|Sažetak (engleski)|| |
This dissertation is focused on the analysis and design (modeling) of electromagnetic structures based on the Gap Waveguide technology. There are several commercial software tools available for solving wide spectrum of electromagnetic problems, including different types of periodic structures (Gap Waveguide contains two-dimensional periodic surface of metallic elements (scatters) inside parallel-plate waveguide), however, the analysis based on those commercial software tools is very complex and especially very time consuming. Based on these arguments, it was necessary to develop specialized software (code) dedicated for solving problems related to the structures based on the Gap Waveguide technology. In order to design concrete structures, such software code can make the whole process of optimization much easier. In this dissertation, the procedure for obtaining two different software codes is explained. First code is based on the Method of Moments and the other on the Mode Matching technique. The first code showed good results, however the complexity and the running time grew very quickly when the structure was enlarged. On the other hand, the software code based on the Mode Matching technique gave even better results and it was much faster in the calculation of some important parameters of several different structures based on the Gap Waveguide technology. Furthermore, a new type of Laky Wave antenna is also presented. The antenna is based on the Gap Waveguide technology, or to be more precise it is a modified Groove Gap Waveguide. Classic Leaky Wave antennas, until now, have been designed in order to have separated transmitting and radiating part. Proposed design, represents a compact form of the Leaky Wave antenna, where transmitting and radiating part are not separated and the final result is a significant reduction of the overall antenna dimensions. The first part of the Introduction discusses modern trends in communication industry. These trends are mostly related with the increase of the operational frequencies, which sets additional requirements on the existing components (components that are already incorporated in various communication systems). However, current components exhibit problems on higher frequencies, such as unwanted leakage of energy. New components based on the Gap Waveguide technology show very good properties on higher frequencies and, because of its novelty, they represent a very interesting research field. This explains the reason why this thesis is focused on that kind of technology. In the continuation of the first chapter, a brief introduction to the Gap Waveguide technology is presented and first examples of different structures based on that technology are highlighted from the available literature. After that basic operating properties of those structures are explained. Namely, Gap Waveguides are structures which consist of two-dimensional periodic surfaces made of metallic elements (pins) situated between two metal plates (parallel-plate waveguide). It is shown that Gap Waveguide ensures frequency stop band, meaning that in a particular frequency band the structure stops electromagnetic waves, i.e. there is no propagation of the electromagnetic energy along the structure. Of course, in order to achieve frequency stop band certain conditions related to the dimensions and periodicity of the structure have to be fulfilled. Frequency stop band is used in order ensure minimum lateral leakage of energy but the structure can be modified in order to make some transmition paths where energy will be guided. This is achieved by adding or removing the metallization inside the structure so that the electromagnetic waves can freely propagate. In that way, strong localization of energy along the desired paths can be achieved. According to that, some examples of transmition structures based on the Gap Waveguide technology are presented and certain advantages and disadvantages are highlighted in comparison with standard electromagnetic components. The complexity of these structures shows the necessity for detailed analysis of Gap Waveguide structures. That kind of structures can be analyzed with many commercial software tools (full wave analysis) but this procedure is mostly very time consuming and demands significant computing resources. Therefore, it is useful to develop adequate software support. This is the key part of this work which was mostly oriented to develop simple and fast software code for the analysis of structures based on the Gap Waveguide technology. The main advantage of that kind of software is the ability to calculate certain parameters very fast (in comparison with commercial software) and with sufficient accuracy. At the end of the of the Introduction the structure of the whole dissertation is presented. The beginning of the second chapter describes some basic electromagnetic structures (Coaxial Line, Rectangular Waveguide, Planar Line). Relevant specifications of these structures are highlighted including their advantages, disadvantages and limitations. Those limitations show the necessity for the introduction of some different structures such as Gap Waveguides. Detailed operating principles of Gap Waveguides are explained and their advantages are clarified. Gap Waveguides are structures that ensure stop frequency band in which energy cannot freely propagate through the structure. The important thing is that stop band can be adjusted by choosing specific dimensions of the metallic elements inside the structure. The stop band is crucial in order to reduce the leakage of energy, i.e. to achieve localized transmition of energy with minimum losses. Based on that idea, some practical applications are shown. For example, Gap Waveguides can be used as transmition structures, where energy is guided along the desired paths with minimum of lateral leakage of energy. After that, Gap Waveguides can be used as radiating structures, i.e. top metal plate with radiating slots must be located exactly above some kind of transmition structure mentioned earlier. Gap Waveguides can be used to design resonant structures (filters) where resonant cavity is surrounded with metallic elements from all sides. The dimensions of the resonant cavity determine resonant frequency. Finally, interesting application of the Gap Waveguides is for the protection of sensitive electronic equipment, i.e. for packaging. For example, inside the package (cavity metal box), instead of using absorbers, metallic elements (which ensure stop band) can be used in order to suppress resonating cavity modes which can harm sensitive electronic equipment. In the third chapter the complete analysis procedure for the Gap Waveguides is presented. First algorithm is based on the Method of Moments (MoM), and the second is based on the Mode Matching (MM) technique. In both cases, the very first problem was to solve electromagnetic scattering from one single metallic element placed between two metal plates. The MoM algorithm implies that correct current distribution, on the element of interest, must be solved in order to compute the scattered field form that element. After that, all other elements (two-dimensional array of elements) can be included in the analysis procedure. The developed program code, based on the Method of Moments, was very slow because expressions that represent the scattered field from all elements inside the structure were extremely complex (multiple number of slow converging summations). Because of that, it was difficult to quickly and correctly (in comparison