An antenna or aerial is an electrical device designed to transmit or receive radio waves or, more generally, any electromagnetic waves. Antennas are used in systems such as radio and television broadcasting, point-to-point radio communication, radar, and space exploration. Antennas usually work in air or outer space, but can also be operated under water or even through soil and rock at certain frequencies for short distances.

Physically, an antenna is an arrangement of conductors that generate a radiating electromagnetic field in response to an applied alternating voltage and the associated alternating electric current, or can be placed in an electromagnetic field so that the field will induce an alternating current in the antenna and a voltage between its terminals. Some antenna devices (parabola, horn antenna) just adapt the free space to another type of antenna.

Antennas were used for the first time, in 1889, by Heinrich Hertz (1857-1894) to prove the existence of electromagnetic waves predicted by the theory of James Clerk Maxwell. He even placed the emitter dipole in the focal point of a parabolic reflector. He published his work and installation drawings in Annalen der Physik und Chemie (vol. 36, 1889).

There are many variations of antennas that have various configurations. These configurations contain space or medium which tends to confine the energy within specified boundaries along a predetermined path (known as "restricted space"), such as wave guides, hollow resonators, and conductive wires. Below are a few common models. More can be found in .

The isotropic radiator is a purely theoretical antenna that radiates equally in all directions. It is considered to be a point in space with no dimensions and no mass. This antenna cannot physically exist, but is useful as a theoretical model for comparison with all other antennas. Most antennas' gains are measured with reference to an isotropic radiator, and are rated in dBi (decibels with respect to an isotropic radiator).

The dipole antenna is simply two wires pointed in opposite directions arranged either horizontally or vertically, with one end of each wire connected to the radio and the other end hanging free in space. Since this is the simplest practical antenna, it is also used as reference model for other antennas; gain with respect to a dipole is labeled as dBd. Generally, the dipole is considered to be omnidirectional in the plane perpendicular to the axis of the antenna, but it has deep nulls in the directions of the axis. Variations of the dipole include the folded dipole, the half wave antenna, the groundplane antenna, the whip, and the J-pole.

The Yagi-Uda antenna is a directional variation of the dipole with parasitic elements added with functionality similar to adding a reflector and lenses (directors) to focus a filament lightbulb.

Loop antennas have a continuous conducting path leading from one conductor of a two-wire transmission line to the other conductor. "Symmetric" loop antennas have a plane of symmetry running along the feed and through the loop. "Planar" loop antennas lie in a single plane which also contains the conductors of the feed. "Three-dimensional" loop antennas have wire which runs in all of the x,y, and z directions. By definition they are not planar. They may, however, be symmetric about planes which contain the feed.

The (large) loop antenna is similar to a dipole, except that the ends of the dipole are connected to form a circle, triangle (delta loop antenna) or square. Typically a loop is a multiple of a half or full wavelength in circumference. A circular loop gets higher gain (about 10%) than the other forms of large loop antenna, as gain of this antenna is directly proportional to the area enclosed by the loop, but circles can be hard to support in a flexible wire, making squares and triangles much more popular. Large loop antennas are more immune to localized noise partly due to lack of a need for a groundplane. The large loop has its strongest signal in the plane of the loop, and nulls in the axis perpendicular to the plane of the loop.

The small loop antenna, also called the magnetic loop antenna is a loop of wire (in other words, both ends of the wire connect to the radio) less than a wavelength in circumference. Typically, the circumference is less than 1/10 for a receiving loop, and less than 1/4 for a transmitting loop. Unlike nearly all other antennas in this list, this antenna detects the magnetic component of the electromagnetic wave. As such, it is less sensitive to near field electric noise when properly shielded. The received voltage can be greatly increased by bringing the loop into resonance with a tuning capacitor. The small loop has a maximum output when the magnetic field is normal to the plane of the loop, and since this field is transverse to the direction of the wave, has a maximum in the plane of the loop. This is the same mechanism as the large loop.

