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What Is an Orthomode Transducer?

By Geisha A. Legazpi
Updated May 16, 2024
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An orthomode transducer is a passive microwave device that allows the microwave transmitter and receiver to transmit and receive, respectively, on the same feed horn or antenna. In microwave communication circuits, the transmission line, as well as the antenna, deals with the electromagnetic (EM) wave. The orthomode transducer is used mostly on transceivers used by satellite data and satellite television.

In microwave transceivers, the waveguide is a device that acts as the transmission plumbing, which has the same function as a transmission line for lower radio frequency (RF) applications. At frequencies more than a few tens of gigahertz (GHz), where 1 GHz is a billion cycles per second, using a coaxial cable, or transmission line, to connect to the antenna loses all the signal in the coaxial cable. One approach is to skip the transmission line to take the microwave transceiver to where the antenna is, which is usually mounted on high elevation on a radio tower. The waveguide was later used as a transmission line equivalent, although it does not carry pure electrical energy as do the other transmission lines. It carries an EM wave, which is the same energy format as the over-the-air propagation of radio waves.

The orthomode transducer uses the principle that two microwave EM emissions of the same frequency will interfere if these two EM waves are polarized the same way. Polarization has to do with the relative position of the magnetic component of the emission. A vertically polarized emission will have a vertical magnetic field and a horizontal electric field. Moreover, the orthomode transducer operates on two emissions of very near frequencies where one is orthogonally, or 90 degrees, separated from the other in EM orientation. Orthomode transducers maintain the orthogonal separation and allow the transmitted and received signals to coexist on the same antenna.

Orthomode transducers have three ports: feed horn or antenna port, transmitter port, and receiver port. They are the equivalent of frequency duplexers that use the so-called tuned radio cavities that have geometries correlated to the over-the-air geometries of RFs. These geometrical sizes of RFs are referred to as wavelengths. If the electric field sees a conductive geometry that is a multiple of half-wavelengths, it just passes through, but if it sees a conductive geometry that is an odd multiple of a quarter-wavelength, it gets shorted out.

In the waveguide and in the orthomode transducer, the dimension of the hollow cross section has to be a suitable function of the signal wavelength. The orthomode transducer routes transmitted energy to the antenna or feed horn, and protects the receiver by preventing the transmitted energy from reaching the receiver. It takes the incoming signal from the feed horn and routes it to the receiver. Orthomode transducers also make use of the polarization difference to isolate the transmitted and received signals.

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