Radio: A Form Of Communication Essay

This essay has a total of 2971 words and 16 pages.

Radio: A Form Of Communication

Radio: A Form of Communication


Physics
Idoh Gersten
Mr. Zambizi
Physics
March 12, 1995

Radio is a form of communication in which intelligence is transmitted without
wires from one point to another by means of electromagnetic waves. Early forms
of communication over great distances were the telephone and the telegraph. They
required wires between the sender and receiver. Radio, on the other hand,
requires no such physical connection. It relies on the radiation of energy from
a transmitting antenna in the form of radio waves. These radio waves, traveling
at the speed of light (300,000 km/sec; 186,000 mi/sec), carry the information.
When the waves arrive at a receiving antenna, a small electrical voltage is
produced. After this voltage has been suitably amplified, the original
information contained in the radio waves is retrieved and presented in an
understandable form. This form may be sound from a loudspeaker, a picture on a
television, or a printed page from a teletype machine.

HISTORY

Early Experimenters

The principles of radio had been demonstrated in the early 1800s by such
scientists as Michael Faraday and Joseph Henry. They had individually developed
the theory that a current flowing in one wire could induce (produce) a current
in another wire that was not physically connected to the first.

Hans Christian Oersted had shown in 1820 that a current flowing in a wire sets
up a magnetic field around the wire. If the current is made to change and, in
particular, made to alternate (flow back and forth), the building up and
collapsing of the associated magnetic field induces a current in another
conductor placed in this changing magnetic field. This principle of
electromagnetic induction is well known in the application of transformers,
where an iron core is used to link the magnetic field of the first wire or coil
with a secondary coil. By this means voltages can be stepped up or down in value.
This process is usually carried out at low frequencies of 50 or 60 Hz (Hertz, or
cycles per second). Radio waves, on the other hand, consist of frequencies
between 30 kHz and 300 GHz.

In 1864, James Clerk Maxwell published his first paper that showed by
theoretical reasoning that an electrical disturbance that results from a change
in an electrical quantity such as voltage or current should propagate (travel)
through space at the speed of light. He postulated that light waves were
electromagnetic waves consisting of electric and magnetic fields. In fact,
scientists now know that visible light is just a small portion of what is called
the electromagnetic spectrum, which includes radio waves, X rays, and gamma rays
(see electromagnetic radiation).

Heinrich Hertz, in the late 1880s, actually produced electromagnetic waves. He
used oscillating circuits (combinations of capacitors and inductors) to transmit
and receive radio waves. By measuring the wavelength of the waves and knowing
the frequency of oscillation, he was able to calculate the velocity of the waves.
He thus verified Maxwell's theoretical prediction that electromagnetic waves
travel at the speed of light.

Marconi's Contribution

It apparently did not occur to Hertz, however, to use electromagnetic waves for
long-distance communication. This application was pursued by Guglielmo Marconi;
in 1895, he produced the first practical wireless telegraph system. In 1896 he
received from the British government the first wireless patent. In part, it was
based on the theory that the communication range increases substantially as the
height of the aerial (antenna) is increased.

The first wireless telegraph message across the English Channel was sent by
Marconi in March 1899. The use of radio for emergencies at sea was demonstrated
soon after by Marconi's wireless company. (Wireless sets had been installed in
lighthouses along the English coast, permitting communication with radios aboard
nearby ships.) The first transatlantic communication, which involved sending the
Morse-code signal for the letter s was sent, on Dec. 12, 1901, from Cornwall,
England, to Saint John's, Newfoundland, where Marconi had set up receiving
equipment.

The Electron Tube

Further advancement of radio was made possible by the development of the
electron tube. The diode, or valve, produced by Sir Ambrose Fleming in 1905,
permitted the detection of high-frequency radio waves. In 1907, Lee De Forest
invented the audion, or Triode, which was able to amplify radio and sound waves.

Radiotelephone and Radiotelegraph

Up through this time, radio communication was in the form of radio telegraphy;
that is, individual letters in a message were sent by a dash-dot system called
Morse Code. (The International Morse Code is still used to send messages by
shortwave radio.) Communication of human speech first took place in 1906.
Reginald Aubrey Fessenden, a physicist, spoke by radio from Brant Rock, Mass.,
to ships in the Atlantic Ocean.