The electrically short antenna is an open-end wire far less than 1/4 wavelength in length - in other words only one end of the antenna is connected to the radio, and the other end is hanging free in space. Unlike nearly all other antennas in this list, this antenna detects only the electric field of the wave instead of the electromagnetic field - think of the free end of the wire as measuring the voltage of that point in space, as opposed to measuring both the voltage and the magnetic field. Its receiving aperture cannot be changed by adding lumped components, but more efficient power transfer can be achieved by impedance matching with such circuits. Electrically short antennas are typically used where operating wavelength is large and space is limited, e.g. for mobile transceivers operating at long wavelengths.

The parabolic antenna is a special antenna where a reflector dish is used to focus the signal from a directional antenna feeder. Antennas of this type are commonly found as Satellite television antennas, Wi-fi / WLAN, radio astronomy, radio-links, mobile phone backhaul and military tactical radio link -antennas. They are characterized by high directionality and gain but can only be used at UHF to microwave and higher frequencies due to dimensions getting too large at lower frequencies.

The microstrip antenna consists of a patch of metalization on a ground plane. These are low profile, light weight antennas, most suitable for aerospace and mobile applications. Because of their low power handling capability, these antennas can be used in low-power transmitting and receiving applications. Microstrip antennas are the most commonly used antennas in mobile communications, satellite links, W-LAN and so on because circuit functions can be directly integrated to the microstrip antenna to form compact transceivers and spatial power combiners.

The quad antenna is an array of square loops that vary in size. The quad is related to the loop in exactly the same way the yagi is related to the dipole. Typically, the quad needs fewer elements to get the same gain as a yagi. Variations of the quad include the delta loop antenna which uses a triangle instead of a square, requiring fewer supports for large wavelength antennas.

The random wire antenna is simply a very long (greater than one wavelength) wire with one end connected to the radio and the other in free space, arranged in any way most convenient for the space available. Folding will reduce effectiveness and make theoretical analysis extremely difficult. (The added length helps more than the folding typically hurts.) Typically, a random wire antenna will also require an antenna tuner, as it might have a random impedance that varies nonlinearly with frequency.

The Beverage antenna is a form of directional long-wire antenna which uses a resistive termination at one end and feed from the other.

The endfire helical antenna is a directional antenna suited for receiving signals that are either circular polarized or randomly polarized. These are usually used with satellites, and are frequently used for the driven element on a dish.

The Phased array antenna is a group of independently fed active elements in which the relative phases of the respective signals feeding the elements are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions. In plain language, this is a directional antenna that can be aimed without moving any parts.

Synthetic aperture radar uses a series of observations separated in time and space to simulate a very large antenna. Interferometry allows the monitor to combine signals from several radio receivers or a single moving receiver.

A trailing wire antenna is used by submarines when submerged. These antennas are designed to pick up transmissions in the low frequency (LF) and very low frequency (VLF) ranges.

An evolved antenna refers to an antenna fully or substantially designed using a computer algorithm based on Darwinian evolution.

A dielectric resonator is a variation on the conventional antenna in which an insulator with a large dielectric constant is used to modify the electromagnetic field. It is claimed that the dielectric contains the antenna's near field and therefore prevents it from interfering with other nearby antennas or circuits, making it suitable for miniature equipment such as mobile phones.

A feed horn is an antenna system that handles the incoming waveform from the dish to the focal point. It usually comprises of a series of rings with decreasing radius in order to drive the signal to the polarizer.

The electrical field created by an electric charge $scriptstyle$ is

$vec\; E=left\{vec\; e\_\{r\text{'}\}over\; r\text{'}^2\}+\; \{r\text{'}over\; c\}\{d\; over\; dt\}left(\{vec\; e\_\{r\text{'}\}over\; r\text{'}^2\}\; ight)\; +\; \{1over\; c^2\}\{d^2\; over\; dt^2\}left(vec\; e\_\{r\text{'}\}\; ight)\; ight,$

$scriptstyle$ is the speed of light in vacuum.

