15 July 2010 A compact L-band Ortho Mode Junction
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We describe the design construction and performance of a L-band (1300-1800 MHz) Ortho Mode Junction for the L-P dual-band receiver to be installed on the 64 m Sardinia Radio Telescope (SRT), a new radio telescope which is being built in Sardinia, Italy. The Ortho Mode Junction (OMJ) separates two orthogonal linearly polarized signals propagating in a 172 mm diameter circular waveguide and couple them into four coaxial outputs. The OMJ is part of an OMT (Ortho Mode Transducer), which includes two 1800 hybrids allowing to recombine the out-of-phase signals from the balanced OMJ outputs. The OMJ consists of four probes arranged in symmetrical configuration across the circular waveguide. A shaped tuning stub with cylindrical profile is placed a quarter wavelength away from the probes to guarantee broadband operation with low reflection coefficient across L-band. The four identical probes have a cylindrical structure, each consisting of three concentric cylinders that attach to the central pin of standard 50 Ω 7/16-type coaxial connectors. The OMJ will be cooled at 80 K inside a compact dewar together with directional couplers and Low Noise Amplifiers. The two linearly polarized signals from an input 190 mm diameter room temperature L-band feed couple into the cryogenic dewar through a vacuum window located across the waveguide. Inside the dewar, the 190 mm diameter circular waveguide is tapered down to 172 mm using a conical transition (length 85 mm) filled with a Styrodur® foam that provides mechanical support for a 0.125 mm thick Kapton vacuum barrier. A 0.6 mm air gap across the 172 mm circular waveguide provides thermal decoupling between the ambient temperature and the 80 K OMJ, which is connected to the conical transition output.
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T. Pisanu, T. Pisanu, P. Marongiu, P. Marongiu, A. Navarrini, A. Navarrini, G. Valente, G. Valente, } "A compact L-band Ortho Mode Junction", Proc. SPIE 7741, Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy V, 774124 (15 July 2010); doi: 10.1117/12.856665; https://doi.org/10.1117/12.856665


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