Marantis, L; (2014) Spherical array antennas for LEO Satellite communications. Doctoral thesis , UCL (University College London).

Abstract

The trend in the construction of the latest array antennas is such, that electronic beam scanning can be conducted for a significantly large angular sector. Indeed, as far as circular array antennas are concerned, the quality of the radiation pattern will not be impaired when scanned in the full azimuthal axis. Compared to planar and several different shapes of conformal antenna arrays, spherical array antennas have the unique property of scanning their beam with no deterioration in full spherical coverage (i.e. azimuth and elevation axes). Furthermore, multi-beam functioning and direction finding techniques can be applied to spherical array antennas. In contrast to antennas that use mechanical motors for steering, they exhibit higher speed and stability. Low Earth Orbit (LEO) satellite communications can utilize the aforementioned properties of spherical array antennas. Spherical harmonic (phase mode) theory constitutes the base on which the signal processing of the spherical array antenna is developed. In this analysis, spherical harmonics are employed, which take advantage of the sphere’s symmetry and regenerate themselves in the far-field. The spherical phase mode approach is able to provide considerable improvements and computational simplification in several array processing levels. Specifically, research efforts are concentrated toward the choice of an optimal uniform spherical distribution and the minimum number of array elements that are necessary in the process. Various antenna element distributions are investigated and compared to each other placing special emphasis on the design and fabrication efficiency. Additionally, the direction finding potential of a spherical array antenna is explored by applying one of the main DOA estimation algorithms used with spherical arrays, the spherical ESPRIT algorithm. A spherical array, called LISA, has been designed and a prototype demonstrator has been manufactured. The spherical array has a radius of 40 cm, and operates at 3 GHz, employing circular polarization. A multi-planar approach, with 240 triangular planar tiles approximating the spherical surface, is adopted. Special attention is given to the RF front end of the array, and especially to the development of the microstrip antenna design that is accommodated on each tile (three elements) and a bespoke feed network. Valuable simulation and measurement results are also provided, demonstrating satisfactory performance of the tested tiles and offering essential conclusions for discussion and further investigation.

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