Orthomode Transducer (OMT): Advantages and Disadvantages

This page explores the benefits and drawbacks of Orthomode Transducers (OMTs).

What is an Orthomode Transducer (OMT)?

Introduction:

An Orthomode Transducer is a three-port waveguide component that combines horizontally polarized and vertically polarized signals to create a circularly or elliptically polarized signal. Essentially, each of these signals occupies its own dedicated port.

Conversely, an OMT can also receive a circularly polarized signal and divide it into horizontal and vertical planes with different polarizations.

Because of these capabilities, it’s also known as a polarization duplexer. OMTs are commonly used in terrestrial microwave radio links and ground-based satellite systems.

OMT - Orthomode Transducer

The figure above shows a C-band orthomode transducer (OMT) used in a VSAT system. It’s mounted with a feed horn antenna at the center of the VSAT dish. The circular port connects to the circular horn antenna, and the other two ports connect to two rectangular waveguides.

As shown, the transmit port is connected to the output of a power amplifier (PA) after RF up-conversion, or it can be directly connected to a BUC (Block Up Converter). A BUC contains both a PA and an RF Up-converter.

The receive port is connected to an LNA (Low Noise Amplifier) or an LNB (Low Noise Block Converter). An LNB contains both an LNA and an RF Down Converter.

Orthomode transducers are available for various frequency bands, making them easily adaptable to specific application needs. Common frequency bands for OMTs include C-band, X-band, Ku-band, K-band, and Ka-band.

In summary, the two major functions of an Orthomode Transducer (OMT) are:

• It separates and directs signals of two different polarities.
• It acts as a T-junction device, providing connectivity between two rectangular waveguides and one circular waveguide.
• It determines the isolation required for circular polarization.

A C/Ku-band OMT serves two terminals simultaneously at C-band and Ku-band. It separates incoming signals based on polarization and frequency and delivers the signals to two distinct ports for further processing by the respective C/Ku-band modules.

One such C/Ku-band OMT has been developed by the Space Applications Center division of ISRO Ahmedabad, INDIA.

Benefits or Advantages of Orthomode Transducer (OMT)

The following are the benefits or advantages of an Orthomode Transducer (OMT):

• It allows the same VSAT to be used for simultaneous transmission and reception by functioning as a polarization duplexing device. This increases satellite capacity by enabling frequency re-use at different polarizations.
• Polarization adjustments are not required in circular polarized antenna systems because the polarization is fixed by the OMT.
• It provides superior cross-polarization performance.
• It’s very small in size, minimizing RF signal blockage.
• It’s lightweight and easily installed by a single RF technician, reducing installation time.
• Modern OMTs, such as the C/Ku Orthomode transducer, allow for the combination of separate C-band VSAT and Ku-band VSAT terminals into a single system. This leads to significant cost savings due to reduced infrastructure hardware.
• It offers low insertion loss, good return loss, and good isolation at the desired ports.

Drawbacks or Disadvantages of Orthomode Transducer (OMT)

There aren’t many significant drawbacks or disadvantages to Orthomode Transducers (OMTs).

• Using an OMT in a wireless system can increase the overall cost of the antenna system. However, this factor has become less significant due to advances in manufacturing techniques.
• When selecting an OMT, it’s crucial to choose one with good return loss for all three ports and good isolation between ports. Poor return loss and isolation can degrade the performance of the wireless satellite system using the OMT.