Symmetric Waveguide Orthomode Junctions E.J. Wollack 1 and W. Grammer 2 National Radio Astronomy Observatory 2 949 North Cherry Avenue, Tucson, AZ 85721 Laboratory for Astronomy and Astrophysics 1 NASA/Goddard Space Flight Center Greenbelt, MD 20771 email: edward.wollack.1@gsfc.nasa.gov ABSTRACT – Imaging applications at millimeter and submillimeter wavelengths demand precise characterization of the amplitude, spectrum, and polarization of electromagnetic radiation. The use of a waveguide orthomode transducer (OMT) can help achieve these goals by increasing spectral coverage and sensitivity while reducing exit aperture size, optical spill, instrumental polarization offsets, and lending itself to integration in focal plane arrays. For these reasons, symmetric OMTs are favored over a traditional quasi-optical wire grid for focal plane imaging arrays from a systems perspective. The design, fabrication, and test of OMTs realized with conventional split- block techniques for millimeter-waveguide bands are described. The design provides a return loss of ~20 dB over a full waveguide band. The observed cross-polarization and isolation are better than ~40 dB for tolerances readily achievable in practice. Prototype examples realized in WR10.0 and WR3.7 waveguide bands are presented. Keywords: Orthomode Transducer, Polarization Diplexer, Waveguide Techniques INTRODUCTION: Receiver systems for radio astronomy require high-performance polarization- discrimination components. We report on the fabrication and performance of a wideband linear polarization diplexer for millimeter wavelengths. The designs considered here build on the concepts laid out by Brain (1978) at the Marconi Research Laboratory and first reported in a true split-block configuration by Boifot (1991). This two-fold symmetric junction achieves full waveguide band performance by limiting the excitation