Cryogenic 29–50 GHz Orthomode Transducer for Radio Astronomical Receiver

A larger than 50% fractional bandwidth cryogenic orthomode transducer (OMT) based on turnstile junction is designed and fabricated. The OMT has one circular waveguide of 7.2 mm diameter input port and two standard WR-22 output ports. At room temperature, it exhibits better than 18 dB return losses across 29 to 50 GHz with 0.5 to 1.5 dB insertion loss. Cross-polarization and port isolation are better than 38 dB. The performance of the OMT is further tested under cryogenic condition for radio astronomical receiver application. The measured insertion loss is 0.5 to 2.0 dB while the return losses are better than 10 dB at 77 K between 29 and 50 GHz. Cryogenic cross-polarization is better than 36 dB.