Antenna effective area
Encyclopedia
In telecommunications, antenna effective area or effective aperture expresses an antenna's ability to collect an incident radio wave and deliver it as an electrical current at the antenna's terminals. The symbol Aeff is used to represent this figure, usually expressed in square meters.
Effective area is calculated using this formula:
Where
is the power (usually watt
s) delivered by the antenna, and
is the incident power density (usually watts per square meter) passing the antenna. It is assumed that the antenna is terminated with a matched load to absorb the maximum power.
For example, if an antenna delivers 30 watts of electrical power at its terminals when in the presence of electromagnetic waves whose density is 20 watts per square meter, then the antenna's effective area is 1.5 square meters.
. This is so because an antenna's power gain expresses two of its radiative physical properties: its directivity
and its efficiency
. Directivity describes its preference for certain angles of incidence, and efficiency describes the electrical losses inside it (due to ohmic resistance, losses into the surrounding dielectric, and similar). Both affect an antenna's effective area because both impact the power ultimately delivered at its terminals.
This formula calculates effective area as a function of its power gain:
where G is the antenna's power gain (not its relative gain or dBi), and
is the wavelength.
That formula can be derived as a consequence of electromagnetic reciprocity
which relates the transmit properties of an antenna to the receiving properties. Consequently, it may not hold if the antenna is made with certain non-reciprocal materials.
Like the power gain property on which the equation depends, the effective area varies with direction. If no direction is specified, the direction that produces the maximum gain and maximum effective area is assumed.
of well-known wire antennas can be calculated from their (constant) power gain figures:
where
is the aperture efficiency, 
is the physical size of the aperture, and 
is the effective aperture.
Federal Standard 1037C 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. However, very high aperture efficiency is not always desirable because such antennas tend to have large side lobe
s.
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'.
Effective area is calculated using this formula:
Where
is the power (usually wattWatt
The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...
s) delivered by the antenna, and
is the incident power density (usually watts per square meter) passing the antenna. It is assumed that the antenna is terminated with a matched load to absorb the maximum power.For example, if an antenna delivers 30 watts of electrical power at its terminals when in the presence of electromagnetic waves whose density is 20 watts per square meter, then the antenna's effective area is 1.5 square meters.
Relationship to antenna gain
An antenna's effective area depends on its power gainAntenna gain
In electromagnetics, an antenna's power gain or simply gain is a key performance figure which combines the antenna's directivity and electrical efficiency. As a transmitting antenna, the figure describes how well the antenna converts input power into radio waves headed in a specified direction...
. This is so because an antenna's power gain expresses two of its radiative physical properties: its directivity
Directivity
In electromagnetics, directivity is a figure of merit for an antenna. It measures the power density the antenna radiates in the direction of its strongest emission, versus the power density radiated by an ideal isotropic radiator radiating the same total power.An antenna's directivity is a...
and its efficiency
Antenna efficiency
In electromagnetics, antenna efficiency or radiation efficiency is a figure of merit for an antenna. It measures the electrical losses that occur throughout the antenna while it is operating at a given frequency, or averaged over its operation across a frequency band...
. Directivity describes its preference for certain angles of incidence, and efficiency describes the electrical losses inside it (due to ohmic resistance, losses into the surrounding dielectric, and similar). Both affect an antenna's effective area because both impact the power ultimately delivered at its terminals.
This formula calculates effective area as a function of its power gain:
where G is the antenna's power gain (not its relative gain or dBi), and
is the wavelength.That formula can be derived as a consequence of electromagnetic reciprocity
Reciprocity (electromagnetism)
In classical electromagnetism, reciprocity refers to a variety of related theorems involving the interchange of time-harmonic electric current densities and the resulting electromagnetic fields in Maxwell's equations for time-invariant linear media under certain constraints...
which relates the transmit properties of an antenna to the receiving properties. Consequently, it may not hold if the antenna is made with certain non-reciprocal materials.
Like the power gain property on which the equation depends, 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 effective areas are generally larger.Wire antennas
In the case of wire antennas, there is no simple relationship between physical area and effective area. However, the effective areas for wavelength of well-known wire antennas can be calculated from their (constant) power gain figures:| Wire antenna | Power gain | Effective area |
|---|---|---|
| Short dipole aka Hertzian dipole | 1.5 | 0.1194  2 |
| Half-wave dipole Dipole antenna A dipole antenna is a radio antenna that can be made of a simple wire, with a center-fed driven element. It consists of two metal conductors of rod or wire, oriented parallel and collinear with each other , with a small space between them. The radio frequency voltage is applied to the antenna at... |
1.64 | 0.1305  2 |
| Monopole Monopole antenna A monopole antenna is a class of radio antenna consisting of a straight rod-shaped conductor, often mounted perpendicularly over some type of conductive surface, called a ground plane. The driving signal from the transmitter is applied, or for receiving antennas the output voltage is taken,... * aka Quarter-wave |
3.28** | 0.2610  2 |
- Assuming the monopole antenna has an infinite ground/counterpoise.
- Monopoles have twice the power gain of dipoles because they do not radiate into the space below their ground plane.
Aperture antennas
In the case of aperture antennas (horns and parabolic reflectors) considered in their direction of maximum radiation, the aperture efficiency is the ratio of effective area to physical area:where
is the aperture efficiency, is the physical size of the aperture, and is the effective aperture.Federal Standard 1037C 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. However, very high aperture efficiency is not always desirable because such antennas tend to have large side lobe
Side lobe
In antenna engineering, side lobes or sidelobes are the lobes of the far field radiation pattern that are not the main lobe....
s.
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'.
See also
- Antenna apertureAntenna apertureIn electromagnetics and antenna theory, antenna aperture or effective area is a measure of how effective an antenna is at receiving the power of radio waves. The aperture is defined as the area, oriented perpendicular to the direction of an incoming radio wave, which would intercept the same...
, especially Antenna effective length - Antenna (radio)Antenna (radio)An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...