C-, Ku- and Ka-band communications and broadcast satellites use
so-called dual-gridded reflector antennas for linear polarisation (LP) to
provide independent reflector surfaces and/or independent feeds for the two
orthogonal polarisations.
C-, Ku- and Ka-band communications and broadcast satellites use
so-called dual-gridded reflector antennas for linear polarisation (LP) to
provide independent reflector surfaces and/or independent feeds for the two
orthogonal polarisations.
ASC proposed and lead an initial work to extend this concept to circular polarisation (CP). The effort was supported by ESA, but originated from the strong reflector surface modelling team that was a part of the EU FP6 Network of Excellence ACE. First we defined preliminary specifications (supported by experts from main European satellite operators) and identified several antenna concepts based upon planar transmission-type sheet polarisers. The left-hand side figure shows one antenna concept with a rotated LP diplexer, a planar CP polariser and a possibly large solid reflector. Next we identified a number of polarisers of which the most promising are the meander-line, the L+C strip-grids and the parallel-plate polariser. Finally, analyses and preliminary designs were carried out of the sheet polarisers - on the antenna level as part of a reflector antenna system, on the polariser level, and on the sheet level inside the polarisers. Rotating the CP polariser ±45° provides switching between dual LP and dual CP operation
Part of the work identifying a number of antenna and polariser concepts is summarised in:
Current Ka-band multiple spot beam satellite
antenna systems often use one feed per beam, but involve a high number of
reflector antennas (four for transmit and four for receive, or four for
transmit/receive) to realise both acceptable crossover levels and spillover
losses. Several developments go on to reduce the number of reflectors to one
for transmit and one for receive at the cost of much complexity in the feed
array and beamforming. Orthogonal efforts aim at developing antennas that both
receive and transmit.
ASC developed with ESA a new concept (French patent FR2897722 and US patent US7522116), where a shaped reflector reduces both the number of feeds per beam and the number of reflectors to one. The price paid is an oversizing of the reflector making the reflector area comparable to the total area of the conventional four-reflector solution. The figure to the left shows superimposed pattern cuts in various pattern planes for a number of beams. Some scan degradations occur for the large reflector (due to astigmatism in the large offset reflector) and the gain variation is rather high, but these problems may be overcome. A higher performance is achieved in the receive band, where the reflector is electrically larger.
A summary of the initial work is available in:
Currently there is an interest in
large Ku-band satellite reflector antenna systems
with multiple frequency reuses by polarisation and sidelobe isolation of 30-33 dB over
regional or "linguistic" beams tailored to specific geographical areas. While the
sidelobe isolation is achieved by specifying a minimum distance between beams that
reuse the same frequency band, the reflector offset limits the polarisation isolation.
Most methods for reducing the crosspolarisation are not suitable due to the large
reflector diameter.
ASC investigated for ESA several concepts aiming at solving the issue. Some of the results obtained are described in the publications:
Multibeam antennas (MBAs) may be defined as antennas able to generate multiple independent beams simultaneously from a single aperture. MBAs play increasingly important roles, e.g. in mobile base stations, on-board satellites and in advanced radars. They add more functions to the systems that they are a part of, increase the capacity of the systems, preserve the available frequency spectrum and other limited assets, and reduce interference.
Peter Balling contributed an overview article on MBAs to the 6-volume Encyclopedia of RF and Microwave Engineering edited by Kai Chang. Excerpts from an early version of the article is available here