with commercial software tool) calculate wanted parameters. In order to calculate the parameters of the Gap Waveguide structures faster and more correctly, Mode Matching technique was applied in the analysis instead of the Method of Moments. The basic advantages of the Mode Matching technique are: (a) higher accuracy (using Method of Moments the surface current on the top of the metallic elements is neglected), (b) the dimension of the system of linear equations, which has to be solved, is smaller (there is no need to first solve the surface current distribution on the metallic elements; it is sufficient to directly solve the scattering field and (c) the elapsed time for the calculation is much smaller (there is smaller number of unknowns and accordingly the elapsed time for solving the linear system of equations is smaller; it is sufficient to calculate scattered filed from some elements only once). It is important to mention that for calculation of some specific parameters (scattering S parameters), it was necessary to include models of the real sources in the analysis (sources bring energy to the structure). It was done through a combination of the Method of Moments (model of source) and Mode Matching technique (analysis of the periodic part of the structure made of metallic elements). At the end of the chapter some practical structures are analyzed with the developed program code based on the Mode Matching technique. First, dispersion diagram was calculated for the structure with thin metallic elements (thin in comparison with wavelength). After that, transmition line based on the Groove Gap Waveguide was calculated, i.e. dispersion diagram and scattering parameters (reflection and transmition) were calculated for that structure. There was a good agreement between results obtained with developed program code and the results obtained with commercial software tool. However, when thicker elements were used for the design of the transmition line, there was a significant difference between those two results in the dispersion diagram. The problem was solved by calculating the dispersion diagram with the developed program code in a different way; it was desirable to observe the amount of the field that penetrates through the structure in the desired frequency band - transition). By observing the boundaries of the stop band in that way there was a good agreement between results obtained with the developed code and the results obtained with commercial software, i.e. it was possible to correctly determine lower and upper frequency of the stop band. Calculation of the scattering parameters (reflection and transmition) of that structure also showed good agreement between Mode Matching and commercial software results. The conclusion was that developed software is a good tool for calculation of some important parameters of the Gap Waveguide structures. At the end of the chapter important differences between two procedures (Method of Moments and Mode Matching technique) are shown and at the same time the benefits of the Mode Matching technique in this specific case are stressed. In the fourth chapter a new type of Leaky Wave antenna based on the Gap Waveguide technology is presented. The presented antenna represents a compact solution where transmitting part (groove waveguide) and radiating part (one row of periodic metallic elements which supports the leakage of energy outside of the structure) are not separated. The developed software code, explained in the third chapter, was very useful in this case, because it was possible to calculate some parameters of the antenna very fast. After that, commercial software was used for the final design. The chapter covers the detailed explanation of the design procedure, fabrication and measurements of the two prototypes of the antenna. The idea was to design Leaky Wave antenna that operates around 10 GHz (X band). First task was to pick appropriate dimensions to achieve stop band around 10 GHz. After that, those dimensions were used to construct transmition line (Groove Gap Waveguide). That structure does not leak energy, i.e. there are no radiation losses and most of the inserted energy at one port of the structure arrives to the second port of the structure (transmition parameter is very high). However, in order to produce radiation, it was important to modify that structure by removing some parts (some rows of elements). In that way, the structure starts to radiate energy (leakage occurs) and the structure becomes Leaky Wave antenna. First prototype (design) of the antenna didn’t give satisfying results in terms of radiation properties. Namely, the amount of radiated power was too small which means that the antenna was not efficient, i.e. the gain of the antenna was to small in the whole frequency band. Moreover, the gain of the antenna was decreasing as frequency was increased which is not a desired behavior. Also, the position of the main lobe changed with the frequency, which is a normal property of the Leaky Wave antennas in general. Due to all of the above, it was necessary to design a new antenna with better radiation properties. Accordingly, it was very important to thoroughly study radiation losses for this kind of antennas. For this purpose, an approximate formula was developed that can help predict the radiation properties of the antenna before the final design, i.e. at the beginning of the design procedure. The formula shows that the width of the groove has the strongest influence on the radiation properties of the antenna. During the design procedure of the second antenna, special attention was devoted to achieve constant gain in the desired frequency band. That was successfully achieved by gradually changing the width of the groove. Moreover, the second prototype (design) is twice longer than the first prototype. It all together resulted with better radiation properties of the new antenna prototype. Gain of the second antenna was much larger than the gain of the first antenna in the whole frequency band. Comparison between the simulations and the measurements were very good for both antennas. Last chapter represents the recapitulation of the whole dissertation where some conclusions are presented. Also, in this chapter some future research plans are highlighted. In the future there is a possibility to improve the developed program code in order to have the ability to calculate particular parameters much faster. The improvement regarding the Leaky Wave antenna is the intention to move to higher frequencies (30 GHz or 60 GHz). Those frequencies will be used in the next generation of mobile communication systems and that is the main reason why those frequency bands are now interesting. However, on those high frequencies, all the dimensions of the antenna are very small (metallic elements are very small) and it is important to have very good manufacturing precision, which is very challenging task. In this dissertation these three contributions are fulfilled: • Analytical model of the waveguide which contains the periodical structure which provides frequency stop band. The model is based on the Mode Matching technique and on the analysis of the system with many scatterers. • The expansion of the analytical model on the class of electromagnetic structures (transmition lines, antennas, resonators, filters) which include Gap Waveguides. The method is based on the hybrid combination of the Method of Moments and Mode Matching technique. • New type of the antenna based on the Gap Waveguide technology which has integrated transmitting and radiating part.