Optimum Aperture Radius of R-Band Rectangular Resonator for Use in Electromagnetic Material Characterization

The most critical part of a resonator design for electromagnetic material characterization is determining the optimum excitation aperture radius of the resonator. 11 pairs of galvanized iron plates with apertures of various radii ranging from 3 mm to 13 mm, increasing in average radial steps of 1 mm, were fabricated for use as the excitation walls of the resonator. In measurements, each pair of plates with an aperture hole was attached between two waveguide adapters and a waveguide section. Then, S-parameters were measured using a network analyser. CST simulations for each case were also performed and compared with the measured ones which is not seen frequently in literature for a resonance structure. The nature of the excitation (under, critical, and over couplings) was discussed from the measured and simulated results. The quality factor values were calculated using S_21 resonance curves obtained from both the measurements and the simulations for each radius. The optimum excitation aperture radius value for the R-band resonator was found to be approximately 7 mm for the chosen resonator dimensions. For this value of aperture, a maximum loaded quality factor was obtained, and the resonator was critically coupled. For validation, theoretical radius values were also calculated and compared with the measured and simulated ones. This is a reliable and new method for showing that the obtained values are consistent.