High-Performance Reverse-Coupler OMT With Submicron Alignment for the 209–281 GHz Band

We describe the design, construction, and performance of a waveguide orthomode transducer (OMT) for the 209–281 GHz frequency band. The device is one of three candidates being considered for deployment in the upgraded Atacama Large Millimeter/submillimeter Array (ALMA) Band 6 receiver, known as “Band 6v2,” currently under development by the National Radio Astronomy Observatory (NRAO). The OMT is based on a symmetric reverse coupler structure. It has a circular waveguide input port (diameter 1.29 mm) and two single-mode oval waveguide output ports with full-radius corners matched to WR3.7 rectangular waveguide (0.94 mm × 0.47 mm). A circular-to-square waveguide transition is used on the input side. The two oval waveguide outputs have E-plane orientations parallel to each other and are located on opposite sides of the OMT module. The device was optimized using a commercial 3D electromagnetic simulator. The OMT consists of a split-block assembly, fabricated using a conventional CNC micromilling machine. It was tested at room temperature using a commercial vector network analyzer equipped with WR-3.4 frequency extension modules. An initial OMT design demonstrated excellent performance but was susceptible to micron alignment shifts. These shifts, caused by thermal contraction during cryogenic cooling, resulted in inconsistent isolation and cross-polarization. To overcome these problems, innovative alignment and module locking techniques were developed to ensure stable OMT operation with thermal cycling. This article details these methods and their successful implementation. Across the 209–281 GHz band, the measured input and output return losses exceed 17 dB, the room temperature insertion losses are less than 0.5 dB, isolations surpass 52 dB and cross-polarizations are greater than 40 dB for both polarization channels. The device meets the requirements set for the ALMA Band 6v2 OMT. The OMT design is scalable to higher frequencies, and the alignment and locking techniques are suitable for submillimeter applications.