Orthomode transducer

Lore & 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.

Reader's Guide

The orthomode transducer is significant for enabling dual-polarized operation in VSAT systems and terrestrial microwave links, allowing simultaneous transmission and reception on orthogonal polarizations over a single waveguide. In VSAT applications, the OMT provides essential protection to the sensitive LNB by isolating the high-power BUC output (1, 2, or 5 watts) from the low-power received signal (on the order of μV), with typical isolation of 40 dB in Ku and Ka bands. Further isolation up to 100 dB can be achieved with additional radio frequency filtering. The device is used in dual-polarized VSATs, sparsely populated areas, radar antennas, radiometers, and communications links. Its legacy lies in its role as a fundamental junction element in outdoor units, enabling frequency reuse through polarization diversity and protecting receiver front-ends from damage. In terrestrial links, the OMT supports high-capacity point-to-point paths by allowing two radios to share one dish, increasing throughput or providing redundancy. The characterization of OMTs remains a delicate measurement challenge, requiring careful mechanical alignment to avoid errors in cross-polarization discrimination measurements.

Did You Know?

• An orthomode transducer is also called a polarization duplexer.
• In VSAT Ku band operation, a typical OMT provides -40 dB isolation, meaning only 0.01% of transmitter output is cross-fed into the receiver port.
• The OMT protects the LNB from burn-out by the BUC's output signal, which can be 1, 2, or 5 watts for common VSAT equipment.
• In terrestrial microwave links, a T-shaped OMT can separate a feed signal into horizontal and vertical polarity radios to increase data throughput.