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Abstract : |
In this paper, a complete design procedure is presented for an original structure of evanescent-mode filters which does not require special tuning techniques or subsequent optimization procedures beyond the design method itself. The method covers the cases of both rectangular and circular waveguide structures. The structure is composed of an air-filled waveguide containing a number of dielectric inserts. It is designed such that the air-filled sections are evanescent, while those with dielectric inserts operate in the normal propagative mode. Furthermore, the interfaces with the external source and load are achieved via a pair of air-filled guides of a propagative nature. The technique consists in finding a scattering description of all the filter sections and junctions relying on an exhaustive electromagnetic study of the field quantities and implying modal analysis methods. This leads to a scattering description of the entire structure and, subsequently, to an approximate lumped equivalent circuit model. This, in turn, is identified with a bandpass filter prototype and the parameters of the waveguide filter (cross section, lengths of sections, and permittivity) are evaluated such that they lead to a close agreement with the response of the prototype. The bandwidth over which the quality of approximation is acceptable is studied in detail and several examples are given for both rectangular and circular filters. Experimental results are also presented indicating that the approach proposed is at the same time accurate and very efficient. (C) 1998 John Wiley Sons, Inc., |
