Circularly Polarized multi-beam Antenna System for High-Altitude-Platforms (2013)

High Altitude Platform Stations (HAPS) are unmanned platforms positioned at stratospheric altitude, aiming to provide mainly telecommunication services as broadcasting repeaters.Communication services delivered by HAPS require the use of high gain circularly polarized multi-beam antennas.Multiple beams allow switching between different antenna footprints while increasing the capacity of the communication channel.On the other hand, circular polarization is the standard solution to overcome misalignments between transmitter and receiver and to mitigate multipath problems.In this context, the objective of this thesis is to contribute to the development of solutions for HAPS antenna subsystems.From the frequency point of view, the migration towards higher frequencies, providing larger bandwidths and hence higher channels capacities, is a continuous trend.Nowadays, the Ka-band (around 30 GHz) is considered very promising for this type of application.At these frequencies, one of the most useful implementations of the antenna subsystem is probably the use of a dielectric lens fed by several feeds.Taking into account a wide range of technical concerns, we have designed, prototyped and measured a multibeam dielectric lens antenna suitable for typical HAPS scenarios.In this type of antenna subsystem, the circular polarization can be obtained essentially in two ways: either the elementary radiators feeding the lens are already circularly polarized or they are linearly polarized and circular polarization is generated after the lens, with the help of an external polarizer.In the former situation, a polarizer has been optimized numerically, whereas in the second one, we have implemented a design procedure based on the combination of transmission line models with unit cell full wave analysis including periodic boundary conditions.Thus, we propose two effective designs, highlighting advantages and drawbacks of each one of these solutions.Our investigations are validated by prototype measurements in agreement with the predicted values.When a multibeam antenna system is mounted on platforms flying above Earth, the shape of the ground footprints has to be carefully controlled for different elevation angles.In this thesis, we propose a Ka-band multi-beam dielectric lens antenna design in which the beam symmetry is perturbed by acting on the lens shape, with the goal of generating circular footprints in the entire covered area.Such radiation characteristics are useful not only for HAPS, but for any scenario where the antenna system is mounted on platforms flying above Earth and, in general, for any application where the shape of the footprints has to be carefully controlled for different elevation angles.The new ellipsoidal shape is obtained with a set of simple analytical design equations, eventually enhanced by full-wave optimizations.Radiation pattern measurements show that such a lens minimizes the natural beam footprint v vi elongation unavoidable with traditional spherical lenses and confirm the validity of the proposed system.All the above results pave the way for future research oriented to the development of a complete on-board antenna payload for HAPS.