C-band

C-band is a band of frequencies used for terrestrial radio relay and satellite transmissions in satellite communications. In satellite communications C-band is commonly called 6/4 GHz to illustrate that the uplink frequencies are around 6 GHz and the downlink frequencies are around 4 GHz. The band of frequencies are between 5.925-6.425 GHz for uplinks and 3.7-4.2 GHz for downlinks.

In C-band the communications equipment is relatively inexpensive since it is similar to the terrestrial microwave, troposcatter, and radar equipment that has been developed over the past several decades. Consequently, C-band was the first part of the spectrum to be applied extensively to satellite communications and is still the most widely used due to the low cost and wide availability of components. The background noise of the sky (due to galactic, solar and terrestrial sources) is small—cosmic noise it at its lowest levels in the range 1–10 GHz. C-band frequencies are also low enough that they are not appreciably attenuated by rainfall, which is the principal cause of signal degradation in the atmosphere. Other propagation losses are also small.

Each C-band satellite covers approximately 500 MHz of bandwidth with a number of 36-MHz transponders, although 72-MHz transponders are also available. Transponders usually have 5-10W TWTA; the more linear SSPAs of up to 8.5 W are also available. Early C-band Earth stations commonly used 32-m antennas. As more power has become possible on board the satellite, small antennas (around 3 m) are possible, particularly for receive-only applications.

Despite considerable advantages, the most significant disadvantage of C-band is that the band is effectively full and interference considerations limit satellite-based applications since existing microwave radio relay links are already assigned frequencies to operate within the band. Frequency coordination is therefore very important in C-band and satellite communications Earth stations, and their receiving antennas also must be located carefully so that they do not receive or cause interference with terrestrial microwave radio relay links or other satellite communications links that are using the same frequencies.