A Compact L-Band Orthomode Transducer for Radio Astronomical Receivers at Cryogenic Temperature

We describe the design, construction and performance of a compact orthomode transducer (OMT) for the L-band receiver (1.3 GHz - 1.8 GHz) of the ardinia Radio Telescope (SRT). The complete OMT consists of a cylindrical orthomode junction (OMJ), which is presented in this paper, and two identical 180° hybrid power combiners in double ridged waveguide, which have been proposed in a previous work. The OMT operates at the cryogenic temperature of 20 K to reduce its thermal noise contribution to the noise of the receiving chain. Therefore, particular care has been taken in the design of the OMJ to minimize its dimension and insure a good thermalization. The proposed OMT has been designed and optimized by using CST Microwave studio, and then manufactured, tested at room temperature, and installed on the L-band receiver of SRT. The measured results fully comply with the design specifications. In particular, the isolation between the OMT output ports is more than 40 dB, and the cross polarization is less than -35 dB for both polarization channels.