Understanding Orthomode Transducers (OMT) in VSAT Systems

An OMT, or Orthomode Transducer, is a critical device in VSAT (Very Small Aperture Terminal) satellite communication systems. Its primary function is to separate or combine signals that have different polarizations.

Satellite signals are transmitted using specific polarization states, such as vertical and horizontal. The OMT is engineered to take these orthogonally polarized signals and split them into separate output ports, allowing each signal to be directed to the appropriate transmitter or receiver. This enables simultaneous transmission and reception of signals with different polarizations using a single antenna.

A typical VSAT system comprises a BUC (Block Upconverter), LNB (Low Noise Block Converter), satellite modem, antenna, and the OMT.

Transmit and Receive Process in VSAT

Here’s a breakdown of the transmit and receive processes within a VSAT system:

• Transmit:

  1. The satellite modulator (part of the modem) converts baseband information into a modulated IF (Intermediate Frequency) signal.
  2. This modulated IF signal is then upconverted and amplified by the BUC.
  3. The BUC output is connected to the antenna. The BUC functions as both an RF upconverter and a power amplifier.

• Receive:

  1. The RF signal received from the antenna is fed into the LNB.
  2. The LNB amplifies and downconverts the received signal. It essentially integrates the functionality of a Low Noise Amplifier and a downconverter.
  3. The downconverted signal is demodulated by the demodulator within the satellite modem.

The OMT serves as a passive microwave device, enabling both the transmitter and receiver components to interface with the same feed horn. The diagram below illustrates this setup.

Applications of OMT

OMTs find use in VSAT satellite communication and satellite TV applications. They feature connecting ports for two rectangular waveguides and one circular waveguide. In VSAT systems, OMTs handle uplink and downlink signals, each having different linear polarization orientations. The OMT at the VSAT terminal helps separate these signals, ensuring efficient communication.

Key Technical Specifications for OMTs

When selecting an OMT for VSAT and other applications, consider the following technical specifications:

• Waveguide Interfaces (Transmitter Port, Feedhorn Port, LNB Port)
• Frequency of operation in transmit and receive direction
• Return loss for transmit port and receive port
• Isolation between transmit and receive ports with same polarisation and orthogonal polarisation
• Insertion Loss between transmit port to receive port and between receive port to feedhorn port
• Maximum input power (Transmit port to feedhorn port)
• Operational temperature
• Weight

OMT Application in C-Band VSAT

OMTs are available for various frequency bands, including C-band, Ku-band, and Ka-band. Let’s examine a C-band VSAT application.

As depicted in the figure above, the OMT interfaces with two rectangular waveguide flanges and one circular flange. A BUC operating in the 6 GHz frequency band with horizontal polarization is used on the transmit side, while an LNB operating in the 4 GHz frequency band with vertical polarization is used on the receive side. The feed horn connects to the circular port of the OMT. The OMT functions as a transmit reject filter when the antenna is receiving the downlink signal.

OMT Manufacturers

Here are some notable manufacturers of OMTs:

• Smiths Microwave (www.millitech.com)
• Pasquali Microwave Systems (https://www.pasquali-microwavesystems.com/)
• Parabolic Networks (https://www.parabolicnetworks.com/)
• COMTECH EF DATA
• paradise
• Global Invacom Ltd. (https://www.globalprofessional.co.uk/)

Benefits of Using OMTs

Here are the key advantages of using OMTs in VSAT systems:

• Single Antenna Operation: OMTs enable the use of a single antenna to handle different polarizations (e.g., horizontal and vertical), reducing the need for multiple antennas. This simplifies the system and lowers hardware costs.
• Improved Communication Quality: By preventing polarization cross-talk and minimizing signal degradation, OMTs improve the overall quality of communication.
• Simplified System Design: OMTs simplify the design and installation of satellite communication systems.
• Versatility: OMTs support signals in various frequency bands, making them versatile components in VSAT and other satellite communication systems.
• Space and Cost Savings: OMTs contribute to space and cost savings by consolidating transmit and receive paths.

Conclusion

Overall, the Orthomode Transducer plays a vital role in maintaining signal integrity and minimizing interference in VSAT satellite communication systems by effectively managing signals with different polarizations. OMTs are also used in radio astronomy, remote sensing, and other microwave applications.