Image antenna
Encyclopedia
In telecommunication
s and antenna
design, an image antenna is an electrical mirror-image of an antenna
element formed by the radio waves reflecting from a conductive surface called a ground plane
, such as the surface of the earth. It is used as a geometrical technique in calculating the radiation pattern
of the antenna.
When a radio antenna is mounted above a conductive surface such as the earth
, the radio waves directed down toward the surface reflect off it. The radiation received at a distant point is the sum of two contributions: the waves that travel directly from the antenna to the point, and the waves that reach the point after reflecting off the ground plane. Because of the reflection, these second waves appear to come from a second antenna behind the plane, just as a visible object in front of a flat mirror
forms a virtual image
that seems to lie behind the mirror. The radiation pattern
of the antenna is exactly the same as it would be if the ground plane were replaced by a mirror image of the antenna, located an equal distance behind the plane. This second apparent source of radio waves is the image antenna.
The image antenna is helpful in calculating electric field
vectors, magnetic field
vectors, and electromagnetic field
s emanating from the real antenna, particularly in the vicinity of the antenna and along the ground. Each charge and current in the real antenna has its counterpart in the image, and may also be considered as a source of radiation
.
To form an image of the antenna above it, the ground plane
need not be grounded to the Earth.
Telecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
s and antenna
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...
design, an image antenna is an electrical mirror-image of an antenna
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...
element formed by the radio waves reflecting from a conductive surface called a ground plane
Ground plane
In electrical engineering, a ground plane is an electrically conductive surface.-Radio antenna theory :In telecommunication, a ground plane structure or relationship exists between the antenna and another object, where the only structure of the object is a structure which permits the antenna to...
, such as the surface of the earth. It is used as a geometrical technique in calculating the 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 ....
of the antenna.
When a radio antenna is mounted above a conductive surface such as the earth
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....
, the radio waves directed down toward the surface reflect off it. The radiation received at a distant point is the sum of two contributions: the waves that travel directly from the antenna to the point, and the waves that reach the point after reflecting off the ground plane. Because of the reflection, these second waves appear to come from a second antenna behind the plane, just as a visible object in front of a flat mirror
Mirror
A mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection...
forms a virtual image
Virtual image
In optics, a virtual image is an image in which the outgoing rays from a point on the object always diverge. It will appear to converge in or behind the optical device . A simple example is a flat mirror where the image of oneself is perceived at twice the distance from oneself to the mirror...
that seems to lie behind the mirror. The 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 ....
of the antenna is exactly the same as it would be if the ground plane were replaced by a mirror image of the antenna, located an equal distance behind the plane. This second apparent source of radio waves is the image antenna.
The image antenna is helpful in calculating electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
vectors, magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
vectors, and electromagnetic field
Electromagnetic field
An electromagnetic field is a physical field produced by moving electrically charged objects. It affects the behavior of charged objects in the vicinity of the field. The electromagnetic field extends indefinitely throughout space and describes the electromagnetic interaction...
s emanating from the real antenna, particularly in the vicinity of the antenna and along the ground. Each charge and current in the real antenna has its counterpart in the image, and may also be considered as a source of radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
.
To form an image of the antenna above it, the ground plane
Ground plane
In electrical engineering, a ground plane is an electrically conductive surface.-Radio antenna theory :In telecommunication, a ground plane structure or relationship exists between the antenna and another object, where the only structure of the object is a structure which permits the antenna to...
need not be grounded to the Earth.