Antenna (radio)

A stack of "fishbone" and Yagi–Uda television antennas
Animation of a half-wave dipole antenna radiating radio waves, showing the electric field lines. The antenna in the center is two vertical metal rods connected to a radio transmitter (not shown). The transmitter applies an alternating electric current to the rods, which charges them alternately positive (+) and negative (−). Loops of electric field leave the antenna and travel away at the speed of light; these are the radio waves. In this animation the action is shown slowed down enormously.
Electronic symbol for an antenna
Antennas of the Atacama Large Millimeter/submillimeter Array.
An automobile's whip antenna, a common example of an omnidirectional antenna.
Half-wave dipole antenna
Diagram of the electric fields ( blue ) and magnetic fields ( red ) radiated by a dipole antenna ( black rods) during transmission.
Cell phone base station antennas
Standing waves on a half wave dipole driven at its resonant frequency. The waves are shown graphically by bars of color ( red for voltage, V and blue for current, I ) whose width is proportional to the amplitude of the quantity at that point on the antenna.
Typical center-loaded mobile CB antenna with loading coil
Polar plots of the horizontal cross sections of a (virtual) Yagi-Uda-antenna. Outline connects points with 3 dB field power compared to an ISO emitter.
The wave reflected by earth can be considered as emitted by the image antenna.
The currents in an antenna appear as an image in opposite phase when reflected at grazing angles. This causes a phase reversal for waves emitted by a horizontally polarized antenna (center) but not for a vertically polarized antenna (left).
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Antenna or aerial is the interface between radio waves propagating through space and electric currents moving in metal conductors, used with a transmitter or receiver.

- Antenna (radio)
A stack of "fishbone" and Yagi–Uda television antennas

65 related topics

Alpha

A Yagi antenna with one driven element (A) called a folded dipole, and 5 parasitic elements: one reflector (B) and 4 directors (C). The antenna radiates radio waves in a beam toward the right.

Passive radiator

A Yagi antenna with one driven element (A) called a folded dipole, and 5 parasitic elements: one reflector (B) and 4 directors (C). The antenna radiates radio waves in a beam toward the right.
A Yagi antenna for UHF TV reception with 22 parasitic elements; 4 reflectors attached to the vertical bracket at left, and 18 directors attached to the horizontal beam at right. The driven element is attached to the black box next to the reflectors. The antenna is most sensitive to radio waves coming from the right, parallel to the antenna's axis.
Comparison of a Yagi with parasitic elements to a log periodic, with all active elements

In a radio antenna, a passive radiator or parasitic element is a conductive element, typically a metal rod, which is not electrically connected to anything else.

Schematic representation of the elementary components of a transmission line.

Telegrapher's equations

Electrical transmission line with distance and time.

Electrical transmission line with distance and time.

Schematic representation of the elementary components of a transmission line.
Schematic showing a wave flowing rightward down a lossless transmission line. Black dots represent electrons, and the arrows show the electric field.
In the presence of losses the solution of the telegrapher's equation has both damping and dispersion, as visible when compared with the solution of a lossless wave equation.
Changes of the signal level distribution along the single dimensional transmission medium. Depending on the parameters of the telegraph equation, this equation can reproduce all four patterns.

It can also be used to electrically model wire radio antennas as truncated single-conductor transmission lines.

Yagi-Uda antenna polar pattern showing pattern of alternating lobes and nulls

Null (radio)

Yagi-Uda antenna polar pattern showing pattern of alternating lobes and nulls

In radio electronics, a null is a direction in an antenna's radiation pattern where the antenna radiates almost no radio waves, so the far field signal strength is a local minimum.

Array of 4 axial-mode turnstiles for portable military satellite communication terminal

Turnstile antenna

Array of 4 axial-mode turnstiles for portable military satellite communication terminal

A turnstile antenna, or crossed-dipole antenna, is a radio antenna consisting of a set of two identical dipole antennas mounted at right angles to each other and fed in phase quadrature; the two currents applied to the dipoles are 90° out of phase.

Ground screen, similar to a counterpoise, at base of mast antenna of AM radio station KTBS

Counterpoise (ground system)

Ground screen, similar to a counterpoise, at base of mast antenna of AM radio station KTBS
Diagram of counterpoise under the antenna mast of an AM radio station. It consists of a network of radial copper wires suspended above the ground, connected to the transmitter feedline ground. It is suspended about 8 feet above ground, so technicians can get access to the helix house at the foot of the tower.
Antenna used in Lodge-Muirhead wireless system around 1900, the first counterpoise.
Inverted-L antenna with counterpoise, in a powerful amateur radio station, Colorado, 1920. The counterpoise is the lower grid of horizontal wires, suspended below the antenna.

