Voloshyn A. Micromechanically tunable UHF antenna elements

The thesis is devoted to the development of methods of micromechanical tuning of the antenna elements operating frequency on the basis of dielectric resonators and microstrip lines and to establish dependencies of influence of electrophysical and geometric parameters of antenna elements on the frequency, energy and radiation characteristics of antennas. Unlike electrical, magnetic and optical tunable methods, the micromechanical method does not introduce additional losses, has a wide tuning range, and allow combining the advantages of electrical and mechanical methods using modern electric actuators. To determine the features of micromechanical tuning of dielectric resonators, a system of two parallel infinite dielectric plates with an air gap between them is considered, which is the simplest dielectric resonance structure suitable for electromechanical control using electric actuators. The electrodynamic problem for one-dimensional dielectric heterogeneity is solved analytically in terms LM- and LE-modes using the method of partial domains. The problem is reduced to the problem of eigenvalues and eigenvectors. Eigenvalues determine the resonant frequencies of the corresponding types of oscillations, and eigenvectors determine the amplitudes of the electromagnetic field components in partial regions. The dependencies of resonance frequency tuning due to the displacement of composite parts of the dielectric resonator are established. Based on the analysis of the influence of electrophysical and geometrical parameters on the frequency and energy characteristics of the resonator, the con-ditions for increasing the resonance frequency sensitivity to displacements and broadening the frequency tuning range are formulated. The established dependencies are generalized by theoretical and experimental studies of three-dimensional dielectric resonance structures. The relation between the tuning range and the sensitivity of the resonant frequency change to the micro-displacements of the signal electrode from the parameters of the microstrip resonator turned on as the final load of the microstrip line is established. Based on the theory of distributed circuits, a circuit model of a microstrip resonator is proposed. The parameters of the circuit model components are obtained solving the electrodynamic problem by the two-dimensional finite element method. The circuit model analysis results are in good agreement with obtained based on a rigorous elec-trodynamic model and solved by the three-dimensional finite element method. The circuit model simplifies the process of designing an antenna element based on such resonator and optimizes its characteristics. The investigation of energy and radiation characteristics of dielectric resonator and microstrip antennas shows non-degradation of characteristics while operating frequency tuning. The radiation efficiency of antenna elements increases while the increase in the thickness of the air gap between the parts of the tunable resonant elements due to an increase in the unloaded Q-factor of the structure because of the redistribution of the electromagnetic field energy in favor of an air gap where are practically no losses. Since the input impedance of the radiating element changes while tuning the operating frequency of the antennas, this can lead to the mismatch of the radiating element with the exciting feeder and an increase in reflective losses. However, given the radiation ef-ficiency increase while the operating frequency tuning, the allowable total antenna efficiency can be ensured over a wider operating frequencies range. The method of finding the optimal width of the antenna feeder, which provides minimal reflective losses in a given tuning range, is proposed. It is shown that the distribution of the electromagnetic field in the far-field area remains almost the same while tuning the operating frequency of the antenna elements, and a slight change in the radiation pattern is caused solely by some increase in the gain, which is determined by the radiation efficiency. Key words: microwave antenna element, micromechanical frequency tuning, dielectric resonator, microstrip resonator, effective dielectric permittivity.