Symmetric Reverse-Coupling Waveguide Orthomode Transducer for the 3-mm Band

We describe the design, construction, and performance of a waveguide orthomode transducer (OMT) for the 3-mm band (84-116 GHz). The OMT is based on a symmetric backward coupling structure and has a square waveguide input port (2.54 mm times 2.54 mm) and two single-mode waveguide outputs: a standard WR10 rectangular waveguide (2.54 mm times 1.27 mm), and an oval waveguide with full-radius corners. The reverse coupling structure is located in the common square waveguide arm and splits one polarization signal in two opposite rectangular waveguide sidearms using broadband -3-dB .E-plane branch-line hybrid couplers. The device was optimized using a commercial 3-D electromagnetic simulator. The OMT consists of two mechanical blocks fabricated in split- block configuration using conventional CNC milling machine. From 84 to 116 GHz, the measured input reflection coefficient was less than -17 dB, the cross polarization was less than -30 dB, the isolation between the outputs was greater than 50 dB, and the insertion loss was less than 0.35 dB at room temperature for both polarization channels. The device is suitable for scaling to higher frequency.