Because satellite signals (especially those from geostationary satellites ) suffer very large path losses the signals reaching the earth station are at a very low level. To achieve reasonable signal levels for processing in the receiver the path losses need to be overcome with a very high gain receiving antenna.
The need for high gain antennas has led satellite system designers to look to higher frequency bands. At the higher frequency bands it become practical to produce very high gain parabolic antennas. For a structure of, at most, a few metres in diameter an antenna of many tens of wavelengths can be constructed.
Note:
Some 1-2 GHz satellite systems (eg Mobilesat and satellite DSB)
have been planned using much lower gain whip antennas. In this
case the power flux density per Hz of the satellite system must
be higher. Although these antennas have low gain they have an
omni directional receiving pattern which is important for mobile
reception. High gain dishes need a fixed mounting (or motor controlled
servo systems) to maintain accurate pointing and thereby derive
benefit from the high on-axis gain they produce.
A parabolic antenna focuses incident RF signals to the focus of the dish in much the same way as the optical parabolic lenses used in optical telescopes do. The theoretical gain of a parabola is related to its size in relation to a wavelength.
In practice gain is more conveniently expressed in dB and wavelength is replaced by frequency. This leads to the following formula.
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