This open-access review presents the design, fabrication, and testing of a broadband millimeter-wave orthomode transducer (OMT) for the CARUSO cryogenic focal-plane array receiver at the Sardinia 64-m radio telescope.

Key points:

• Operating band: 70–116 GHz, covering an extended W-band range.
• Purpose: Separate an incoming signal into two orthogonal linear polarizations for dual-polarization radio astronomy measurements.
• Architecture: A turnstile junction receives the signal through a custom circular-waveguide interface and provides two orthogonally polarized outputs through standard WR10 waveguide ports.
• Receiver context: The OMT is intended for each of CARUSO’s 16 dual-polarization pixels.
• Design approach: Electromagnetic optimization used proprietary design software together with a commercial 3-D electromagnetic field solver.
• Manufacturing approaches compared:
  1. Platelet construction
  2. Split-block direct machining
• Testing: Both versions were fabricated and characterized at room temperature.
• Final selection: The split-block design was chosen for integration into CARUSO because it offered advantages in machining, assembly, compactness, and suitability for reliable multiple-unit production.
• Motivation: Compared with free-standing or dielectric-supported polarization grids, waveguide OMTs are more compact, mechanically robust, and better suited to densely packed cryogenic focal-plane arrays.

The work addresses the main engineering challenge of achieving wide bandwidth, low insertion loss, good polarization isolation, compact packaging, and repeatable fabrication at millimeter wavelengths.