In telecommunications, antenna effective area or effective aperture is the functionally equivalent area from which an antenna directed toward the source of the received signal gathers or absorbs the energy of an incident electromagnetic wave.
Note 1: Antenna effective area is usually expressed in square meters.
Note 2: In the case of parabolic and hornparabolic antennas, the antenna effective area is about 0.35 to 0.55 of the geometric area of the antenna aperture.
Where P_{0} is the power absorbed by the antenna in watts, and P is the power density incident on the antenna in watts per square meter. It is assumed that the antenna is terminated with a matched load to absorb the maximum power.
Source: from Federal Standard 1037C
Relationship to antenna gain
The effective area is related to the antenna gain by
where G is the antenna gain (not in decibels) and λ is the wavelength. Note that G is the antenna gain with respect to the isotropic radiator. This formula can be derived as a consequence of electromagnetic reciprocity which relates the transmit properties of an antenna to the receiving properties. It may not hold if the antenna is made with certain nonreciprocal materials. Like the antenna gain, the effective area varies with direction. If no direction is specified, the direction that produces the maximum gain and maximum effective area is assumed.
Relationship to physical area
Simply increasing the size of antenna does not guarantee an increase in effective area; however, other factors being equal, antennas with higher maximum effective area are generally larger.
In the case of wire antennas, there is no simple relationship between physical area and effective area. In the case of aperture antennas (for example, horns and parabolic reflectors) considered in their direction of maximum radiation, the aperture efficiency is the ratio of effective area to physical area:
where e_{ap} is the aperture efficiency, A_{phys} is the physical size of the aperture, and A_{eff} is the effective aperture.
Note 2 in the definition section above, derived from the Federal Standard, implies that the aperture efficiency is 0.35 to 0.55, which is true for simple designs. However, carefully designed and constructed reflector antennas can easily have efficiencies in the 0.65 to 0.75 range; and values as high as 0.85 have been reported in the literature. Very high aperture efficiency is not always desirable, since such antennas tend to have high side lobe levels.
Factors limiting the aperture efficiency are non uniform illumination of the aperture, phase variations of the aperture field (typically due to surface errors in a reflector and high flare angle in horns), and scattering from obstructions. The incident wavefront may also not be completely phase coherent due to variations in the propagating medium; this results in an increase in the effective area of an antenna not resulting in a commensurate increase in signal power, an effect known as 'aperture loss'.
This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MILSTD188).
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