Research on Low Insertion Loss L-band Feed Front-end for FAST Core Array

A low insertion loss L-band feed front-end system have been designed to operate over 1.15-1.75GHz bandwidth for the FAST Core Array radio telescope. The L-band feed front-end system includes two parts: Quadruple-ridge Ortho-mode transducers (OMTs) and choke horn. The coaxial output of OMT extracts two mutually orthogonal polarization signals by coupling the input of a circular waveguide. Two non-cryogenic low temperature LNAs with average noise of only 10K at room temperature connected to coaxial outputs respectively. Because it operates at room temperature, the new developed OMT is required to have lower insertion loss. This paper focuses on the 1.15–1.75GHz L-Band OMT and choke horn design, which measured value of its return loss within the entire bandwidth is better than -20dB, with insertion loss better than 0.09dB. Considering the large quantity required for array applications, the design of OMT balances performance, adjustability, and ease of manufacture. The designed choke horn has high aperture efficiency throughout the entire bandwidth with symmetry far field pattern.