$scriptstyle$ is the permittivity of free space.

$scriptstyle$ is the distance from the observation point (the place where $scriptstyle$ is evaluated) to the point where the charge was $scriptstyle$ seconds before the time when the measure is done.

$extstyle$ is the unit vector directed from the observation point (the place where $scriptstyle$ is evaluated) to the point where the charge was $scriptstyle$ seconds before the time when the measure is done.

$vec\; E=left(vec\; e\_$

$P=$ watts.

$dE\_\; heta(t+\; extstyle)=displaystyle,$

$I=I\_circ\; e^$

$scriptstyle$ is the amplitude of the current.

$scriptstyle$ is the angular frequency.

$scriptstyle$

$dE\_\; heta(t+\; extstyle)=displaystyle\; e^,$

$dE\_\; heta(t)=\; e^,$

At frequencies used in antennas, the ground behaves mainly as a dielectric. The conductivity of ground at these frequencies is negligible. When an electromagnetic wave arrives at the surface of an object, two waves are created: one enters the dielectric and the other is reflected. If the object is a conductor, the transmitted wave is negligible and the reflected wave has almost the same amplitude as the incident one. When the object is a dielectric, the fraction reflected depends (among others things) on the angle of incidence. When the angle of incidence is small (that is, the wave arrives almost perpendicularly) most of the energy traverses the surface and very little is reflected. When the angle of incidence is near 90° (grazing incidence) almost all the wave is reflected.

Most of the electromagnetic waves emitted by an antenna to the ground below the antenna at moderate (say < 60°) angles of incidence enter the earth and are absorbed (lost). But waves emitted to the ground at gracing angles, far from the antenna, are almost totally reflected. At grazing angles, the ground behaves as a mirror. Quality of reflection depends on the nature of the surface. When the irregularities of the surface are smaller than the wavelength reflection is good.



This means that the receptor "sees" the real antenna and, under the ground, the image of the antenna reflected by the ground. If the ground has irregularities, the image will appear fuzzy.

If the receiver is placed at some height above the ground, waves reflected by ground will travel a little longer distance to arrive to the receiver than direct waves. The distance will be the same only if the receiver is close to ground.

In the drawing at right, we have drawn the angle $scriptstyle$ far bigger than in reality. Distance between the antenna and its image is $scriptstyle$.

Situation is a bit more complex because the reflection of electromagnetic waves depends on the polarization of the incident wave. As the refractive index of the ground (average value $scriptstyle$) is bigger than the refractive index of the air ($scriptstyle$), the direction of the component of the electric field parallel to the ground inverses at the reflection. This is equivalent to a phase shift of $scriptstyle$ radians or 180°. The vertical component of the electric field reflects without changing direction. This sign inversion of the parallel component and the non-inversion of the perpendicular component would also happen if the ground were a good electrical conductor.


This means that a receiving antenna "sees" the image antenna with the current in the same direction if the antenna is vertical or with the current inverted if the antenna is horizontal.

For a vertical polarized emission antenna the far electric field of the electromagnetic wave produced by the direct ray plus the reflected ray is:

$extstyle$

$extstyle$

$scriptstyle$ is the electrical field radiated by the antenna if there were no ground.

$scriptstyle$ is the wave number.

$scriptstyle$ is the wave length.

$scriptstyle$ is the distance between antenna and its image (twice the height of the center of the antenna).

Current circulating in any antenna induces currents in all others. One can postulate a mutual impedance $scriptstyle$ between two antennas that has the same significance as the $scriptstyle$ in ordinary coupled inductors. The mutual impedance $scriptstyle$ between two antennas is defined as:

$Z\_=$

$scriptstyle$ is the voltage applied to the antenna $scriptstyle$

$scriptstyle$ is the impedance of antenna $scriptstyle$

$scriptstyle$ is the mutual impedance between antennas $scriptstyle$ and $scriptstyle$

$scriptstyle$