Design and development of spherical array antennas

Array antennas have been developed that can achieve electronic beamsteering over wide angles. With circular array antennas, a radiation pattern can be steered over 360 azimuthal degrees without pattern deterioration. Some applications may require an omnidirectional beamsteering capability in azimuth as well as in elevation, with antenna characteristics that are independent from the direction of the beam. To achieve this without mechanical actuators requires an antenna that is homeomorphic to the sphere. Spherical array antennas are the 3-dimensional equivalent of circular array antennas and offer a full omnidirectional beamsteering coverage. Using standard Fourier techniques, the analysis and synthesis of a circular array can be simplified. The technique is known as phase mode theory and allows a number of signal process ing techniques such as electronic direction finding to be demonstrated. The symmetry found in spherical array antennas can equally be exploited using spherical harmonics as a basis for Fourier analysis. Spherical array antennas have not enjoyed the same attention as circular arrays, nor have they been the subject, with the exception of SONAR arrays, of a profound study that is equivalent to phase mode theory. This thesis aims to put spherical arrays on the same foot as circular arrays, by using spherical modes to build the theoretical framework for the analysis, synthesis and design of spherical array antennas. Using a technique for conformal array antennas, an holistic model for the spherical array antenna is derived to analyse the radiation from complex spherical arrays, based on analytically obtained expressions. Mutual coupling effects are quantified and included in the calculations. Also some typical antenna characteristics and design considerations are reviewed in the light of modal theory, in order to obtain design formula's for the spherical array antenna. Apart from the electromagnetic approach, attention is given to the processing of the signals. Modal theory lends itself for the development of smart antennas, and the foundations to do that with spherical arrays are laid down. A particular problem arises when one wants to distribute the array elements evenly over the spherical surface. This topic is also given an in-depth treatment, for distributions of single elements as well as for more advanced subarraying techniques. To back up the findings from theory and simulation, as well as to demonstrate the unique capabilities of the spherical array antenna, a prototype array has been designed and partially constructed. The design and development of this antenna is covered to the point where a partially completed spherical array could be measured. Results of these measurements are encouraging and sufficient to show the potential of spherical array antennas. Finally, suggestions are given for future experiments and how to further advance spherical array technology