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Numerička karakterizacija efikasnosti zaštite kućišta sa otvorima na bazi sprege sa žičanim strukturama
Cvetković, Tatjana M.
Dončov, Nebojša 1970-
Marković, Vera 1956-
Maleš Ilić, Nataša 1968-
Janković, Milan 1950-
One of the fundamental tasks in designing electronic systems is to fulfill conditions of electromagnetic compatibility - EMC. EMC defines the capability of an electronic system to satisfactorily function in its electromagnetic (EM) environment not disturbing the operation of other neighboring devices. EMC has become one of the main aspects that has to be considered in the design of electronic systems because of the increasing number and rapid growth of EM intereference sources. It has been noticed that foreseeing the protection measures during the design phase is simpler and much more efficient than applying those measures on a completed system, which often becomes impossible. The fulfillment of EMC requirements is regulated by a set of standards. In Europe, all standards have to be harmonized with the Directive for Electromagnetic Compatibility (2004/108/EC). Those standards regulate limits of emission (the highest emission level of a source of EM interference), limits of immunity (the highest level of EM interference on a device with no degradation or with acceptable degradation of its performances) and the measuring conditions and methods. One of the ways to reduce the emission and increase the immunity of an electronic system in its real environment is to place it in a protective metal enclosure. Characteristics of the enclosure regarding EMC are evaluated by a parameter called shielding effectiveness - SE. SE is the ratio between the field strength without and with the enclosure interposed, at the same observation point. SE can be defined for the electric field (the so-called electric SE) and for the magnetic field (the so-called magnetic SE). Regarding the EMC, performances of the electronic system, i.e. SE level, are influenced by: construction, shape, size and wall thickness of the enclosure, electric and magnetic characteristics of the construction materials (electric conductivity, relative permeability, magnetic susceptibility, etc.), frequency range of the system operation, size, shape and number of apertures, their position on its walls, parameters of excitation plane wave and coupling mechanisms between externaly generated EM interference and EM radiation generated within the various parts of the system. The paths by which external EM interference penetrates into the system are usually the same with the paths by which internally generated disturbances are radiated outside of enclosure. The protective enclosure walls contain apertures of various shapes and purposes, most often used to access and control the system, retract the input and output cables, for ventilation, cooling, etc. Through those apertures, EM radiation penetrates into the enclosure and also into the outer space, disturbing enclosure protective function. In addition to coupling through the apertures, there are also coupling by diffusion through shield walls, coupling with wire structures within the enclosure and with other parts of the system. For proper functioning of the system it is necessary to analyze and determine the nature and level of EM emissions generated by various parts of the system, the influence of externally generated EM interefrence on the system and its parts, as well as their mutual coupling, in order to undertake measures for reducing or eliminating the coupling paths. For determining the SE, various analytical and numerical methods are used alongside with experimental measurements. Analytical methods may be applied only in simple cases and with appropriate approximations. For an example an empty enclosure with apertures may be represented as an equivalent waveguide circuit, in which the enclosure is represented as a shorted waveguide, while the opening is represented as a coplanar strip line short circuited at both ends. The application of analytical methods on complex structures gives only approximate results. Numerical methods, which gained in importance by the use of high performance computers, have become an indispensable tool in modeling and simulation of various realistic EMC problems. By using these methods, research time can be significantly shortened, since the construction of prototypes which do not fulfill the established EMC requirements and thus need additional measurements and subsequent corrections in design can be avoided. The subject of the PhD scientific research is a numerical characterization of the metal enclosures’ protective characteristics, as well as analyses of the shielding effectiveness behavior in EMC relevant frequency range, depending on some of the previously mentioned factors (polarization type of the excitation plane wave and the azimuth and elevation angles under which the plane wave comes across the enclosure walls with apertures, the number of wall apertures and their mutual distance as well as their position with respect to the point in which the SE of enclosure is determined, etc.). Furthermore, the coupling of EM waves that penetrate into the enclosure interior with the wire structures, that can significantly affect the total EM field distribution within the shield and consequently affect its protective function expresses as SE, will be studied. As a numerical tool, a modeling method based on electric transmission lines (transmission line matrix method - TLM method) is used. Thanks to its characteristics, the TLM method has been widely used in solving various EM field propagation problems. The TLM method belongs to the group of differential numerical techniques in the time domain. It is based on the analogy between EM field components and electric currents and voltages in the transmission line network by which the concept of EM field is reduced to the concept of electric circuit theory. Enhancements of the TLM method, in the form of so-called compact models, suitable for efficient modeling of the mutual interaction between the excited EM field and geometrically small but in electrical sense important structures (thin wire structures, complex wire junctions, slots, apertures, etc.) have empowered this metod to be efficiently used for practical EMC problem solving. For an example, besides enabling the modeling of very thin wire structures