Zoom antennas for multiple spotbeam coverage of Europe

One significant feature of an elliptical orbit is that the coverage region of the satellite changes with altitude; the change being both angular and positional. To optimise the spacecraft system a zoom antenna is required rather than the more usual fixed focus variety. The purpose of the zoom antenna is to maximise the flux for all altitudes and to minimise the dynamic range over the coverage. This is achieved by matching the antenna beam shape to the curvature of the Earth. A study has been carried out for ESA to investigate various methods for implementing a `zoom capability´ in antennas for the Archimedes mission. The authors present an overview of a phased array antenna system that provides seven spot beams over Europe at L-band frequencies