Coaxial feed array for a short focal-length reflector

The design of array feeds for deep dishes is complicated by the need to closely pack the elements to minimize scan gain loss and coma sidelobes. An example is the design of a 4-element multibeam feed for the Lovell radio telescope at Jodrell Bank, UK, which is prime-focus fed reflector with a diameter (D) of 76.2 m and f/D=0.3, where f is the focal length. The multibeam feed was developed to reduce the time required for large-area surveys of the sky near the 21 cm wavelength (1.33-1.43 GHz). A suitable multibeam feed for the Lovell radio telescope has been designed by CSIRO. However, the design approach could be used in other applications requiring an array feed for a short focal-length reflector. To design the multibeam feed, new software that analyses circular/coaxial horn arrays by a mode-matching method was used. The effects of mutual coupling between the feeds, discontinuities and steps in the elements and a finite-sized ground plane are included in the analysis. This paper describes the design of the feed array and gives the predicted and measured results for a C-band model. Results are described also for when the array is used as a feed for the Lovell radio telescope.