Armstrong's Contributions

Much of the improvement of radio receivers is the result of work done by the
American inventor Edwin Armstrong. In 1918 he developed the superheterodyne
circuit. Prior to this time, each stage of amplification in the receiver had to
be adjusted to the frequency of the desired broadcast station. This was an
awkward operation, and it was difficult to achieve perfect tuning over a wide
range of frequencies. Using the heterodyne principal, the incoming signal is
mixed with a frequency that varies in such a way that a fixed frequency is
always produced when the two signals are mixed. This fixed frequency contains
the information of the particular station to which the receiver is tuned and is
amplified hundreds of times before being heard at the loudspeaker. This type of
receiver is much more stable than its predecessor, the tuned-radio-frequency
(TRF) receiver.

In order to transmit speech the radio waves had to be modulated by audio sound
waves. Prior to 1937 this modulation was done by changing the amplitude, or
magnitude, of the radio waves, a process known as amplitude modulation (AM). In
1933, Armstrong discovered how to convey the sound on the radio waves by
changing or modulating the frequency of the carrier radio waves, a process known
as frequency modulation (FM). This system reduces the effects of artificial
noise and natural interference caused by atmospheric disturbances such as
lightning.

Radiobroadcasting

The first regular commercial radio broadcasts began in 1920, but the golden age
of broadcasting is generally considered to be from 1925 to 1950. NBC was the
first permanent national network; it was set up by the Radio Corporation of
America (RCA). Radio was also being used in the 1930s by airplane pilots, police,
and military personnel.

Significant changes in radio occurred in the 1950s. Television displaced the
dramas and variety shows on radio; they were replaced on radio by music, talk
shows, and all-news stations. The development of the transistor increased the
availability of portable radios, and the number of car radios soared.
Stereophonic were initiated in the early 1960s, and large numbers of stereo FM
receivers were sold in the 1970s. A recent development is stereo AM, which may
lead to a similar boom for this type of receiver in the 1980s.

OPERATION

Frequency Allocations

In the United States the Federal Communications Commission (FCC) allocates the
frequencies of the radio spectrum that may be used by various segments of
society. Although each user is assigned a specific frequency in any particular
area, general categories are identified. Some representative allocations are
indicated in the table that follows the article.

The Transmitter

The heart of every transmitter is an oscillator. The oscillator is used to
produce an electrical signal having a frequency equal to that assigned to the
user. In many cases the frequency of oscillation is accurately controlled by a
quartz crystal, which is a crystalline substance that vibrates at a natural
resonant frequency when it is supplied with energy. This resonant frequency
depends on its thickness and the manner in which it is cut. By means of the
piezoelectric effect, the vibrations are transformed into a small alternating
voltage having the same frequency. After being amplified several thousand times,
this voltage becomes the radio-frequency carrier. The manner in which this
carrier is used depends upon the type of transmitter.

Continuous Wave. If applied directly to the antenna, the energy of the carrier
is radiated in the form of radio waves. In early radiotelegraph communications
the transmitter was keyed on and off in a coded fashion using a telegraph key or
switch. The intelligence was transmitted by short and long bursts of radio waves
that represented letters of the alphabet by the Morse code's dots and dashes.
This system, also known as interrupted continuous wave (ICW) or, simply,
continuous wave (CW), is used today by amateur radio operators, by beacon buoys
in harbors, and by airport beacons.

Amplitude Modulation. In radio-telephone communication or standard broadcast
transmissions the speech and music are used to modulate the carrier. This
process means that the intelligence to be transmitted is used to vary some
property of the carrier. One method is to superimpose the intelligence on the
carrier by varying the amplitude of the carrier, hence the term amplitude
modulation (AM). The modulating audio signal (speech or music) is applied to a
microphone. This produces electrical signals that alternate, positively and
negatively. After amplification, these signals are applied to a modulator. When
the audio signals go positive, they increase the amplitude of the carrier; when
they go negative, they decrease the amplitude of the carrier. The amplitude of
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