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 roduced.
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. 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, cientists 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 oon 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 he 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 s 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 ramas 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.
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 ociety. 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 epends 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 witch. 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 eaks 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 Antenna An 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 pproximately 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 Receiver When 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) eceivers 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 requency 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 FM Although 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. Both AM and FM receivers incorporate automatic gain control (AGC), sometimes called automatic volume control (AVC). If a strong station is tuned in, the volume of the sound would tend to be overwhelming if the volume control had previously been set for a weak station. This drawback is overcome by the use of negative feedback–a DC voltage is developed at the detector and used to reduce utomatically the gain, or amplification, of the IF amplifiers.
The prime advantage of FM, in addition to its fidelity, is its immunity to electrical noise. Lightning storms superimpose noise on an AM signal by increasing the amplitude of the signal. This effect shows up in a receiver as a crackling noise. An FM receiver, because it decodes only the frequency variations, has a limiter circuit that restricts any amplitude variations that may result from added noise. Single Sideband Systems When an audio signal of 5 kHz is used to amplitude-modulate a carrier, the utput of the transmitter contains sideband frequencies in addition to the carrier frequency.
The upper sideband frequencies extend to 5 kHz higher than the carrier, and the lower sideband frequencies extend to 5 kHz lower than the carrier. In normal AM broadcasts both sidebands are transmitted, requiring a bandwidth in the frequency spectrum of 10 kHz, centered on the carrier frequency. The audio signal, however, is contained in and may be retrieved from either the upper or lower sideband. Furthermore, the carrier itself contains no useful information. Therefore, the only part that needs to be transmitted is one of the idebands.
A system designed to do this is called a single sideband suppressed carrier (abbreviated SSBSC, or SSB for short). This is an important system because it requires only half of the bandwidth needed for ordinary AM, thus allowing more channels to be assigned in any given portion of the frequency spectrum. Also, because of the reduced power requirements, a 110-watt SSB transmitter may have a range as great as that of a 1,000-watt conventional AM transmitter. Almost all ham radios, commercial radiotelephones, and marine-band radios, as well as citizens band radios, use SSB systems.
Receivers for such ystems are more complex, however, than those for other systems. The receiver must reinsert the nontransmitted carrier before successful heterodyning can take place. Radio has become a sophisticated and complex area of electrical engineering, especially when compared to its elementary origin. Every day new radio applications are being found, ranging from digital radio-controlled garage-door openers to weather satellites and from tracking systems for polar bear migrations to radio telescope investigations of the universe. This multiplicity of uses demonstrates the important part radio plays in the world today.
