Antenna (radio)
Interface between radio waves and electric currents.
An antenna (or aerial, in British English) is a device that bridges the gap between electrical currents in metal wires and radio waves traveling through space. When transmitting, it takes an alternating current from a radio transmitter and turns it into electromagnetic waves that radiate outward. When receiving, it does the opposite: it captures some of the energy from passing radio waves and converts it back into an electrical current, which is then sent to a receiver for amplification. Every piece of radio equipment relies on an antenna to function.
An antenna is built from conductive segments, or elements, that are electrically linked to a transmitter or receiver. Some antennas are omnidirectional, meaning they send or receive radio waves equally well in all horizontal directions. Others are directional (also called high-gain or beam antennas), focusing their energy in a specific direction. To shape the radio waves into a beam or other pattern, an antenna may include parts that are not directly connected to the transmitter, such as parabolic reflectors, horns, or parasitic elements. Achieving strong directivity and efficient transmission is difficult if the antenna’s size is much smaller than half the wavelength of the radio waves it handles.
The first antennas were built in 1886 by German physicist Heinrich Hertz, who used them in experiments that confirmed the existence of electromagnetic waves, which had been predicted by James Clerk Maxwell’s 1867 theory. Hertz placed dipole antennas at the focal points of parabolic reflectors for both sending and receiving. Starting in 1895, Guglielmo Marconi began developing practical antennas for long-distance wireless telegraphy, and by 1898 he had opened a factory in Chelmsford, England, to manufacture them.
The terms “antenna” and “aerial” are often used interchangeably, though “aerial” sometimes specifically refers to an elevated horizontal wire antenna. The word “antenna” in wireless technology is credited to Marconi. During the summer of 1895, while testing his wireless system outdoors on his father’s estate near Bologna, he experimented with long wires suspended from a pole. In Italian, a tent pole is called *l’antenna centrale*, so the pole with the wire became simply *l’antenna*.
Quick Facts
- Inventor
- Heinrich Hertz
- Invention Year
- 1886
- First Produced
- 1898
- First Produced By
- Marconi Company
Facts from the source article.
Lore & Background
The first antennas were built in 1886 by German physicist Heinrich Hertz in his pioneering experiments to prove the existence of electromagnetic waves predicted by the 1867 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 and opened a factory in Chelmsford, England, to manufacture his invention in 1898.
The origin of the word antenna relative to wireless apparatus is attributed to Italian radio pioneer Guglielmo Marconi. In the summer of 1895, Marconi began testing his wireless system outdoors on his father's estate near Bologna and soon began to experiment with long wire "aerials" suspended from a pole. In Italian a tent pole is known as l'antenna centrale, and the pole with the wire was simply called l'antenna. Until then wireless radiating transmitting and receiving elements were known simply as "terminals". Because of his prominence, Marconi's use of the word antenna spread among wireless researchers and enthusiasts, and later to the general public.
Antennas can be classified as omnidirectional, radiating energy approximately equally in all horizontal directions, or directional, where radio waves are concentrated in some direction(s). A half‑wave dipole antenna is a balanced element, with equal and opposite voltages and currents at its two terminals, and serves as a simple reference for analyzing many other antenna designs. More complex antennas increase the directivity of the antenna; additional elements in the antenna structure, which need not be directly connected to the receiver or transmitter, increase its directionality.
Reader's Guide
Antennas are required by any radio receiver or transmitter to couple its electrical connection to the electromagnetic field. Radio waves are electromagnetic waves that carry signals through space at the speed of light; in free space they propagate without material absorption, apart from geometric spreading loss. Since antennas obey reciprocity, the same radiation pattern applies both to transmission as well as reception of radio waves.
Antenna "gain" describes the concentration of radiated power into a particular solid angle of space. Unlike amplifier gain, which generally implies a net increase in signal power, antenna gain reflects a redistribution of power so that more is radiated in some directions and less in others, without increasing the total radiated power. Unlike amplifiers, antennas are electrically "passive" devices which conserve total power, and there is no increase in total power above that delivered from the power source (the transmitter), only improved distribution of that fixed total.
A phased array consists of two or more simple antennas which are connected together through an electrical network. Log‑periodic and other frequency‑independent antennas employ self‑similar structures in order to operate effectively over a wide range of frequencies. Even greater directionality can be obtained using aperture antennas such as the parabolic reflector or horn antenna. Since high directivity in an antenna depends on it being large compared to the wavelength, highly directional antennas (thus with high antenna gain) become more practical at higher frequencies (UHF and above).
Did You Know?
- Guglielmo Marconi opened a factory in Chelmsford, England, to manufacture antennas in 1898.
- The word 'antenna' for wireless apparatus originated from Marconi's use of the Italian term for a tent pole.
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