LETTER
A novel design method for electromagnetic bandgap based waveguide filters with periodic dielectric loading

https://doi.org/10.1016/j.aeue.2011.07.007Get rights and content

Abstract

An extremely rapid, versatile prototyping methodology is developed for waveguide filters with periodic dielectric loading which enables direct dimensional synthesis of the unit cell starting from given design objectives. A prototype filter has been designed using a low dielectric constant material and built for the purposes of testing the feasibility of the proposed approach. Our numerical results are compared with an independent commercial software package and also with experimental measurements. Simulation results are also presented for a filter loaded high dielectric constant material. In both cases the results are in excellent agreement with the theory and demonstrate the validity and the applicability of the proposed method.

Introduction

Investigation of wave propagation in periodically stratified media goes back to the end of the 19th century [1]. Ever since, the interest of scientists and engineers in periodic structures has increased steadily due to substantial advantages they provide in certain applications involving slow waves or frequency selective spectral characteristics [2], [3].

Periodically loaded waveguides are important for numerous engineering applications such as filters [4], [5], slow-wave structures [6], phase shifters and impedance matching devices [7], [8], antennas [9], antenna feeds [10], and pulse compressors [11]. Propagation characteristics of periodic structures may be specified via generalized scattering matrix (GSM) approach. Different techniques are developed for obtaining GSM representations [12], [13], [14]. For our purposes it is more convenient to construct the GSM representation in terms of the scattering amplitudes of all propagating and a sufficiently large subset of evanescent modes of the loaded/unloaded waveguide. GSM representations provide a rigorous analytical framework for investigating the general features and the fine details of the propagation phenomena in periodically loaded waveguides [15], [16], [17], [18], [19], [20], [21], [22], [23], and also for addressing certain inverse problems [24].

The art and science of microwave filter design has extended to a vast territory encompassing numerous different topologies and design approaches that have matured over the years [25], [26]. The classical approach for designing periodically loaded waveguide filters is based on network representations for the unit cell which describe a mapping between the parameters of the unit cell and of the corresponding “equivalent” network element(s) [27]. The first outcome of this design procedure is a network topology which meets required design specifications. At the second step with the aid of full-wave analysis or a suitable approximate method the network topology is mapped into the physical dimensions defining the specific unit cell topology chosen for implementation. Of course, both mappings involve certain approximations which may require a final optimization step using a full-wave analysis tool for dimensional fine tuning.

An alternative approach, which has attracted considerable attention in recent literature [28], [29], is waveguide filter synthesis using dimensional optimization schemes. However, in all reported investigations on dimensional optimization schemes the initial topology of the dielectric loading had to be rather close to the optimal unit cell configuration, in order to reduce the computational burden to manageable limits.

In what follows we propose a different, computationally efficient, direct dimensional synthesis approach for electromagnetic bandgap based waveguide filters without utilizing network representations in the design process.

Section snippets

Methodology

The main features of the propagation characteristics in periodically loaded waveguides may be inferred by introducing the assumption of strict periodicity which corresponds to an “infinite” or “match-terminated” (i.e. terminated in Bloch impedance [30]) structure. The calculation of the dispersion diagram and the identification of passband/stopband regions of the periodic structure then reduce to determining the GSM representation for the unit cell and imposing Floquet conditions on its ports.

A design example

In order to demonstrate the application of the proposed synthesis approach we consider the design of an X-band filter which is required to suppress 10.2–11.2 GHz band by more than 35 dB.

Another design example

The proposed synthesis method makes use of the “effective” dielectric constant estimated by considering only the propagation of dominant mode in each section. The question arises whether this method can be used in designing filters loaded with higher dielectric contrast materials, so that, the filter sections support several propagating modes within the design bandwidth. In this section, we outline the design of a filter using Rogers TMM10i as dielectric material which is specified by ɛr =9.8,

Conclusions

An efficient synthesis method is developed for determining the physical dimensions of certain dielectric loading configurations in waveguides directly from given design objectives. It is shown that the proposed method may also be used for detailed investigation of the sensitivity of filter response on machining tolerances. A prototype filter has been built using a low dielectric constant material and our simulation results are compared with those obtained via an independent commercial software

Acknowledgement

We would like to thank Dr. Richard Snyder of RS Microwave Inc. for accurate experimental measurements and Oksana Manzhure for manufacturing of prototype filter.

Serkan Şimşek received the B.S. degree in electrical and electronics engineering from Istanbul University, Istanbul, Turkey, in 2001, the M.S. and Ph.D. degrees in electronics and communication engineering from Istanbul Technical University (I.T.U.), Istanbul, Turkey, in 2003 and 2008 respectively. He is currently an Assistant Professor with the ECE Department of I.T.U. His current research interests are guided-wave theory, microstrip antennas, and microwave measurement techniques.

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    Serkan Şimşek received the B.S. degree in electrical and electronics engineering from Istanbul University, Istanbul, Turkey, in 2001, the M.S. and Ph.D. degrees in electronics and communication engineering from Istanbul Technical University (I.T.U.), Istanbul, Turkey, in 2003 and 2008 respectively. He is currently an Assistant Professor with the ECE Department of I.T.U. His current research interests are guided-wave theory, microstrip antennas, and microwave measurement techniques.

    Ercan Topuz received the M.Sc. and Ph.D. degrees in electronics and communication from Istanbul Technical University, Istanbul, Turkey, in 1965 and 1973, respectively. From 1965 to 2005 he was with the ECE Department of I.T.U. Since 2005, he is with ECE Department, Dogus University, Istanbul, Turkey. Prof. Topuz has authored or coauthored over 70 technical papers in books, journals, and conferences. His research interests include theoretical and computational electromagnetics, microwave/optical devices and systems. Dr. Topuz is a member of the Electromagnetics Academy.

    Edip Niver received the B.Sc., M.Sc., and Ph.D. degrees in electrical engineering from the Middle East Technical University, Ankara, Turkey, in 1970, 1973, and 1979, respectively. From 1979 to 1982, he was with the Polytechnic Institute of New York as a Post-Doctoral Researcher. In 1982, he joined the Electrical Engineering Department, New Jersey Institute of Technology (NJIT), Newark, where he is currently a Professor. His interests are antennas and wave propagation, microwave and lightwave engineering, and experimental methods. He has authored or coauthored over 60 publications and conference presentations in the above areas. Dr. Niver is a member of IEEE and Sigma Xi.

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