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Difference between OMT (Orthomode transducer) and TRF (Transmit Reject Filter)

This page compares OMT (Orthomode transducer) vs TRF (Transmit Reject Filter) and mentions difference between OMT and TRF.

What is Orthomode Transducer (OMT) ?

It is used along with the feed horn antenna to receive the EM waves from satellite or to transmit EM waves towards satellite. It changes circular waveguide shape into two rectangular waveguides. One of these is connected with Tx chain and the other is connected with Rx chain. OMT can be used to convert circular polarized signal into linear polarized signals of horizontal and vertical types.

OMT vs TRF

The figure depicts OMT (Orthomode Transducer) for C band VSAT terminal. OMT has three ports. As shown, port-1 is connected with BUC, port-2 is connected with LNA or LNB via TRF and port-3 is connected with circular tube of the feed horn antenna. TRF blocks transmit power to reach LNA (Low Noise Amplifier) or LNB (Low Noise Block Converter).

Following are the typical specifications of a C Band OMT.
• Frequency Band : Tx/Rx frequencies in C band
• Insertion Loss : 0.3 to 0.6 dB
• Isolation : 30 to 35 dB

What is Transmit Reject Filter (TRF) ?

The filter used to reject frequencies designed to be used for transmission towards receive path of the satellite earth station is known as TRF or Transmit Reject Filter. Hence TRF prevents undesired frequencies being reached to receiver modules such as LNA or LNB (combination of LNA and down converter). Hence it prevents damage to LNA/LNB. The figure-2 depicts C Band Transmit Reject Filter (TRF) used for INSAT.

Transmit Reject Filter

Following are the typical specifications of a C Band TRF.
• Pass Band : 4.5 to 4.8 GHz (Receive Band used in INSAT)
• Reject Band : 6.725 to 7.025 GHz (Transmit Band used in INSAT)
• Insertion Loss : Approx. 0.5 dB (Maximum)
• Rejection : 80 dB (minimum)
• VSWR : 1.3: 1 (Maximum)
• Dimensions, Operating Temperature range, Flanges etc.

Difference between OMT and TRF

Following points summarize difference between OMT and TRF.
➨OMT has three ports and it is connected between horn feed, Tx path and Rx path. TRF has two ports and it is connected between Rx port of OMT and LNA/LNB.
➨OMT passes both the Transmit/Receive frequencies at different polarizations i.e. horizontal and vertical linear polarizations. TRF blocks transmit frequencies to prevent damage to LNA in the receiver chain.
➨OMT is basically a polarizaton duplexing device where as TRF is a passive RF filter.



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