C BAND
Satellite Communications Glossary
Satellite Communications — Satellite Frequencies — C band
Information on C band
The C band is a band of frequencies used
for terrestrial radio relay and satellite transmissions in satellite communications.
The C band in satellite communications 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 travelling wave tube (TWT) amplifiers; the more linear solid-state
power amplifiers of up to 8.5W 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
3m) 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 have to 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.
Other topics in our resources on Satellite Communications related to C band include:
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