Genetic algorithm optimisation for evanescent mode waveguide filter design

Filters are an integral part of any communication or electronic warfare system. They are used extensively in electronic countermeasures, instrumentation and satellite communication. There are numerous types of filters that can be used at microwave frequencies, nevertheless the main drawback of a waveguide filter is its large size at lower microwave frequencies. For this reason, the one structure that can provide smaller size is the evanescent mode filter. A complete design procedure is presented for an evanescent mode filter, which does not require special tuning techniques. This method covers the case of a rectangular waveguide structure, which is composed of an air-filled waveguide containing a number of dielectric inserts operating in the normal propagative mode, while air-filled sections are evanescent. The technique consists in finding a scattering description of all the filter sections and junctions. This search stems from an exhaustive electromagnetic study of the field quantities and requires modal analysis methods. A genetic algorithm optimisation has been developed and included and rules are provided to assist designers in their search for best results. Examples of waveguide filters with various orders are simulated for given cross section, permittivity and step size. The filters' section lengths are evaluated such that the best response according to designer specifications is achieved.