The wideband feed system on the Australia Telescope

From the outset the Australia Telescope has been designed to permit operation over a very wide frequency range covering frequency bands within the range 1.25-116 GHz. The lower end of this range is determined by the requirement to observe red shifted hydrogen which has a line at 1.42 GHz while the upper limit is to encompass the carbon monoxide line at 115.3 GHz. Extending the operation of the telescope down to the deuterium line at 327 MHz is also anticipated as is the inevitable increase in the number of bands required for observations within this extensive frequency range. Given a 22-m dia. Cassegrain antenna is to be the array element, the optics and feed system necessary to cover the very wide frequency range provides a formidable challenge to the antenna designer. The authors give an overview of the feed system that was developed to meet these demands