K-band orthomode transducer with waveguide ports and balanced coaxial probes

We describe the design and test results of a K-band orthomode transducer consisting of orthogonal balanced coaxial probes in a 0.455-in-diameter circular waveguide linked by coaxial transmission lines to balanced coaxial probes in orthogonal WR42 waveguides. We built this device to model at large scale (∼10 : 1) the performance of a compact millimeter-wave transducer for use in an astronomical receiver. The identical construction of the dual channels results in the same phase and insertion-loss characteristics for both polarizations. Measurements show that, from 18 to 26 GHz, the orthomode transducer has a reflection coefficient of below -15dB (standing-wave ratio <1.4) and a cross-polarization coupling from -20to-40dB. The insertion loss over most of the band is ∼0.4dB at room temperature. To scale this device for operation at 200-270 GHz, one must connect corresponding probes by coaxial lines with phase imbalance less than 15°, equivalent to a line-length difference less than 0.0015 in, to mitigate periodical resonances. Probes must be fabricated and positioned with 0.001-in accuracy. The predicted insertion loss would be 0.25 dB at 4 K.