Antenna aperture
Encyclopedia
In electromagnetics and antenna
theory, antenna aperture or effective area is a measure of how effective an antenna is at receiving the power of radio wave
s. The aperture is defined as the area, oriented perpendicular to the direction of an incoming radio wave, which would intercept the same amount of power from that wave as is produced by the antenna receiving it. At any point, a beam of radio waves has an irradiance
or power flux density (PFD) which is the amount of radio power passing through a unit area. If an antenna delivers an output power of Po watt
s to the load connected to its output terminals when irradiated by a uniform field of power density PFD watts per square metre, the antenna's aperture Aeff in square metres is given by:
.
So the power output of an antenna in watts is equal to the power density of the radio waves in watts per square metre, multiplied by its aperture in square metres. The larger an antenna's aperture is, the more power it can collect from a given field of radio waves. To actually obtain the predicted power available Po, the polarization of the incoming waves must match the polarization of the antenna, and the load (receiver) must be impedance matched to the antenna's feedpoint impedance.
Although this concept is based on an antenna receiving a radio frequency wave, knowing Aeff directly supplies the (power) gain
of that antenna. Due to reciprocity
, an antenna's gain in receiving and transmitting are identical. Therefore Aeff can just as well be used to compute the performance of a transmitting antenna. Note that Aeff is a function of the direction of the radio wave relative to the orientation of the antenna, since the gain of an antenna varies according to its radiation pattern
. When no direction is specified, Aeff is understood to refer to its maximum value, that is, the direction that the antenna would be pointed to take advantage of its maximum gain.
and horns
, have a physical aperture (opening) which collects the radio waves. In these aperture antennas, the effective aperture Aeff is always less than the area of the physical aperture Aphys of the antenna. An antenna's aperture efficiency, ea is defined as the ratio of these two areas:
The aperture efficiency is a dimensionless parameter between 0 and 1.0 that measures how far the antenna falls short of using all the radio power entering its physical aperture. If the antenna were perfectly efficient, all the radio power falling within its physical aperture would be converted to electrical power delivered to the load attached to its output terminals, so these two areas would be equal Aeff = Aphys and the aperture efficiency would be 1.0. But all antennas have losses, such as power dissipated as heat in the resistance of its elements, nonuniform illumination by its feed
, and radio waves scattered by structural supports and diffraction at the aperture edge, which reduce the power output. Aperture efficiencies of typical antennas vary from 0.35 to 0.70 but can range up to 0.90.
of an antenna, its ability to direct radio waves in one direction or receive from a single direction, is measured by a parameter called its gain
, which is the ratio of the power received by the antenna to the power that would be received by a hypothetical isotropic antenna
, which receives power equally well from all directions.
It can be shown that the aperture of a lossless isotropic antenna, which by definition has unity gain, is:
where λ is the wavelength
of the radio waves. So the gain of any antenna is proportional to its aperture:
So antennas with large effective apertures are high gain antennas, which have small angular beam widths. Most of their power is radiated in a narrow beam in one direction, and little in other directions. As receiving antennas, they are most sensitive to radio waves coming from one direction, and are much less sensitive to waves coming from other directions. Although these terms can be used as a function of direction, when no direction is specified, the gain and aperture are understood to refer to the antenna's axis of maximum gain, or boresight
.
:.
where
and dipoles
consisting of thin rod conductors
, the aperture bears no obvious relation to the size or area of the antenna. An alternate measure of antenna gain that has a greater relationship to the physical structure of such antennas is effective length leff measured in metres, which is defined for a receiving antenna as:
where
The longer the effective length the more voltage and therefore the more power the antenna will receive. Note, however, that an antenna's gain or Aeff increases according to the square of leff, and that this proportionality also involves the antenna's radiation resistance
. Therefore this measure is of more theoretical than practical value and is not, by itself, a useful figure of merit relating to an antenna's directivity.
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...
theory, antenna aperture or effective area is a measure of how effective an antenna is at receiving the power of radio wave
Radio Wave
Radio Wave may refer to:*Radio frequency*Radio Wave 96.5, a radio station in Blackpool, UK...
s. The aperture is defined as the area, oriented perpendicular to the direction of an incoming radio wave, which would intercept the same amount of power from that wave as is produced by the antenna receiving it. At any point, a beam of radio waves has an irradiance
Irradiance
Irradiance is the power of electromagnetic radiation per unit area incident on a surface. Radiant emittance or radiant exitance is the power per unit area radiated by a surface. The SI units for all of these quantities are watts per square meter , while the cgs units are ergs per square centimeter...
or power flux density (PFD) which is the amount of radio power passing through a unit area. If an antenna delivers an output power of Po watt
Watt
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 to the load connected to its output terminals when irradiated by a uniform field of power density PFD watts per square metre, the antenna's aperture Aeff in square metres is given by:
.So the power output of an antenna in watts is equal to the power density of the radio waves in watts per square metre, multiplied by its aperture in square metres. The larger an antenna's aperture is, the more power it can collect from a given field of radio waves. To actually obtain the predicted power available Po, the polarization of the incoming waves must match the polarization of the antenna, and the load (receiver) must be impedance matched to the antenna's feedpoint impedance.
Although this concept is based on an antenna receiving a radio frequency wave, knowing Aeff directly supplies the (power) gain
Antenna 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...
of that antenna. Due to 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...
