Design of Waveguide Applicators Using a Quarter-Wave Transformer Prototype
In this paper, we propose a design methodology for waveguide applicators to maximize microwave power deposition into human tissues. The optimized applicators can be used in the experimental studies of the biological effects of exposure to electromagnetic radiation in the frequency range from 6 GHz to 100 GHz. The design methodology relies on the provision of reflectionless matching of a dissipative waveguide load, achieved by employing a matching network based on a quarter-wave transformer prototype. The prototype is synthesized by knowledge of the voltage standing wave ratio (VSWR) evaluated in the unmatched loaded waveguide. A key difference from the conventional synthesis procedure is that in our design approach, the characteristic impedance of the first transformer section is given, and we have to not only determine the characteristic impedances of the remaining sections, but also establish the output load. A solution of this synthesis problem and the process of converting the transformer prototype into a waveguide structure are described. The physical structure can be implemented according to provided sample models of waveguide WR137 applicators employing symmetric inductive or capacitive posts. The matched waveguide applicators are easy to manufacture, and according to the results from computational simulations, they demonstrate superior performance compared to the unmatched waveguides. Limitations of our designs (narrow bandwidth, dependence on the type of tissues encountered, limited potential for miniaturization) are discussed.