A compact planar orthomode transducer

In this paper, the design of a compact orthomode transducer (OMT) using substrate integrated waveguide technology (SIW) is presented. The orthomode signals come from a square waveguide and are coupled to four SIW lines through four wide slots placed on the end wall of the square waveguide. The out-of-phase signals from each opposite slot pairs are combined using a simple planar configuration to provide the two final uncoupled orthomode signals. This arrangement provides a compact and low insertion loss circuit. A back-to-back OMT having a central operating frequency of 30 GHz was fabricated. Measured insertion losses, including the losses due to two microstrip to SIW transitions and two SMA connectors used in the measurements, are less than 3.8 dB over a 1 GHz frequency band. Over this bandwidth, the return losses are lower than 10 dB and the isolations are better than 24 dB. Each OMT has a compact size of 1.1mm × 20.9mm × 20.7 mm when designed on a 20 mil thick Rt/duroid 5880 substrate with a dielectric constant of 2.2. Design guidelines are also provided.