Analytical Modeling and Experimental Studies on Tapered Post Re-Entrant Cavity Resonator

Compact re-entrant cavities, widely used as microwave resonators, can be employed as high-performance sensors. A new approach is proposed here to calculate the resonance frequency, its sensitivity, and quality factor (Q-actor) of these cavities, having diverse taper shapes of the re-entrant post. The proposed closed-form expressions are based on the chain matrix approach for tapered coaxial transmission lines, and the calculated results for the resonance frequency and quality factor have been verified with full-wave electromagnetic simulations and experiments. The formulation is mathematically simple and accurate (<; 4%). Since a cavity with taper toward the gap-end is found to be more sensitive to gap variations, a robust, tunable re-entrant cavity resonator with a tapered post and an adjustable gap was fabricated. The performance of this cavity as a displacement sensor was evaluated by numerically calculating the deflection of a thin diaphragm from measured resonance frequencies using the expressions derived here. This design of the sensor is found to be immune to manufacturing tolerance in the re-entrant gap.