Chernov A. Tunable resonant elements based on the coplanar transmission lines

The thesis is devoted to the research of methods for the micromechanical tuning of resonant elements based on the coplanar and slot lines and establishment of the resonant elements’ geometrical and electrophysical parameters influence on their frequency and energy characteristics during tuning. The micromechanical tuning stands out from other tuning methods due to wide tuning range, high Q-factor and absence of additional losses during tuning. The presented tuning method is realized by lifting either signal electrode or dielectric plate above the substrate for the coplanar line and dielectric or metal plate above the electrodes for the slot line. This leads to the redistribution of the electromagnetic field in the line, change of which, due to its complexity, was described by using effective dielectric permittivity and characteristic impedance. These parameters determine the wavelength in the line and relation between the electric and magnetic field amplitudes, respectively. The methods for computation of the coplanar and slot line’s equivalent parameters and losses were proposed. The proposed methods are based on the reduction of the system of Maxwell's equations to a homogeneous Poisson equation, in case of the TEM-mode of the coplanar line, or to the problem of eigenvalues and eigenvectors in case of the TE-mode of the slot line. The obtained problem was solved by two-dimensional finite element method applied to the cross-section of the line. The reliability of the proposed methods is confirmed by good agreement with experimental and well-known literature data. The presented methods have no restrictions on the electrophysical parameters or shape of the line's components and can be used to analyze the change in effective dielectric permittivity, losses and characteristic impedance of the micro mechanically tunable coplanar and slot lines. The proposed tuning method allows achieving the effective dielectric permittivity’s change of more than 60% for the displacements of tens micrometres. The ways to reduce required displacements and increase tuning range by changing the line’s electrophysical and geometrical parameters were established. The criteria for choosing a better tuning method for the coplanar lines depending on the line's geometrical parameters was shown. It was shown that this method doesn’t bring additional losses during tuning, moreover, the losses caused by dielectric and metallic parts of the line decrease with the growth of the distance between the substrate and line’s movable parts. The structures of the tunable resonators based on the slot and coplanar lines are presented. The slot line etched in the ground electrode of the coplanar was considered as a short-ended stub serially plugged in the line’s ground electrode forming a slot resonator. The resonant frequency of the slot resonator depends on its size and equivalent parameters, changing which allows obtaining tunable resonator. Tuning of the effective dielectric permittivity of the slot resonator is realized by moving dielectric or metal plates above the resonator’s electrodes. The structure of the coplanar line based tunable stub resonator was shown. Frequency tuning of the proposed stub resonator is achieved by lifting the dielectric plate above the stub electrodes. Both types of tunable resonators provide up to 80% resonant frequency tuning range with no distortion in frequency characteristics during tuning for the displacement of less than 100 micrometres. The ways to increase the tuning range are similar to ones for the effective dielectric permittivity of the tunable slot and coplanar lines. The use of the tunable slot resonators for the creation of a tunable antenna is presented. Slot resonators are etched in the ground electrode of the antenna’s feeder line and act as a filter. Changing filter’s frequency characteristic is realized by moving metal surfaces deposited on the dielectric plate above the slot resonators that leads to the shift of the antenna’s operating frequency. The strip of ionic polymer metal composite with low control voltage was used for the change of the position of the dielectric plate with metal surfaces. The frequency shift of the proposed tunable antenna is 130% with stable radiation pattern.