A compact L-band Ortho Mode Junction

T. Pisanu; P. Marongiu; A. Navarrini; G. Valente

doi:10.1117/12.856665

15 July 2010 A compact L-band Ortho Mode Junction

T. Pisanu, P. Marongiu, A. Navarrini, G. Valente

Proceedings Volume 7741, Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy V; 774124 (2010) https://doi.org/10.1117/12.856665
Event: SPIE Astronomical Telescopes + Instrumentation, 2010, San Diego, California, United States

Abstract

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.

Citation Download Citation

T. Pisanu, P. Marongiu, A. Navarrini, and 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); https://doi.org/10.1117/12.856665

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