<P> In radio an antenna 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 an electromagnetic 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 radio equipment, and are used in radio broadcasting, broadcast television, two - way radio, communications receivers, radar, cell phones, satellite communications and other devices . </P> <P> An antenna is an array of conductors (elements), electrically connected to the receiver or transmitter . During transmission, the oscillating current applied to the antenna by a transmitter creates an oscillating electric field and magnetic field around the antenna elements . These time - varying fields radiate energy away from the antenna into space as a moving transverse electromagnetic field wave . Conversely, during reception, the oscillating electric and magnetic fields of an incoming radio wave exert force on the electrons in the antenna elements, causing them to move back and forth, creating oscillating currents in the antenna . </P> <P> 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 antennas). An antenna may include parasitic elements, parabolic reflectors or horns, which serve to direct the radio waves into a beam or other desired radiation pattern . </P> <P> The first antennas were built in 1888 by German physicist Heinrich Hertz in his pioneering experiments to prove the existence of electromagnetic waves predicted by the theory of James Clerk Maxwell . Hertz placed dipole antennas at the focal point of parabolic reflectors for both transmitting and receiving . He published his work in Annalen der Physik und Chemie (vol. 36, 1889). </P>

Antennas that transmit an equal amount of energy in the horizontal direction are called