VSAT

A Very-Small-Aperture Terminal (VSAT), is a two-way satellite communication system generally with a dish shaped antenna with a diameter under 4.5m
A VSAT terminal communicates via a geosynchronous orbit satellite in a ‘star configuration’ to a master ‘Hub’ or Earth Station which will usually consist of a much larger satellite dish from 4.5m up to as large as 18m in diameter. A VSAT may also communicate directly with another VSAT as well as the ‘Hub’ which is known as a ‘mesh configuration’.
VSATs are most commonly used to transmit narrowband data applications such as point of sale transactions, RFID data, SCADA information, or broadband data services for corporate data networks and internet access.

Typical components of a VSAT terminal will consist of the following:

1. Antenna
2. Block Up Converter (BUC)
3. Low-noise block downconverter (LNB)
4. Orthomode transducer (OMT)
5. Interfacility link cable (IFL)
6. Indoor unit (IDU)

The outdoor components on the dish are collectively known as the ODU (Outdoor Unit), consisting of the OMT, BUC and LNB. The IDU is generally a satellite modem (Modulator and Demodulator) which transforms the analogue signal received from the satellite into a digital data stream typically through an ethernet interface.

1. Antenna

The parabolic shape of a dish reflects the signal to the dish’s focal point. Mounted on brackets at the dish’s focal point is a device called the feedhorn. This feedhorn is essentially the front-end of a waveguide that gathers the signals at or near the focal point and ‘conducts’ them to a low-noise block downconverter or LNB. Similarly, the feedhorn conducts the transmitted signal from a block up converteror BUC and transmits the signal onto the dish which then retransmits the amplified signal to the satellite.

2. Block Up Converter (BUC)

A block upconverter (BUC) is used in the transmission (uplink) of satellite signals. It converts a band (or “block”) of frequencies from a lower frequency to a higher frequency. Modern BUCs convert from the L band to Ku band, C band and Ka band. Older BUCs convert from a 70 MHz intermediate frequency (IF) to Ku band or C band.
Most BUCs use phase-locked loop local oscillators and require an external 10 MHz frequency reference to maintain the correct transmit frequency.
BUCs used in remote locations are often 2 or 4 W in the Ku band and 5 W in the C band. The 10 MHz reference frequency is usually sent on the same feedline as the main carrier. Many smaller BUCs also get their direct current (DC) over the feedline, using an internal DC block.
BUCs are generally used in conjunction with low-noise block converters (LNB). The BUC, being an up-converting device, makes up the “transmit” side of the system, while the LNB is the down-converting device and makes up the “receive” side. An example of a system utilizing both a BUC and an LNB is a VSAT system, used for bidirectional Internet access via satellite.
The block upconverter is assembled with the LNB in association with an OMT, orthogonal mode transducer to the feed-horn that faces the reflector parabolic dish.

3. Low-noise block downconverter (LNB)

A low-noise block downconverter (or LNB) is the receiving device of a VSAT terminal. The LNB is a combination of Low-noise amplifer, block downconverter and IF amplifier. The LNB converts the signals from electromagnetic or radio waves to electrical signals and shifts the signals from the downlinked C-band and/or Ku-band to the L-band range which is then fed into the indoor Unit via the IFL cable.

4. Orthomode transducer (OMT)

An orthomode transducer commonly referred to as an OMT, or a polarisation duplexer forms part of the VSAT antenna feed. The OMT combines or separates the transmit and the receive signal paths. The OMT serves in an essential role as the junction element of the outdoor, unit (ODU). It protects the LNB from burn-out by the power of the output signal generated by the BUC through a transmit reject filter. The BUC is also connected to the feed horn through a wave guide port of the OMT.
Wherever there are two polarisations of radio signals Horizontal and Vertical. The transmitted and received radio signal to and from the antenna are said to be “orthogonal”. This means that the modulation planes of the two radio signal waves are at 90 degrees angles to each other. Various configurations of OMT are available to support co-pol, cross-pol, linear and circular polarizations, the choice is dependant on the satellite payload configuration.

5. Interfacility link cable (IFL)

The IFL for a VSAT is generally a length of coaxial cable connecting the BUC and LNB to the IDU/Satellite modem. This cable will normally carry either an IF or L-Band signal as well as a 10MHz reference signal as well as a 24V dc voltage to supply power to the BUC and LNB. Careful consideration is needed in the choice of cable and considerations must be made for the connector types required, the RF losses in the cable and the current load capacity of the cable.

6. Indoor unit (IDU)

The IDU for most VSAT applications will consist of a satellite modem processing the analogue signals into digital data. The satellite modem will normally provide the dc power for the BUC and LNB as well as the 10MHz reference signals.

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