cated bandpass filter is at 2.47 GHz and the fractional bandwidth is 5%. The measured insertion loss and return loss of the filter is -3.68 and -14.92 dB, respectively. The measured center fre- quency, insertion loss and return loss of the fabricated filter are in agreement with simulated results. ACKNOWLEDGMENT The authors are grateful to the National Science Council of R.O.C. for financial support under the project No. NSC 94-2215-E-214- 007. REFERENCES 1. M.L. Hsieh, L.S. Chen, S.M. Wang, C.H. Sun, M.H. Weng, M.P. Houng, and S.L. Fu, Low-temperature sintering of microwave dielec- trics (Zn,Mg)TiO 3 , Jpn J Appl Phys 44 (2005), 5045-5048. 2. R. Levy, R.V. Snyder, and G. Matthaei, Design of microwave filters, IEEE Trans Microwave Theory Tech 50 (2002), 783-793. 3. J.S. Hong and M.J. Lancaster, Couplings of microstrip square open-loop resonators for cross-coupled planar microwave filters, IEEE Trans Mi- crowave Theory Tech 44 (1996), 2099-2109. 4. J.T. Kuo, M.J. Maa, and P.H. Lu, A microstrip elliptic function filter with copact miniaturized hairpin resonators, IEEE Microwave Guided Wave Lett 10 (2000), 94-95. 5. C.S. Ahn, J. Lee, and Y.S. Kim, Design flexibility of an open-loop resonator filter using similarity transformation of coupling matrix, IEEE Microwave Wireless Compon Lett 15 (2005), 262-264. 6. J.S. Hong and M.J. Lancaster, Design of highly selective microstrip bandpass filters with a single pair of attenuation poles at finite frequen- cies, IEEE Trans Microwave Theory Tech 48 (2000), 1098-1107. 7. J.S. Hong and M.J. Lancaster, Microstrip filters for RF/microwave applications, Wiley, New York, 2001, p. 318. 8. Zeland Software, IE3D 6.0, New York, 1999. © 2007 Wiley Periodicals, Inc. RAPID PROTOTYPING OF CERAMIC MILLIMETERWAVE METAMATERIALS: SIMULATIONS AND EXPERIMENTS Yoonjae Lee, 1 Xuesong Lu, 2 Yang Hao, 1 Shoufeng Yang, 2 Rich Ubic, 2 Julian R. G. Evans, 2 and Clive G. Parini 1 1 Department of Electronic Engineering, Queen Mary, University of London, Mile End Road, London E1 4NS, United Kingdom 2 Department of Materials, Queen Mary, University of London, Mile End Road, London E1 4NS, United Kingdom Received 29 January 2007 ABSTRACT: Rapid prototyping by an extrusion freeforming technique, of ceramic metamaterials based on a woodpile structure at millimeter- wave frequencies has been performed. The finite difference time domain technique is applied for the design and characterization of the proposed metamaterials. The transmittance of the millimeterwave metamaterials is measured in the range of 75–110 GHz. Both measurement and simula- tion results are in good agreement. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 2090 –2093, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop. 22697 Key words: millimeterwave; EBG; metamaterial; woodpile; freeforming fabrication 1. INTRODUCTION Millimeterwave systems are becoming increasingly important in many scientific, civil, and military applications because they can provide wider bandwidth for transmitting large amount of data and better resolution in radar systems. In recent years, it has been demonstrated by various groups that novel devices can be realized using electromagnetic bandgap (EBG) structures, a class of meta- materials. EBG structures, also known as photonic bandgap struc- tures (PBG) [1, 2] in optics, are now finding numerous applications at microwave and millimeterwave frequencies [3, 4]. In general, EBG structures consist of periodic dielectric or metallic elements, and exhibit forbidden frequency bands (bandgap). The full poten- tial of EBG structures can be utilized with a full three-dimensional (3D) bandgap. Thus, rapid and cost-effective fabrication tech- niques for 3D EBG structures are of significant importance. The woodpile structure shown in Figure 1, also called a layer-by-layer structure, consists of stacked diffraction gratings, in which adja- cent layers are perpendicular to each other. This structure pos- sesses face-centered-tetragonal symmetries and provides a full 3D bandgap. Such a periodic structure can be easily fabricated for microwave applications using columns of individually machined dielectric materials with preferable dimensions. However, at mil- limeterwave frequencies, conventional machining would not be convenient because of small dimensions (50 –500 m). Various sophisticated microfabrication techniques such as silicon lithogra- phy and wafer fusion are available for microstructures, but those are more appropriate for terahertz and photonic wavelength appli- cations, and would be costly to fabricate 3D structures with large number of layers for applications at W-band (75–110 GHz). The team at University of Michigan [5] has used indirect solid free- forming to make alumina woodpile structures by casting ceramic slurry into a solid freeformed mould. In this letter, we present a direct rapid prototyping method for constructing 3D EBG materi- als for millimeterwave applications, with a possible extension to higher frequencies based on extrusion freeforming of ceramic materials [6]. The proposed fabrication method can also be versa- tile for constructing curved geometries and creating defects in layered structures. Figure 6 Simulated and measured frequency responses of the bandpass filter 2090 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 49, No. 9, September 2007 DOI 10.1002/mop