Directional antenna in the context of "Antenna (radio)"

Pioneer 10 (originally designated Pioneer F) is a NASA space probe launched in 1972 that completed the first mission to the planet Jupiter. Pioneer 10 became the first of five artificial objects to achieve the escape velocity needed to leave the Solar System. This space exploration project was conducted by the NASA Ames Research Center in California. The space probe was manufactured by TRW Inc.

Pioneer 10 was assembled around a hexagonal bus with a 2.74-meter (9 ft 0 in) diameter parabolic dish high-gain antenna, and the spacecraft was spin stabilized around the axis of the antenna. Its electric power was supplied by four radioisotope thermoelectric generators that provided a combined 155 watts at launch.

In radio-frequency engineering, an antenna (American English) or aerial (British English) is an electronic device that converts an alternating electric current into radio waves (transmitting), or radio waves into an electric current (receiving). It is the interface between radio waves propagating through space and electric currents moving in metal conductors, used with a transmitter or receiver. In transmission, a radio transmitter supplies an electric current to the antenna's terminals, and the antenna radiates the energy from the current as electromagnetic waves (radio waves). In reception, an antenna intercepts some of the power of a radio wave in order to produce an electric current at its terminals, that is applied to a receiver to be amplified. Antennas are essential components of all radio equipment.

An antenna is an array of conductor segments (elements), electrically connected to the receiver or transmitter. Antennas can be designed to transmit and receive radio waves in all horizontal directions equally (omnidirectional antennas), or preferentially in a particular direction (directional, or high-gain, or "beam" antennas). An antenna may include components not connected to the transmitter, parabolic reflectors, horns, or parasitic elements, which serve to direct the radio waves into a beam or other desired radiation pattern. Strong directivity and good efficiency when transmitting are hard to achieve with antennas with dimensions that are much smaller than a half wavelength.

In radio and telecommunications a dipole antenna or doubletis one of the two simplest and most widely used types of antenna; the other is the monopole. The dipole is any one of a class of antennas producing a radiation pattern approximating that of an elementary electric dipole with a radiating structure supporting a line current so energized that the current has only one node at each far end. A dipole antenna commonly consists of two identical conductive elementssuch as metal wires or rods. The driving current from the transmitter is applied, or for receiving antennas the output signal to the receiver is taken, between the two halves of the antenna. Each side of the feedline to the transmitter or receiver is connected to one of the conductors. This contrasts with a monopole antenna, which consists of a single rod or conductor with one side of the feedline connected to it, and the other side connected to some type of ground. A common example of a dipole is the rabbit ears television antenna found on broadcast television sets. All dipoles are electrically equivalent to two monopoles mounted end-to-end and fed with opposite phases, with the ground plane between them made virtual by the opposing monopole.

The dipole is the simplest type of antenna from a theoretical point of view. Most commonly it consists of two conductors of equal length oriented end-to-end with the feedline connected between them.Dipoles are frequently used as resonant antennas. If the feedpoint of such an antenna is shorted, then it will be able to resonate at a particular frequency, just like a guitar string that is plucked. Using the antenna at around that frequency is advantageous in terms of feedpoint impedance (and thus standing wave ratio), so its length is determined by the intended wavelength (or frequency) of operation. The most commonly used is the center-fed half-wave dipole which is just under a half-wavelength long. The radiation pattern of the half-wave dipole is maximum perpendicular to the conductor, falling to zero in the axial direction, thus implementing an omnidirectional antenna if installed vertically, or (more commonly) a weakly directional antenna if horizontal.

A Yagi–Uda antenna, or simply Yagi antenna, is a directional antenna consisting of two or more parallel resonant antenna elements in an end-fire array; these elements are most often metal rods (or discs) acting as half-wave dipoles. Yagi–Uda antennas consist of a single driven element connected to a radio transmitter or receiver (or both) through a transmission line, and additional passive radiators with no electrical connection, usually including one so-called reflector and any number of directors. It was invented in 1926 by Shintaro Uda of Tohoku Imperial University, Japan, with a lesser role played by his boss Hidetsugu Yagi.

Reflector elements (usually only one is used) are slightly longer than the driven dipole and placed behind the driven element, opposite the direction of intended transmission. Directors, on the other hand, are a little shorter and placed in front of the driven element in the intended direction. These parasitic elements are typically off-tuned short-circuited dipole elements, that is, instead of a break at the feedpoint (like the driven element) a solid rod is used. They receive and reradiate the radio waves from the driven element but in a different phase determined by their exact lengths. Their effect is to modify the driven element's radiation pattern. The waves from the multiple elements superpose and interfere to enhance radiation in a single direction, increasing the antenna's gain in that direction.

An antenna radiation pattern (or antenna pattern or far-field pattern) is the directional (angular) dependence of the field strength (sometimes also the phase) of the radio waves from the antenna or other source.

Particularly in the fields of fiber optics, lasers, and integrated optics, the term radiation pattern may also be used as a synonym for the near-field pattern or Fresnel pattern. This refers to the positional dependence of the electromagnetic field in the near field, or Fresnel region of the source. The near-field pattern is most commonly defined over a plane placed in front of the source, or over a cylindrical or spherical surface enclosing it.

The Croydon transmitting station is a broadcasting and telecommunications facility on Beaulieu Heights in Upper Norwood, London, England (grid reference TQ332696), in the London Borough of Croydon, owned by Arqiva. It was established in 1955 and initially used a small lattice tower. The present tower is 152 metres (499 ft) high and was built in 1962.

It was originally used to broadcast the London ITV signal on VHF Band III. When UHF broadcasting began, the nearby Crystal Palace transmitting station was used. VHF television was discontinued in 1985, and the Croydon transmitter was not used for regular TV broadcasting until 1997, when a new directional UHF antenna, designed to avoid interference with continental transmitters, was installed to carry the newly launched Channel 5 in the London area. It carried Channel 5's analogue signal, and the digital terrestrial signal is transmitted from Crystal Palace. Croydon also had reserve transmitters for BBC1, BBC2, ITV and Channel 4, but these were used only in the event of engineering works or a failure at Crystal Palace. Since the digital switchover in April 2012 no television has been broadcast from Croydon, but it is still used as a backup for Crystal Palace for the BBC A & B, Digital 3&4 and COM 4, 5 and 6 multiplexes.