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What is OMT | Orthomode Transducer used in VSAT

This page covers VSAT OMT (Orthomode Transducer) basics.It mentions Orthomode Transducer (OMT) application note used in VSAT. The link to VSAT basics is provided.

Refer VSAT tutorial covering following.

• Satellite Terminology  • VSAT system overview  • SCPC  • MCPC 

As mentioned OMT is the short form of Orthomode Transducer.

As we know VSAT system composed of BUC, LNB, Satellite modem, Antenna and OMT. Let us understand transmit and receive process in VSAT.
• Satellite modulator(part of modem) converts baseband information to modulated IF frequency. This Modulated IF is upconverted and amplified using BUC (Block Up converter). The same BUC output is interfaced with antenna. BUC has functionality of both RF up converter and power amplifier.

• received RF signal from antenna is fed to LNB (Low Noise Block Converter). This LNB includes both LNB and Down converter functionality. Hence received signal is amplified and down converted. The down converted signal is demodulated using demodulator part of satellite modem. Refer VSAT system overview for detailed understanding of interfacing between various VSAT subsystems.

OMT-Orthomode Transducer

In order to use same antenna between transmit and receive systems, OMT is used. It is a passive microwave device which allows interfacing of transmitter part as well as receiver part to be connected with the same feed horn. It is used in VSAT to satellite communication as well as satellite TV applications. It has connecting ports for two rectangular and one circular waveguide. The figure-1 depicts sketch of OMT.

Following technical specifications are considered for purchasing the OMT for VSAT and other applications.
• 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 VSAT Application note

OMT VSAT application note

As we know OMT is available for various frequency bands as per need such as C band, Ku band and Ka band. We will understand OMT used for C band VSAT application.

As shown in the figure-2, OMT is interfaced with two rectangular waveguide flanges and one circular flange. BUC with 6 GHz frequency band with horizontal polarisation is used in transmit side and LNB with 4 GHz frequency band with vertical polarisation is used in receive side. Feed horn is interfaced with circular port of the OMT(Orthomode Transducer). OMT acts as transmit reject filter when antenna is receiving the downlink signal.

OMT Manufacturers

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

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