Orthomode transducer

Background

The orthomode transducer (OMT) is a contraction of orthogonal mode. In VSAT and satellite Earth station applications, the transmission and reception paths are at 90° to each other, orthogonally polarized. This orthogonal shift provides approximately 40 dB isolation in the Ku band and Ka band.

The OMT serves as the junction element of the outdoor unit (ODU) of a VSAT modem, protecting the low-noise block downconverter (LNB) from burn-out or saturation by the power of the output signal from the block up converter (BUC). The BUC is connected to the feed horn through a waveguide port of the OMT. The device unites both uplink and downlink paths, which are at 90° to each other, at the feed-horn focal point of the parabolic antenna.

In terrestrial microwave radio links, a T-shaped orthomode transducer is mounted at the rear of the feed on each parabolic dish, separating the signal into two separate radios—one operating in horizontal polarity and the other in vertical polarity. This arrangement increases aggregate data throughput or provides fault-tolerance redundancy. Some outdoor microwave radios have integrated orthomode transducers and perform cross-polarization interference cancellation (XPIC) within the radio unit. The OMT may also be built into the antenna, allowing connection of separate radios or separate ports of the same radio.

An orthomode transducer can be modeled as a 4-port device, with two single-polarization ports (H and V) and a dual-polarized port embodying degenerate modes. The scattering parameters form a 4×4 matrix, symmetrical for a reciprocal OMT. An ideal OMT exhibits perfect matching, unitary direct transmission, and infinite cross-polarization discrimination (XPD) and isolation. Characterization of a manufactured OMT is delicate; mechanical misalignment can introduce artificial leakage that affects measured XPD.