Antenna
An antenna is one of the fundamental parts of a radio; it is not only needed to receive radio waves, but to transmit them as well.
In 1887 Heinrich Hertz (1857-1894) discovered that electromagnetic radio waves were produced by the oscillations of alternating current in a wire. Using a simple dipole antenna (two metal plates connected to a rod), he was able to detect what he called " Hertzian waves." His Hertzian dipole antenna allowed him to measure both the shape and intensity of the invisible waves. The rate of the oscillation of the current that produces radio waves, known as the frequency, is now measured in kilohertz or megahertz in honor of Hertz's discovery.
A transmitting antenna converts the oscillating electric current into radio waves, whose frequency is identical to that of the original electrical oscillations. The waves radiate outward from the antenna in all directions like ripples on water. When intercepted by a receiving antenna, the oscillation of the waves sets up a weak electric current which exactly matches the frequency of the waves transmitted. This weak electric signal can be increased with an amplifier.
In 1889 Édouard Branly (1844-1940) discovered that metal filings in a container cohered in the presence of Hertzian waves. While not a true antenna, this was an early radio wave detector. Immediately on his heels came Oliver Lodge who improved the "coherer," making it a more efficient detector. Lodge was an early proponent of "tuning" frequencies to obtain better results and devised a resonant antenna circuit. Russian physicist Aleksandr Popov added a wire to the coherer, thereby inventing the first "modern" antenna, and was able to detect waves over the "immense" distance of 262 ft. (82 m).
The next advancement in antenna technology was invented by Guglielmo Marconi. He discovered that adding a ground wire to his transmitter and receiver allowed him to send radio waves over greater distances. He also discovered he could focus radio waves into beams by putting sheets of metal around his antenna. On December 11, 1901, Marconi amazed the world when he successfully transmitted a Morse code signal 2,137 miles (3,440 km) from England to Newfoundland. His receiver used a single wire (a monopole antenna) 515 ft. (157 m) long, flown aloft by a kite. After Marconi's success, radio (and later television) technology advanced rapidly.
Depending on the type of signal being broadcast, different types of antennae are needed. A directional antenna has a horizontal element on which short cross beams are attached. When aimed at the transmitter, the cross beams receive and radiate the signal. A reflector at the end of the antenna directs the radiated signal back into the cross beams, strengthening the original signal. This helps eliminate interference caused by other nearby sources. In Japan, Dr. Hidetsugu Yagi and Dr. S. uda invented a Very High Frequency (VHF) antenna in 1926 that was eventually put into use for television reception.
In 1928 Karl Jansky, a radio engineer at Bell Telephone Laboratories, built an antenna designed to detect radio signals that were causing interference with radio- telephone calls over the Atlantic Ocean. His discovery set the foundation of radio astronomy. Grote Reber built the world's first radio telescope in 1937.
Andre G. Clavier established a microwave link between two stations in New Jersey in 1930 using a 10 ft. (3 m) parabolic dish antenna. In the following year, he began transmitting across the English Channel.
With the advent of direct satellite broadcasting, parabolic "dish" antennae have sprouted in many suburban and rural areas. They function very much like radio telescopes. Developments such as Digital satellite systems and Satellite Master Antenna Television have built on this technology.
Mobile technology has necessitated a significantly increased dependence on antennae; cellular phone antennae now dot much of the nation's open landscape and devices such as beepers utilize satellite signals. The preponderance of cellular phone antennae in populated areas has generated some controversy, as some observers fear the microwaves radiating from cellular phone antennae may be cancer-causing. Thus, residents sometimes oppose the installation of a mobile phone antennae near their homes.
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