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 (1.3–1.8 GHz) of the Sardinia Radio Telescope (SRT). The complete OMT consists of a cylindrical orthomode junction (OMJ), which is presented in this paper, and two identical 180^\circ hybrid power combiners in a 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 the <?Pub _bookmark Command="[Quick Mark]"?>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. [ABSTRACT FROM PUBLISHER]