A Low Insertion Loss L-band Feed Front-end for the FAST Core Array

We designed a low insertion loss L-band feed front-end system to operate over a 1.15-1.75 GHz frequency range for the Five-hundred-meter Aperture Spherical radio Telescope (FAST) Core Array. The L-band feed front-end system comprises quadruple-ridge ortho-mode transducers and a choke horn. The coaxial output of the transducers extracts two mutually orthogonal polarization signals by coupling the input of a circular waveguide. Two non-cryogenic low temperature low-noise amplifiers with average noise of only 10 K at room temperature connected to coaxial outputs respectively. Because it operates at room temperature, the new developed ortho-mode transducer requires lower insertion loss than in designs intended to operate in extremely cold environments. This paper focuses on the 1.15–1.75 GHz L-Band ortho-mode transducer and choke horn design, with a measured L-band return loss of more than −20 dB, and an insertion loss of more than 0.09 dB. Considering the large quantity of front-end systems required for array applications, our design of ortho-mode transducers balances performance, adjustability, and ease of manufacture, and our choke horn design has high aperture efficiency across the L-band.