Abstract |
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Ultra-Wide Band (UWB) radar systems are widely used for different applications such as imaging systems and Ground Penetrating Radar (GPR). To realize such UWB antennas, different characteristics in terms of geometry and materials need to be used. Three-dimensional metallic antennas attracted great attention for systems which require the miniaturization of antennas. The objective of this thesis is to add insights into the design and analysis of 3-D antennas and arrays for ground-penetrating, imaging and wireless communication applications. This is done by contributing to the development of fast numerical methods able to analyze large antenna arrays and, in particular, by adding the contribution of the ground on antennas placed close to it. The thesis intends to explain the design of 3-D multiband antennas and circular arrays of UWB antennas, based on both numerical simulations and validation with measurement. In the analysis of antenna arrays, the mutual coupling phenomenon has to be seriously taken into consideration. The radiation patterns of a single element in an array differs from that of the same element when it is isolated, due to the mutual coupling contribution. One proposed solution to mitigate the mutual coupling effect is to design the array in a connected, possibly circular, shape. This solution avoids the current distributions flowing along the edges of each individual antenna, and allows them to flow through every consecutive element. The radiation patterns of all elements in the circular array will be the same, except for a rotation with a certain phase shift, which allows us to use the Array Scanning Method (ASM) to analyze large arrays. The results for design and fabrication of various single antenna prototypes are presented with good correspondence between simulation and measurement. The validation of applying the ASM on a circular array of Vivaldi antennas is presented through a comparison of the brute-force MoM system of equation with the ASM solution. The fabrication of a novel circular array of Vivaldi elements is also revealed with very good agreement between simulation and measurement. We named the prototype the ”Wheel-of-Time (WoT)”. The WoT prototype will be used for locating and imaging targets in the area of radiation exposure. An initial near-field image of a metallic tube in the area of radiation exposure is depicted, and the target is localized. The WoT prototype with two different metallic cylinders have been simulated using the ASM applied to the Method of Moments. The resolution between metallic cylinders is investigated experimentally and numerically. Preliminary results are also provided regarding reconstruction of a dielectric cylinder positioned in the area of radiation exposure. An important component in the development of water leak detection is to design a 3-D UWB antenna, i.e., to detect underground leaking pipes. Lowfrequency antenna design is challenging due to the expected large dimensions of the radiator. The bandwidth of interest is set from 0.25 GHz to 2.5 GHz. A UWB Vivaldi antenna design has been proposed with 60411.5 cm dimensions. We took as an objective to minimize the antenna weight by introducing holes in specific positions related to the current distributions. The proposed antenna was fabricated and measured at UCL. The comparison between measurements and simulations is very satisfactory, with a bandwidth from 250 MHz up to 2 GHz under a -10 dB criterion in terms of reflection coefficient. Preliminarily tests have been carried out to examine the effect of the nearby ground on the measurement. The trials have first been carried out using the CST commercial software with two different permittivities, and it shows the effect of the ground appears at low frequencies in terms of reflection coefficient. For the GPR application, the antenna is very close to the medium, such as the lossy ground, which will strongly modify the antenna input impedance and radiated near-fields. This modification can be added on antennas by the explicit use (or pre-computation) of the layered-medium Green’s function. The newly proposed method gives an alternative efficient solution by introducing the effect of the ground via a reflection coefficient defined in a 2-D spatial-spectrum domain. This allows the evaluation of a matrix that exclusively accounts for the contribution of the ground to the MoM impedance matrix of the antenna. This is then added to the free-space MoM matrix, which can be computed once and for all. Moreover the extra matrix including the ground contribution can make use of precomputed radiation patterns of every basis function on the antenna, which are independent from the ground parameters. We name the method Fast Ground Contribution Matrix (FGCM). The number of samples in (complex) spectral domain is dramatically reduced by explicitly compensating truncation with aliasing errors in the spectral integration. The FGCM method is able to simulate the antenna response for arbitrary soil parameters (permittivity, conductivity, permeability, etc.), assuming multi-layered media. To demonstrate the accuracy and efficiency of the proposed method, numerical results for a 3-D metallic Vivaldi and typical dipole antennas are presented. A good agreement among the exact Method of Moments (MoM) solutions, the simulation results, and measured data is observed over an ultra-wide bandwidth. Finally, we gathered the FGCM method with Macro Basis Functions (MBFs) and the Contour-Fast Fourier Transform (C-FFT) methods to analyze large planar antenna arrays placed vertically above a layered substrate. The FGCM method was successfully applied to a regular array of dipole antennas, taking the benefit of accelerated computations of interactions between MBFs using the C-FFT method. |