Tunable Resonator as the Load of a Microstrip Line

Abstract

A microstrip resonator is studied in which frequency tuning is carried out by micromechanical movements of the signal electrode above the substrate. The investigated resonator can be used as a component of a tunable antenna element, turned on as the load of the line. It is shown that the effective dielectric permittivity, which determines the resonant frequency, significantly decreases as a result of the movement of the signal electrode at a distance of several percents from the thickness of the substrate. Absolute values of the displacement usually stand tens or hundreds of micrometers depending on substrate thickness. Such displacements are available for modern piezoelectric or electrostrictive actuators. It opens an opportunity to implement the electrical type of tuning. For resonator performed on the substrate with dielectric permittivity about 10, resonant frequency tuning achieves more than 30 percent while the displacement of the signal electrode above the substrate does not exceed 10 percent. It is established that the variation range and sensitivity of the resonant frequency tuning can be increased by increasing the dielectric permittivity and decreasing the height of the microstrip line's substrate. Although the ratio between the width and the length of the resonator affects the tuning characteristics, the choice of these parameters is conditioned by the requirement for the formation of the required radiation pattern of the antenna element, the component of which is the viewed resonator. An equivalent circuit of a microstrip resonator as an element of a tunable antenna is proposed. The equivalent circuit model gives results that are satisfactorily consistent with those obtained based on a rigorous electromagnetic field model and a three-dimensional finite element method. At the same time, the schematic model simplifies the analysis of the influence of the geometric and electrophysical parameters of the resonant element on its characteristics and allows simplifying the design process of the antenna element and optimize its characteristics. The need to find the optimal parameters at the stage of designing the antenna element, in particular, arises during the matching of the feeder's and the radiator's characteristics and to provide the necessary range of the center frequency tuning.