NEW CLASS OF RECTANGULAR AND CIRCULAR EVANESCENT-MODE WAVE-GUIDE FILTER

In this paper, a complete design procedure is presented for an origina l structure of evanescent-mode filters which does not require special tuning techniques or subsequent optimization procedures beyond the des ign method itself. The method covers the cases of both rectangular and circular waveguide structures. The structure is composed of an air-fi lled waveguide containing a number of dielectric inserts. It is design ed 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 consi sts 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 scatt ering description of the entire structure and, subsequently, to an app roximate 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 eval uated such that they lead to a close agreement with the response of th e prototype. The bandwidth over which the quality of approximation is acceptable is studied in detail and several examples are given for bot h rectangular and circular filters. Experimental results are also pres ented indicating that the approach proposed is at the same time accura te and very efficient. (C) 1998 John Wiley & Sons, Inc.