New antenna configurations based on reflectarrays to generate multi-spot coverage in ka-band from geostationary satellites

Abstract

Today's satellite communications for broadband services, such as Internet access, are predominantly provided by geostationary satellites known as high throughput satellites (HTS). HTS systems split the service area into a large number of small cells by means of multiple beam antennas onboard the satellites. The multi-spot coverage generated from HTS follows a four-color reuse scheme, where the spot-beams are generated at two different frequencies and two orthogonal polarizations. As a result, two adjacent spots never coincide at the same frequency and polarization (same color), so the spots generated with the same color are spatially isolated from each other and the interferences between adjacent spots are reduced. The operation by a four-color multi-spot coverage makes it possible to improve the spectrum utilization, maximizing the throughput of the users, while the offered bandwidth of the system remains unaltered. Current broadband satellite communications commonly operate in Ka-band, generating the four-color coverage simultaneously in transmission (Tx, 19.2-20.2 GHz) and reception (Rx, 29-30 GHz). The coverages are typically formed by around fifty or a hundred of slightly overlapping spot beams, where the beamwidth is around 0.65º. The design of multi-beam antennas in Ka-band for HTS faces some challenges due to the large number of beams generated in different colors with a very small angular separation between adjacent spots (around 0.56º). The most extended antenna farm in HTS is formed by four single-offset reflector antennas in a one-feed-per-beam configuration, operating simultaneously at Tx and Rx frequencies in Ka-band, where each reflector generates the beams in a single color. However, the use of four antennas onboard the satellite implies limitations in terms of accommodation and stowing of the satellite payload. In addition, accurate pointing systems have to be associated to each of the four antennas because a small movement in one of the reflectors or in one of the associated feeding clusters may cause the corresponding beams to move as compared to the other ones and reducing the antenna performance. The main motivation of this PhD thesis has been to develop new antenna solutions based on reflectarray antennas that make it possible to halve the number of antennas and feed chains required onboard the satellite to produce the four-color multispot coverage in Ka-band. In this way, two reflectarrays would produce the total coverage instead of four reflectors, resulting in a significant saving in weight, volume, and cost in the payload. To this end, two different strategies have been evaluated: first, a reflectarray has been proposed to generate a complete four-color coverage in a single band to exclusively operate in transmission or reception. The second strategy is based on the design of a reflectarray to generate one half of the required four-color beams simultaneously in the Tx and Rx bands. In relation to the first strategy, a novel design method has been proposed to generate four spaced beams per feed in four different colors, by operating at two different frequencies and two orthogonal polarizations. The beam squint effect in offset fed reflectarrays has been exploited to overcome the main complexity of the proposed design, which is the independent operation at relatively close frequencies. As a first approach, the proposed design technique has been applied to the preliminary design and simulation of a 1.8 m reflectarray defined with ideal reflectarray cells. The simulations have demonstrated the capability of generating a four-color coverage formed by 108 spots in the transmit band with a single reflectarray illuminated by 27 dual-polarized feeds. Thus, the four reflector antennas commonly used onboard the communications satellites in Ka-band could be replaced by one reflectarray operating in transmission and a second reflectarray operating in reception. A flat 43 cm reflectarray prototype has been designed, manufactured and tested for the first time to experimentally demonstrate the generation of four spaced beams per feed in four different colors. The analysis and design of the prototype has been carried out by a home-made simulation tool based on the Method of Moments in the Spectral Domain (SD-MoM) and the local periodicity approach. The measurements of the 43 cm reflectarray have been used to validate the accuracy of the developed analysis and design tools. Moreover, the results have proven that the beam squint effect can be used to naturally focus spaced beams generated by the same feed at different frequencies while the same phenomenon is partially compensated in each operating sub-band, by optimizing the dimensions of the printed elements in the reflectarray. Concerning the second strategy, an original design method for parabolic reflectarrays has been developed to generate two spaced beams per feed in orthogonal circular polarizations (CP) at Tx and Rx frequencies in Ka-band. The proposed technique makes it possible to design a reflectarray to generate half the required four-color multi-spot coverage. Firstly, a new reflectarray cell has been defined to operate in dual-band and dual-CP by means of the Variable Rotation Technique (VRT). The operating principles of the VRT, typically formulated for normal incidence, have been extended for oblique incidence. Then, a new design method for parabolic reflectarray antennas has been developed based on the previous cells, including an in-band optimization routine to reduce the cross-polar radiation at off-center frequencies and a novel technique to equal the shaping of the beam focused by a parabolic surface at separate frequencies (Tx and Rx in Ka-band). The proposed design method has been firstly adapted to a flat reflectarray to experimentally demonstrate the generation of two spaced beams per feed in orthogonal CP simultaneously at Tx and Rx frequencies in Ka band. A 25-cm reflectarray based on the new dual-band dual-CP cells has been designed, manufactured and tested. The prototype has been designed by an in-house design tool, following the technique developed for parabolic reflectarrays applied to a conventional flat structure, including the new in-band optimization. The measured radiation patterns of the manufactured prototype have satisfactory validated the design techniques and analysis tools for flat reflectarrays. A 1.8-m parabolic reflectarray has been designed by using the developed techniques to generate one half of the required multi-spot coverage simultaneously at Tx and Rx frequencies in Ka-band for broadband satellite communications. The parabolic reflectarray, designed to generate two spaced beams in orthogonal CP per feed, has been illuminated by 27 dual-CP feeds to produce 54 spot-beams in two different polarization colors. The conducted simulations have shown a correct beam distribution at both Tx and Rx frequencies in Ka-band, proving that two parabolic reflectarrays operating at slightly different frequencies can provide a complete coverage of 108 spots within a four-color reuse scheme, which would make it possible to halve the number of feeds and antennas required for current multi-spot satellites in Ka-band, from four reflector antennas to two parabolic reflectarrays. Finally, a model of the real 1.8-m parabolic reflectarray antenna scaled in a factor 1/2 has been designed, manufactured and measured to experimentally demonstrate the generation of two spaced high-gain beams per feed in orthogonal CP simultaneously at Tx and Rx frequencies in Ka-band. The 0.9 m parabolic reflectarray was the final demonstrator of an ESA project, coordinated by the UPM group. The results of this demonstrator have been completely satisfactory and have validated the different techniques developed in this thesis. These techniques include: 1) the optimization procedure to reduce the cross-polar radiation in a prescribed frequency band, overcoming one of the major limitations identified in previous works, and 2) the technique for phase correction at higher frequency, which has been used to equalise the beamwidth in Tx and Rx. The results presented in this thesis have demonstrated new reflectarray configurations to reduce the number of antennas in multi-spot satellites, considering "one feed per beam" architecture, which shows the potential of reflectarray antennas for satellite applications.