, an antenna's gain in receiving and transmitting are identical. Therefore Aeff can just as well be used to compute the performance of a transmitting antenna. Note that Aeff is a function of the direction of the radio wave relative to the orientation of the antenna, since the gain of an antenna varies according to its radiation pattern
Radiation pattern
In the field of antenna design the term radiation pattern most commonly refers to the directional dependence of the strength of the radio waves from the antenna or other source ....
. When no direction is specified, Aeff is understood to refer to its maximum value, that is, the direction that the antenna would be pointed to take advantage of its maximum gain.
Aperture efficiency
In general, the aperture of an antenna is not directly related to its physical size. However some types of antennas, for example parabolic dishesParabolic antenna
A parabolic antenna is an antenna that uses a parabolic reflector, a curved surface with the cross-sectional shape of a parabola, to direct the radio waves. The most common form is shaped like a dish and is popularly called a dish antenna or parabolic dish...
and horns
Horn antenna
A horn antenna or microwave horn is an antenna that consists of a flaring metal waveguide shaped like a horn to direct the radio waves. Horns are widely used as antennas at UHF and microwave frequencies, above 300 MHz...
, have a physical aperture (opening) which collects the radio waves. In these aperture antennas, the effective aperture Aeff is always less than the area of the physical aperture Aphys of the antenna. An antenna's aperture efficiency, ea is defined as the ratio of these two areas:
The aperture efficiency is a dimensionless parameter between 0 and 1.0 that measures how far the antenna falls short of using all the radio power entering its physical aperture. If the antenna were perfectly efficient, all the radio power falling within its physical aperture would be converted to electrical power delivered to the load attached to its output terminals, so these two areas would be equal Aeff = Aphys and the aperture efficiency would be 1.0. But all antennas have losses, such as power dissipated as heat in the resistance of its elements, nonuniform illumination by its feed
Antenna feed
In telecommunications and electronics, an antenna feed refers to the components of an antenna which feed the radio waves to the rest of the antenna structure, or in receiving antennas collect the incoming radio waves, convert them to electric currents and transmit them to the receiver...
, and radio waves scattered by structural supports and diffraction at the aperture edge, which reduce the power output. Aperture efficiencies of typical antennas vary from 0.35 to 0.70 but can range up to 0.90.
Aperture and gain
The directivityDirectivity
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...
of an antenna, its ability to direct radio waves in one direction or receive from a single direction, is measured by a parameter called its gain
Antenna 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...
, which is the ratio of the power received by the antenna to the power that would be received by a hypothetical isotropic antenna
Isotropic radiator
An isotropic radiator is a theoretical point source of electromagnetic or sound waves which radiates the same intensity of radiation in all directions. It has no preferred direction of radiation. It radiates uniformly in all directions over a sphere centred on the source...
, which receives power equally well from all directions.
It can be shown that the aperture of a lossless isotropic antenna, which by definition has unity gain, is:
where λ is the wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
of the radio waves. So the gain of any antenna is proportional to its aperture:
So antennas with large effective apertures are high gain antennas, which have small angular beam widths. Most of their power is radiated in a narrow beam in one direction, and little in other directions. As receiving antennas, they are most sensitive to radio waves coming from one direction, and are much less sensitive to waves coming from other directions. Although these terms can be used as a function of direction, when no direction is specified, the gain and aperture are understood to refer to the antenna's axis of maximum gain, or boresight
Boresight
Boresight is a term used to describe crude adjustments made to an optical firearm sight, or iron sights, to align the firearm barrel and sights. This method is usually used to pre-align the sights, which makes zeroing much faster.Traditional boresighting, as the name suggests involves removing...
.
Friis transmission equation
The fraction of the power delivered to a transmitting antenna that is received by a receiving antenna is proportional to the product of the apertures of both the antennas. This is given by a form of the Friis transmission equationFriis transmission equation
The Friis transmission equation is used in telecommunications engineering, and gives the power received by one antenna under idealized conditions given another antenna some distance away transmitting a known amount of power. The formula was derived in 1945 by Danish-American radio engineer Harald T...
:.
where
- Pr is the power delivered by the receiving antenna in watts
- Pt is the power applied to the transmitting antenna in watts
- Ar is the aperture of the receiving antenna in m2
- At is the aperture of the transmitting antenna in m2
- r is the distance between the antennas in m
- λ is the wavelength of the radio waves in m
Effective length
For antennas which are not defined by a physical area, such as monopolesMonopole 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,...
and dipoles
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...
consisting of thin rod conductors
Electrical conductor
In physics and electrical engineering, a conductor is a material which contains movable electric charges. In metallic conductors such as copper or aluminum, the movable charged particles are electrons...
, the aperture bears no obvious relation to the size or area of the antenna. An alternate measure of antenna gain that has a greater relationship to the physical structure of such antennas is effective length leff measured in metres, which is defined for a receiving antenna as:
where
- V0 is the open circuit voltage appearing across the antenna's terminals
- Es is the electric field strengthField strengthIn physics, the field strength of a field is the magnitude of its vector value.In theoretical physics, field strength is another name for the curvature form...
of the radio signal, in voltVoltThe volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...
s per metre, at the antenna.
The longer the effective length the more voltage and therefore the more power the antenna will receive. Note, however, that an antenna's gain or Aeff increases according to the square of leff, and that this proportionality also involves the antenna's radiation resistance
Radiation resistance
Radiation resistance is that part of an antenna's feedpoint resistance that is caused by the radiation of electromagnetic waves from the antenna. The radiation resistance is determined by the geometry of the antenna, not by the materials of which it is made...
. Therefore this measure is of more theoretical than practical value and is not, by itself, a useful figure of merit relating to an antenna's directivity.