Radio antennas

In radio engineering, an antenna (American English) or aerial (British English) 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 conductors (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. The first antennas were built in 1888 by German physicist Heinrich Hertz in his pioneering experiments to prove the existence of waves predicted by the electromagnetic theory of James Clerk Maxwell. Hertz placed dipole antennas at the focal point of parabolic reflectors for both transmitting and receiving. Starting in 1895, Guglielmo Marconi began development of antennas practical for long-distance, wireless telegraphy, for which he received a Nobel Prize. The words antenna and aerial are used interchangeably. Occasionally the equivalent term "aerial" is used to specifically mean an elevated horizontal wire antenna. The origin of the word antenna relative to wireless apparatus is attributed to Italian radio pioneer Guglielmo Marconi.

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Related exercises (11)

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A coil, made of N rectangular turns of length l and of width w, has a resistance R. The coil, which is moving with a constant velocity

{\bf u} = u\,{\bf \hat{x}}

, enters a region subject to a uniform magnetic field

{\bf B}=-B_0\,{\bf \hat{z}}

. We will neglect the self-inductance of the circuit. Calculate the amplitude and direction of the induced current when : the coil is partially inside the region subject to the field B; the coil is entirely in this region ; the coil starts to go out of the regi

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Calculate and represent the evolution of the current as function of time, from the situation where the coil is completely outside on the left of the field, to the situation where it is completely on the right. We will su

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A coil, made of N rectangular turns of length l and of width w, has a resistance R. The coil, which is moving with a constant velocity

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Radio antennas

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