Untitled Essay, Research Paper
Idoh Gersten
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.HISTORYEarly ExperimentersThe 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 ContributionIt 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 TubeFurther 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 RadiotelegraphUp 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 ContributionsMuch 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.RadiobroadcastingThe 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.OPERATIONFrequency AllocationsIn 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 TransmitterThe 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 the carrier now has
superimposed on it the variation of the audio signal, with peaks and valleys dependent on
the volume of the audio input to the microphone. The carrier has been modulated and, after
further amplification, is sent by means of a transmission line to the transmitting
antenna.The maximum modulating frequency permitted by AM broadcast stations is 5 kHz at carrier
frequencies between 535 and 1,605 kHz. The strongest AM stations have a power output of
50,000 watts.Frequency Modulation. Another method of modulating the carrier is to vary its frequency.
In frequency modulation (FM), on the positive half-cycle of the audio signal the frequency
of the carrier gradually increases. On the negative half-cycle it is decreased. The louder
the sound being used for modulation, the higher will be the change in frequency. A maximum
deviation of 75 kHz above and below the carrier frequency is permitted at maximum volume
in FM broadcasts. The rate at which the carrier frequency is varied is determined by the
frequency of the audio signal. The maximum modulating frequency permitted by FM broadcast
stations is 15 kHz at carrier frequencies between 88 and 108 MHz. This wider carrier
frequency (15 kHz for FM as opposed to 5 kHz for standard AM broadcasts) accounts for the
high fidelity of FM receivers. FM stations range in power from 100 watts to 100,000 watts.
They cover distances of 24-105 km (15-65 mi) because government frequency allocations for
commercial FM are in the VHF range, unlike commercial AM. Television transmitters use AM
for picture signals and FM for sound.The CW system described earlier is used in a modified FM form known as frequency shift
keying (FSK) by high-speed teletype, facsimile, missile-guidance telemetry, and satellite
communication. The carrier is shifted by amounts between 400 and 2,000 Hz. The shifts are
made in a coded fashion and are decoded in the receiver. This keeps the receiver quiet
between the dots and dashes and produces an audible sound in the receiver corresponding to
the coded information.The AntennaAn ANTENNA is a wire or metal conductor used either to radiate energy from a transmitter
or to pick up energy at a receiver. It is insulated from the ground and may be situated
vertically or horizontally.The radio waves emitted from an antenna consist of electric and magnetic fields, mutually
perpendicular to one another and to the direction of propagation. A vertical antenna is
said to be vertically polarized because its electric field has a vertical orientation. An
AM broadcast antenna is vertically polarized, requiring the receiving antenna to be
located vertically also, as in an automobile installation. Television and FM broadcast
transmitters use a horizontal polarization antenna.For efficient radiation the required length of a transmitting (and receiving) dipole
antenna must be half a wavelength or some multiple of a half-wavelength. Thus an FM
station that broadcasts at 100 MHz, which has a wavelength of 3 m (9 ft 10 in), should
have a horizontally polarized antenna 1.5 m (4 ft 11 in) in length. Receiving antennas
(sometimes in the form of "rabbit ears") should be approximately the same length
and placed horizontally.For an AM station broadcasting at 1,000 kHz, the length should be 150 m (492 ft). This is
an impractical length, especially when it must be mounted vertically. In this case, a
quarter-wavelength Marconi antenna is often used, with the ground (earth), serving as the
other quarter wavelength.The ReceiverWhen the modulated carrier reaches the receiving antenna, a small voltage is induced. This
may be as small as 0.1 microvolt in some commercial communication receivers but is
typically 50 microvolts in a standard AM broadcast receiver. This voltage is coupled to a
tunable circuit, which consists of a coil and a variable capacitor. The capacitor has a
set of fixed metal plates and a set of movable plates. When one set of plates is moved
with respect to the other, the capacitance is changed, making the circuit sensitive to a
different, narrow frequency range. The listener thus selects which transmitted signal the
receiver should reproduce.The Crystal Receiver. An early method of detecting radio waves was the crystal receiver. A
crystal of galena or carborundum along with a movable pointed wire called a cat whisker
provides a simple rectifier. This component lets current flow in one direction only, so
that only the upper half of the modulated wave can pass. A capacitor is then used to
filter out the unwanted high-frequency carrier, leaving the audio to operate the
earphones. No external power or amplifiers are used, so the only source of power in the
earphones is the signal. Only strong signals are audible, but with a long antenna and a
good ground, reception of a signal from 1,600 km (1,000 mi) away is sometimes possible.The TRF Receiver. Following the development of the triode, increasing selectivity,
sensitivity, and audio output power in tuned-radio-frequency (TRF) receivers was possible.
This process involved a number of stages of radio-frequency amplification prior to the
detection stage. In early receivers each of these stages had to be separately tuned to the
incoming frequency–a difficult task. Even after single-dial tuning was achieved by
ganging together the stages, the TRF was susceptible to breaking into oscillation and was
not suitable for tuning over a wide range of frequencies. The principle is still used,
however, in some modern shipboard emergency receivers and fixed-frequency microwave
receivers.The Superheterodyne Receiver. Practically all modern radio receivers use the heterodyne
principle. The incoming modulated signal is combined with the output of a tunable local
oscillator whose frequency is always a fixed amount above the incoming signal. This
process, called frequency conversion or heterodyning, takes place in a mixer circuit. The
output of the mixer is a radio frequency that contains the original information at the
antenna. This frequency, called the intermediate frequency (IF), is typically 455 kHz in
AM broadcast receivers. No matter what the frequency that the receiver is tuned to, the
intermediate frequency is always the same; it contains the information of the desired
station. As a result, all further stages of radio-frequency amplification can be designed
to operate at this fixed intermediate frequency.After detection, audio amplifiers boost the signal to a level capable of driving a
loudspeaker.Comparison of AM and FMAlthough the method of detection differs in AM and FM receivers, the same heterodyne
principle is used in each. An FM receiver, however, generally includes automatic frequency
control (AFC). If the frequency of the local oscillator drifts from its correct value the
station will fade. To avoid this problem, a DC voltage is developed at the detector and
fed back to the local oscillator. This voltage is used to change automatically the
frequency output of the local oscillator to maintain the proper intermediate frequency.