Abstract
Broadband communication satellites in Ka-band employ multi-beam antenna systems that produce a high number of slightly overlapping spot beams to provide a cellular coverage. In order to minimize the interference level between adjacent spots, two different frequencies and polarizations (four colors) are used to generate the beams, both in transmission (19.2-20.2 GHz) and reception (29-30 GHz) bands, so that two adjacent spots never present the same color (same frequency and polarization). Typically, four reflector antennas operating in transmission and reception are required to generate the four-color coverage with slightly overlapping spots, one reflector per color, since conventional reflectors cannot provide closely spaced beams. The four reflector antennas can be seen as a not very efficient solution, considering the severe space and weight constraints in satellite systems. As a consequence, different attempts have been made in order to reduce the number of antennas, using direct radiating arrays, active lenses or reflectarrays.
In this Master thesis, a new design concept of a reflectarray antenna is proposed to produce four adjacent beams per feed through the simultaneous use of polarization and frequency discrimination. Beam squint effect in offset-fed reflectarray antennas has been used to generate two adjacent beams in different frequencies, so that, the feed position is computed to produce the beams in the required directions. The other two beams in orthogonal polarization are generated by implementing in the reflectarray a different phase-shift for each polarization.
The proposed concept can be suitable for multiple spot-beam satellites in Ka-band, enabling a reduction in the number of antennas and feeds needed to provide the four-color multi-spot coverage. The coverage could be provided by two reflectarray antennas (one for transmission and one for reception) instead of four reflectors, so the number of apertures would be reduced by a factor of two, just like the number of feeds, since each feed will produce four beams. This change will lead to a substantial mass saving on the spacecraft.
This Master thesis presents the design, manufacturing and measurements of a 43-cm reflectarray antenna prototype, which has demonstrated for first time that a reflectarray is capable of generating four adjacent beams in different polarizations and frequencies (4 colors) with a single dual-polarized feed. Moreover, this thesis includes the preliminary simulations for a real satellite antenna, proposed as a 1.8-m reflectarray, to produce a multi-spot coverage formed by 108 contiguous beams separated 0.56 degree using 27 feeds. Finally, the thesis concludes with an essay of parabolic reflectarray antennas for multi-beam applications in Ka-band.