without the use of an extremely fine mesh around the wire, the compact TLM wire model enables the modeling of the two-ways interaction between the external field and the wire structure. It is based on additional TLM wire nodes introduced to the existing network of TLM nodes, which model signal propagation along the wire and describe the interaction with the EM field. The appropriate numerical TLM model of the enclosure containing apertures and wire and dielectric structures is created in this thesis and then used for the purpose of conducting analyses of their influence on the SE and resonant frequencies of enclosure. In researches a special attention was given to the enclosures with rectangular apertures, since, in the case of unknown plane wave polarization and arbitrary angle under which it faces the wall with apertures, it is more difficult to determine the most critical case from the EMC standpoint (the case when the mere existence of the apertures damages the most the shield’s protective function). The presence of wire structures was considered through several aspects: when the appropriate receiving antenna is brought into the enclosure space in order to measure the EM field level, which corresponds to the procedure of experimental determination of the SE of enclosure; when the current/voltage information induced in the antenna is transmitted through cable to the measuring instrument as well as when the corresponding loaded antenna elements are used for the purpose of damping the resonant frequencies of the enclosure (frequency at which the shielding effectiveness is the lowest). The aim of the scientific research of this PhD is to conduct the detailed assessment of the impact of the parameters of the system’s rectangular apertures, wires and dielectric structures and the plane wave on the SE enclosure and to draw attention to the means of minimizing their disruption of system functioning and complying with EMC standards in their presence. The conclusions arising from these analyses may be of importance in the processes of designing metal enclosures for protecting electronic systems. In addition, the aim is to analyze how the inclusion of additional wire structure within the system, for the purposes of measuring SE or damping the enclosure’s resonant frequencies, affects the protective features of the enclosures and their assessment. Very small, even negative SE enclosure values (which indicates that the shield’s presence enhances, rather than weakens the EM field) usually occur around the resonant frequencies of enclosure. The proposed method for damping the enclosure’s resonant frequencies is investigated in the case when the antenna elements are realized in microstrip technology and is cririclz analyzed from the standpoint of the corrected SE level at resonant frequiencies and possible shift of existing or introduction of additional resonant frequencies. In the experimental procedure for determining the SE enclosure, it is necessary to set up the receiving antenna which is used for the EM field level detection. The antenna of finite dimensions significantly influences the distribution of EM field within the enclosure, and consequently the SE level. For this reason it has been examined how the presence of the receiving antenna and coaxial cable within the enclosure affect the accuracy of measuring the SE of enclosure. The analysis has included the impact of the physical dimensions of the receiving antenna, as well as its position within the enclosure, depending on the parameters of the excitation plane wave (the angle of polarization and the angle of plane wave incidence to the shield defined in the azimuth and elevation plane). The EM field level within the enclosure was evaluated in the dissertation on the basis of the current induced in the receiving antenna, which fully corresponds to the procedure of experimental characterization of the SE of enclosure. This way the influence of the coaxial cable which sends detected signal from the antenna to the measurement instrument was directly involved. The presence of the cable was described through compact TLM wire node which models the wire conductor, but also through appropriate impedance of the cable realized by lumped circuit, which loaded the receiving antenna. The accuracy of the results is verified by comparing with the measured results available in the literature and/or the results obtained by other methods (analytical, numerical). Among the expected results, which represent the scientific contribution of the dissertation, the following can be emphasized: analysis of changes in the SE of empty enclosure with rectangular apertures present on one or adjacent walls, depending on the polarization of the plane wave and its incident angle encountered on the enclosure; efficient characterization of EM coupling of a plane wave with wired elements present inside the enclosure with apertures and the estimation of the influence of this coupling on the SE enclosure using the integrated TLM approach for modeling of thin wire structures, which takes into account the two-way interaction between the wire and the excited EM field; creation of the numerical model of enclosure with receiving antenna that corresponds to the procedure of experimental characterization of SE enclosure where the level of EM field within the enclosure is estimated from the current induced in the receiving antenna; a detailed analysis of the impact of the physical dimensions of the receiving antenna (dipole or monopole antenna) and its position inside the enclosure to the level of detected EM field in the space within the enclosure, and therefore to the level of the SE, and to the position of the enclosure resonant frequencies; analysis of the impact of position and characteristics of the coaxial cable, through which the detected signal is sent from the receiving antenna to the measuring instrument, on the SE of enclosure. Presence of the cable is included directly by specifying a wire conductor or indirectly through matching cable impedance loading the receiving antenna; analysis of damping technique of enclosure resonant frequences (at which the SE of enclosure is the lowest) based on inserting an additional loaded antenna elements, in the form of a dipole antenna realized in microstrip technology, at corresponding positions inside the enclosure.
srpski
2015
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