A satellite feed has three major components: the uplink, the downlink and the satellite. The uplink includes all of the equipment required to send usable data to the satellite, while the downlink includes all of the data required to receive and decode this data. Between these two is the satellite.
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The satellite uplink portion of a feed begins with data entering the modulator/demodulator (MODEM). The baseband or intermediate frequency (IF) carrier then flows from the MODEM to the upconverter. The signal leaves the upconverter at the transmit frequency and enters the high-power amplifier (HPA). The signal leaves the HPA for the antenna, where it is uplinked to the satellite.
The data from the end user first enters the MODEM, also known as the encoder. The MODEM places the information on an IF carrier, typically at 70MHz. The IF from the MODEM enters the upconverter and is multiplied to the transmit frequency. While increasing the frequency is the primary purpose of the upconverter, it also slightly amplifies the signal. The transmit frequency from the upconverter than moves to the HPA, where it is greatly increased in preparation for the long trip to space. Next, the amplified signal at the transmit frequency goes to the antenna, where it is amplified, focused and sent to the satellite.
The three major types of amplifiers are Klystron tubes, travelling wave tube amplifiers (TWTAs) and solid state power amplifiers (SSPAs). Klystron tubes are the oldest of these technologies and without modification are effective for only a frequency span of about 40MHz. TWTAs are a newer technology and are capable of transmitting for the entire 500MHz frequency span of a typical satellite. SSPAs are the newest technology; they are capable of reaching the same power levels as TWTAs while using much less power.
As a general rule, a larger antenna requires less power for a signal to reach a satellite. Also, some antennas are not designed to transmit a signal at all. These antennas are called "receive only," and an example is a satellite television dish on a roof. These antennas cannot transmit because a massive amount of power would be required for signals to reach a satellite, and the signal would be very unfocused, causing power to radiate in a very wide area.
Downlink is basically the reverse of uplink. The signal goes from the satellite to the antenna, where it travels through a low-noise amplifier (LNA). At that point, it is boosted back to a usable level. The signal moves from the LNA to the downconverter, which lowers the signal frequency to IF. The IF signal then travels from the downconverter to the MODEM, where the signal is stripped from the carrier and the data is sent to the end user.
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