A Preliminary Analysis of a Design of the L-Band Multi-Beam Feed for the FAST

In this paper,the theory of defocus is applied to the design of the L-Band Multi-Beam Feed for the FAST.By analyzing the relation between the defocus distance of a feed horn and the deflection angle of the corresponding beam and considering the requirement of the multi-beam radio-telescope scan method, we derive the working-frequency bandwidth, the distance between neighboring horns, and the limit on the aperture of a single horn.Futhermore, we give a detailed analysis of the gains of the main beams and the side beams from the horns at different positions in a regular-hexagon array.The results show the feasibility of adopting a 19-beam feed design for the FAST based on the working-frequency bandwidth and the aperture limit of a single horn.We additionally discuss the choices of the types of the OMT and the horns.This paper effectively provides an outline of the L-Band Multi-Beam Feed for the FAST,which indicates the direction for future work.