In electronics and radio communication, a counterpoise is a network of suspended horizontal wires or cables (or a metal screen), used as a substitute for an earth (ground) connection in a radio antenna system.

Baseband bandwidth. Here the bandwidth equals the upper frequency.

Bandwidth (signal processing)

Difference between the upper and lower frequencies in a continuous band of frequencies.

Difference between the upper and lower frequencies in a continuous band of frequencies.

Baseband bandwidth. Here the bandwidth equals the upper frequency.
The magnitude response of a band-pass filter illustrating the concept of −3 dB bandwidth at a gain of approximately 0.707.

For instance, in the field of antennas the difficulty of constructing an antenna to meet a specified absolute bandwidth is easier at a higher frequency than at a lower frequency.

ANSI and IEC standard schematic symbol for a circulator (with each waveguide or transmission line port drawn as a single line, rather than as a pair of conductors)

Circulator

Passive, non-reciprocal three- or four-port device that only allows a microwave or radio-frequency signal to exit through the port directly after the one it entered.

Passive, non-reciprocal three- or four-port device that only allows a microwave or radio-frequency signal to exit through the port directly after the one it entered.

ANSI and IEC standard schematic symbol for a circulator (with each waveguide or transmission line port drawn as a single line, rather than as a pair of conductors)
A waveguide circulator used as an isolator by placing a matched load on port 3. The label on the permanent magnet indicates the direction of circulation.
Microwave diode reflection amplifier using a circulator

In radar, circulators are used as a type of duplexer, to route signals from the transmitter to the antenna and from the antenna to the receiver, without allowing signals to pass directly from transmitter to receiver.

Modern surface-mount electronic components on a printed circuit board, with a large integrated circuit at the top.

Gain (electronics)

Measure of the ability of a two-port circuit to increase the power or amplitude of a signal from the input to the output port by adding energy converted from some power supply to the signal.

Measure of the ability of a two-port circuit to increase the power or amplitude of a signal from the input to the output port by adding energy converted from some power supply to the signal.

Modern surface-mount electronic components on a printed circuit board, with a large integrated circuit at the top.

The term gain has a different meaning in antenna design; antenna gain is the ratio of radiation intensity from a directional antenna to (red) of an ideal linear amplifier with a voltage gain of 3 with an arbitrary input signal.

A 1980s consumer-grade citizens' band radio (CB) base station

Base station

Base station (or base radio station) is – according to the International Telecommunication Union's (ITU) Radio Regulations (RR) – a "land station in the land mobile service."

Base station (or base radio station) is – according to the International Telecommunication Union's (ITU) Radio Regulations (RR) – a "land station in the land mobile service."

A 1980s consumer-grade citizens' band radio (CB) base station
Basic base station elements used in a remote-controlled installation. Selective calling options such as CTCSS are optional.
The diagram shows a band-pass filter used to reduce the base station receiver's exposure to unwanted signals. It also reduces the transmission of undesired signals. The isolator is a one-way device which reduces the ease of signals from nearby transmitters going up the antenna line and into the base station transmitter. This prevents the unwanted mixing of signals inside the base station transmitter which can generate interference.
A cell tower near Thicketty, South Carolina.
Two GSM mobile phone base station towers disguised as trees in Dublin, Ireland.
A base station disguised as a palm tree in Tucson, Arizona.
Close-up of a base station antenna in Mexico City, Mexico. There are three antennas: each serves a 120-degree segment of the horizon. The microwave dish links the site with the telephone network.
A professional rack-mount iDEN Base Radio at a Cell Site.
Trunked systems have groups of base stations configured as repeaters. The center blocks with frequencies in this trunked block diagram each represent a base station.
136–174 MHz US professional base station antenna examples.
WiMAX base station equipment with a sector antenna and wireless modem on top
Cellular network base station in Yekaterinburg

Technical measures to limit exposure include restricting the radio frequency power emitted by the station, elevating the antenna above ground level, changes to the antenna pattern, and barriers to foot or road traffic.

A typical directional antenna radiation pattern in polar coordinate system representation, showing sidelobes. The radial distance from the center represents signal strength.

Sidelobes

A typical directional antenna radiation pattern in polar coordinate system representation, showing sidelobes. The radial distance from the center represents signal strength.
A typical antenna radiation pattern in cartesian coordinate system representation showing sidelobes.

In antenna engineering, sidelobes are the lobes (local maxima) of the far field radiation pattern of an antenna or other radiation source, that are not the main lobe.