The principal aim was to perform a trade-off along with preliminary designs of innovative multiple beam reconfigurable receive antennas for Ku-band telecommunication missions resulting in a reconfigurable antenna roadmap. The roadmap then classified the most promising concepts with regard to customer mission and payload requirements. During the course of the study various antenna concepts were analysed and compared both from a technical & economical viewpoint against a set of mission scenarios.
For the AFR architecture, two approaches were considered. A fixed reflector with a feed array large enough to generate the flexibility to reconfigure the coverage over the full earth, or a smaller feed array sufficient only to cover an instantaneous 8° area, while reconfigurability of the region is provided by steering the reflector.
The relative complexity, cost and performance of these was compared as well as detailed work to derive and improve crosspolar performance and the pointing performance for an AFR design. For the DRA architecture a trade-off exercise was carried out to determine the optimum radiating element size, number and lattice type (square or triangular). The number of control points and hence beamforming was simplified via decimation using “overlapping sub-arrays” The feasibility of a transparent Digital Processor was assessed to provide the flexibility and reconfigurability for both antenna architectures.
This project has facilitated a detailed design trade-off of two alternative antenna architectures to meet next generation missions in Ku band where coverage reconfigurability will be required. In both cases the technology limitations of analogue beamforming have been shown to be prohibitively complex and a need for high power, fast digital processing has been shown to be the only practical way to proceed.
To limit the complexity and performance demands made on the processor a novel method to reduce the number of control points, yet maintain performance and flexibility of the antenna coverage has been developed. This decimation technique could promote the use of active antennas in a shorter time frame than could previously have been envisaged.
The flexibility features of the Ku reconfigurable Rx antenna included:
- Capability to operate from several orbital slots and provide a back-up solution with several existing spacecrafts,
- Capability to reconfigure polarisation (linear or circular),
- Capability for in-orbit pattern reconfiguration,
- Capability to re-point and zoom beams on the complete visible earth,
- Capability to generate potentially large numbers of spot, elliptical or contoured beams without prohibitive beamformer complexity.
The major focus of the study was the two main active antenna types namely the Array-Fed Reflector (AFR) and the Direct Radiating Array (DRA).
The study was split into four major tasks:
Task 1: concluded with a set of reconfigurable receive antenna requirements.
Task 2: detailed trade-offs were performed on possible Rx antenna architectures and technologies.
Task 3: detailed RF design and analyses were performed for the two selected antenna architectures (one AFR and one DRA antenna).
Task 4: A roadmap for the technologies required in the two selected antenna concepts was derived.
The study has been completed, the major outcomes were:
A set of performance requirements for next generation Ku band Rx reconfigurable antennas was established.
Two antenna architectures were analysed in detail, one being an AFR, the other a DRA. The AFR is simpler than the very sophisticated DRA implementation reported in the study, though bulkier and more limited in performance. While complex in hardware terms, the DRA scales with beam numbers in a manner which brings it within range of digital beamforming. Much simpler DRA implementations are possible, subject to relaxation of assumptions regarding grating lobes suppression and/or electronic reconfiguration of the coverage region.