Please read the abstract in the section 00front of this document / Thesis (PhD (Electronic Engineering))--University of Pretoria, 2007. / Electrical, Electronic and Computer Engineering / PhD / unrestricted

Acquisition of ultra-wideband signals by means of array antennas requires essentially frequency-independent radiation characteristics over the entire bandwidth of the signal in order to avoid distortions. Factors contributing to bandwidth limitation of arrays include array factor, radiation characteristics of the array element, and inter-element mutual coupling. Strictly speaking, distortion-free transmission or reception of ultra-wideband signals can be maintained if the magnitude of the radiated field of the array remains constant while its phase varies linearly with frequency over the bandwidth of interest. The existing wideband-array synthesis methods do not account for all factors affecting the array bandwidth and are often limited to considering the array factor and not the total field of the array in the synthesis process. The goal of this study is to present an ultra-wideband array synthesis technique taking into account all frequency-dependent properties, including array total pattern, phase of the total radiated field, element field, element input impedance, and inter-element mutual coupling. The proposed array synthesis technique is based on the utilization of frequency-adaptive element excitations in conjunction with expressing the total radiated field of the array as a complex Fourier series. Using the proposed method, element excitation currents required for achieving a desired radiation pattern, while compensating for frequency variations of the element radiation characteristics and the inter-element mutual coupling, are calculated. An important consideration in the proposed ultra-wideband array design is that the "phase bandwidth", defined as the frequency range over which the phase of the total radiated field of the array varies linearly with frequency, is taken into account as a design requirement in the synthesis process. Design examples of linear arrays with desired radiation patterns that are expected to remain unchanged over the bandwidth of interest are presented and simulated. Two example arrays, one with a wire dipole as its element and another using an elliptically-shaped disc dipole as the element are studied. Simulation results for far-field patterns, magnitude and phase characteristics, and other performance criteria such as side-lobe level and scanning range are presented. Synthesis of two-dimensional planar arrays is carried out by employing the formulations developed for linear arrays but generalized to accommodate the geometry of planar rectangular arrays. As example designs, planar arrays with wire dipoles and elliptical-shaped disc dipoles are studied. The simulation results indicate that synthesis of ultra-wideband arrays can be accomplished successfully using the technique presented in this work. The proposed technique is robust and comprehensive, nonetheless it is understood that the achieved performance of a synthesized array and how closely the desired performance is met also depends on some of the choices the array designer makes and other constraints, such as number of elements, type of element, size, and ultimately cost. / Ph. D.

Numerical techniques for designing planar arrays with low side lobe level often require memory intensive optimization algorithms and also initialization in the form of some known values of radiation pattern parameters beforehand - information that may not be available when designing arrays. A few analytical methods exist in the literature that can be used to design rectangular lattices of isotropic elements for desired half-power beamwidth and side lobe level, but the number of elements of the array often should be known before the design process. Some array designs based on analytical techniques may suffer from severe performance limitations, an example is the uniformly excited array which cannot produce side lobe levels below ̶13.3 dB. The goal of this study is to present an analytical technique for synthesis of planar arrays that, for specified radiation pattern requirements, not only provides quick solutions for the required number of elements and its distribution along the length and width of the array rectangular lattice, but also produces low side lobes without any limitation. A new class of non-uniformly excited equally spaced planar arrays is introduced and investigated in this study. The new array uses the patterns of uniformly excited linear arrays as its building blocks and has a separable element current distribution, hence making it mathematically convenient to analyze its radiation properties in terms of those of its constituent linear arrays. The proposed planar array does not suffer from the side lobe level limitation of uniformly excited planar arrays, and its synthesis, due to the analytical nature of description of its radiation properties, does not require iterative procedures that are inherent to numerical techniques. Radiation characteristics of the proposed planar array, including directivity, side lobe level, half-power beamwidths, far-field three dimensional radiation patterns, and element excitation currents, are examined and simulation results for several example cases are presented. The analysis culminates with successfully mapping a continuous radiation pattern to discrete element currents in a rectangular lattice geometry. The synthesis procedure is validated by successfully designing various planar arrays with desired requirements in terms of side lobe level and half-power beamwidths in the principal planes. Several design examples are presented. Radiation characteristics of the synthesized arrays are compared with the desired design requirements which were used as input information in the synthesis process. For the cases studied, the achieved performance characteristics are close to the desired ones. / MS / A group of similar antennas that radiate with equal intensity in all directions and each fed with a certain current amplitude is called an isotropic antenna array. When the distance between the adjacent antenna elements in an array is constant, it is called an equidistant array. Furthermore, when the array elements take up the geometry of a rectangular lattice, they are referred to as rectangular planar array. In antenna array design, it is always desirable that the isotropic array is set up in a manner that it establishes a stronger communication link in the desired direction while minimizing any communication in other directions. This can be achieved by changing the spatial footprint of the array’s radiation also known as its array factor. The array factor can be altered by either adjusting the inter-element spacing or changing the current amplitude of each element. A great number of techniques have been proposed over the years that aim to minimize communication in undesired directions. Most of these techniques achieve the objective by employing computationally complex algorithms that require a lot of time and memory. For other synthesis techniques, some design information is required to be known beforehand which may not be possible. The aim of this study is to present an easier analytical approach that designs rectangular planar array for specified radiation pattern requirements. The radiation requirements entail the direction in which the communication link is required to be established and the extent to which radiation in other directions is permissible – characterized by side lobe level of the array factor. The strength of the communication link is measured by directivity and half power beam width of the array factor. It is found that the proposed array in the study minimizes communication in unwanted directions in a much more effective manner than a rectangular array with each element fed with an equal current amplitude – also known as uniformly excited planar array. The performance of the proposed planar array in terms of directivity, half power beam width and side lobe level is first simulated and presented. The design procedure is then validated by designing various planar arrays with desired radiation requirements. Several design examples are presented. Radiation characteristics of the synthesized arrays are compared with the desired design requirements which were used as input information in the synthesis process. For the cases studied, the achieved performance characteristics are close to the desired ones.