh, India; Corresponding author: vaibhavs@iitk.ac.in Received 4 October 2007 ABSTRACT: A C-band bandpass filter is designed using modified ring dielectric resonator in microwave integrated circuit (MIC) environment for improved spurious response. The dielectric resonator (DR) filters are generally made in cavity environment because of its good spurious re- sponse in cavity environment, but its spurious performance degrades as the filter is designed in MIC environment. In MIC environment, the smaller substrate thickness makes DR closer to ground plane and affects its resonance mode spectrum which, in turn, affects closely spaced reso- nant frequencies. The dense resonant mode spectrum of DR in MIC en- vironment limits its application for filter designing. This article intro- duces a comparative study on filter realization with modified ring DR and conventional ring DR to show the improvement of spurious re- sponse in MIC environment. The simulated and measured results of these filters are presented to demonstrate the validity of the design procedure and improvement of spurious response. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 1426–1431, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop. 23391 Key words: dielectric resonator (DR); dielectric resonator filter; modi- fied ring dielectric resonator; spurious response; resonator mode sepa- ration 1. INTRODUCTION The role of dielectric resonator (DR) in miniaturization of micro- wave filter and oscillators is well recognized [1, 2]. The micro- wave filters consist of dielectric resonators that have good in-band performance, but the crowded mode spectrum of dielectric reso- nator gives poor out-of-band response. To get good out-of-band response, mode suppressor [3], and irises [4] have been suggested earlier to filter out the spurious modes. Further, the air-filled cylindrical cavities at the input [5], application of TM 01 mode for bandpass filter [6], mixed mode filter design [7], combine filter [8], and sandwiched conductors DR [9] are the well recognized ap- proaches for improving the spurious performance of dielectric resonator filters. But in all these filters, the attentions are mostly paid to cavity filters, where the dielectric resonators are placed within rectangular or cylindrical metal enclosures (cavity), and it has been found that very limited studies are available on dielectric resonator filters in microwave integrated circuit (MIC) environ- ment [10, 11]. The reason for using the DR in cavity environment is that when DR is placed at the center of cavity, maximum mode separation can be achieved [12], whereas in MIC environment, the -30 -27 -24 -21 -18 -15 -12 -9 -6 -3 0 3 0 30 60 90 120 150 180 210 240 270 300 330 -30 -27 -24 -21 -18 -15 -12 -9 -6 -3 0 3 Left phi_0 Left phi_90 Right phi_0 Right phi_90 Figure 6 Radiation patterns of the proposed antenna on the elevation plane at the resonant frequency of 2.4 GHz 1426 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 50, No. 5, May 2008 DOI 10.1002/mop