Riding on Radio Waves
Dr V B Kamble
In Part-I, we outlined the application of radio waves and the early attempts for their application in what was then called the "wireless" communication. We saw how from the early days of the spark chamber of Heinrich Hertz, and development of sensitive detectors by Bose, and the tuning technique developed by Lodge, along with experiments by Marconi made the first trans-Atlantic radio contact possible on December 13, 1901. In this part, we shall outline the history of radio during the century following this great event. Indeed, this is a saga over nearly one and a half centuries since Maxwell predicted the existence of electromagnetic waves in 1864.
Diode and Audion:
The next major event was the discovery that an electrode operating at a positive voltage inside the evacuated envelope of a heated filament lamp would carry a current. The American inventor Thomas A. Edison had noted that the bulb of such a lamp blackened near the positive electrode, but it was Sir John Ambrose Fleming, professor of electrical engineering at Imperial College, London, who explored the phenomenon and in 1904 discovered the one directional current effect between a positively biassed electrode, which he called the anode, and the heated filament; the electrons flowed from filament to anode only. Fleming called the device a diode because it contained two electrodes, the anode and the heated filament; he noted that when an alternating current was applied, only the positive halves of the waves were passed - that is, the wave was rectified (changed from alternating to direct current). The diode could also be used to detect radio frequency wave and produced a pulsed direct current corresponding to the on and off of the Morse code transmitted signals. Fleming's discovery was the first step to the amplifier tube that in the early part of the 20th century revolutionized radio communication.
Sir John Ambrose Fleming
Fleming failed to appreciate the possibilities he had opened up and it was the American inventor Lee De Forest who in 1906 conceived the idea of interposing an open meshed grid between the heated filament and positively baissed anode, or plate, to control the flow of electrons. De Forest called his invention an Audion (now commonly known as triode). With it he could obtain a large voltage change at the plate for a small voltage change on the grid electrode. This was a discovery of major importance because it made it possible to amplify the radio-frequency signal picked up by the antenna before application to the receiver detector; thus, much weaker signals could be utilized than had previously been possible. Among the major developments of the first two decades of the 20th century was De Forest's discovery in 1912 of the oscillating properties of his Audion tube, a discovery that led to the replacement of the spark transmitter by an electronic tube oscillator that could generate much purer radio waves of relatively stable frequency.
Lee de Forest's father was a minister and hoped that his son would follow in his footsteps. In order to be trained for this calling, de Forest left Alabama for prep school in Massachusetts. His life at school was hard, with chores as well as academics, plus work to supplement his scholarship. Besides, he was not well-liked there. Biographers report he was extremely concerned with getting recognition from his peers, an issue which lasted throughout his life. Alas, he only won acknowledge-ment as "homeliest boy in school."
Despite this, he was confident. During school, de Forest had tried to get money (and fame) by inventing things he might sell or enter in contests, but none were great successes. After receiving a PhD from Yale in 1896 with a dissertation on radio waves, he developed an improved wireless telegraph receiver. By 1902, he had founded the De Forest Wireless Telegraph Company but like other firms he would start, it failed because of poor business practices.
De Forest was extremely creative and energetic, but often was unable to see the potential of his inventions or grasp their theoretical implications. While working on improving wireless telegraph equipment, he modified the work of other inventors and created the Audion, a vacuum tube containing some gas. It was a triode, incorporating a filament and a plate, like ordinary vacuum tubes, but also a grid between the filament and plate. This strengthened the current through the tube, amplifying weak telegraph and even radio signals. De Forest thought the gas was a necessary part of the system. In 1912, others showed that a triode in a complete vacuum would work far better.
This kind of "nearly getting it" would characterize de Forest's life. In 1912, he developed a feedback circuit, which would increase the output of a radio transmitter and produce alternating current. He didn't see the worth of his discovery, though, and by the time he applied for a patent in 1915, it had already been patented by E. Howard Armstrong. De Forest sued, with legal action lasting until 1934. He won, but the radio industry always credited Armstrong with the invention. His other major contribution was to the film industry. In the 1920s, he had been trying to use electricity to improve sound recordings. He found a way to record sound on film, again adapting the work of others and using his Audion. This led directly to the creation of motion pictures with sound. He achieved this in 1921 and tried to push his technology to the film industry. They resisted, but when they were finally won over, used a different method. The De Forest Phonofilm Corporation folded by 1925. Two years later The Jazz Singer appeared in theaters, the first feature-length "talkie." Ironically, the industry later reverted to the sound method De Forest first proposed.
Throughout his tumultuous life - many failed businesses, ongoing lawsuits, patent applications, and four marriages - de Forest promoted radio and later television as a way to raise Americans' cultural awareness. In 1910, he attempted the first live broadcast from New York's Metropolitan Opera House (starring Enrico Caruso). In 1916, he pioneered radio news, broadcasting - although incorrectly - the results of the presidential election. He was disappointed with how radio and television evolved, however, and was deeply critical of its low standards. De Forest wrote an autobiography entitled Father of Radio, but did not get that recognition from the rest of the world. He is remembered as one contributor to an industry that was, in truth, the work of many people.
Discovery of the Ionosphere
Ionosphere is the region of the Earth's atmosphere in which the number of ions, or electrically charged particles - resulting from the action of extraterrestrial (primarily solar) radiation on the neutral atoms and molecules of the air - is large enough to affect the propagation of radio waves. The ionosphere begins at a height of about 50 km above the surface but is most distinct at altitudes above about 80 km.
Discovery of the ionosphere extended over nearly a century. As early as 1839, the German mathematician Carl Friedrich Gauss speculated that an electrically conducting region of the atmosphere could account for observed variations of the Earth's magnetic field. The notion of a conducting region was reinvoked by others notably in 1902 by the American engineer Arthur E. Kennelly (1861 - 1939) and the British physicist Oliver Heaviside (1850 - 1925) to explain the transmission of radio signals around the curve of the Earth's surface, before definitive evidence was obtained in 1925. For some years the ion-rich region was referred to as the Kennelly-Heaviside layer (now called the E region of the ionosphere).
The ionosphere consists of three layers: the D region, the lowest, is strongly ionized during the day and is responsible for the attenuation of shortwave and broadcast band radio signals, an effect that largely disappears at night; the E layer, extending from 90 to 140 km, is a region of ionized molecules and strong electric currents; and the F region, from 140 km up, is characterized by ionized atoms and contains the stratum (designated as F2) in which ion concentration reaches a maximum. This layer is also called the Appleton layer after Sir Edward Victor Appleton (1892-1965) for his investigations of the physics of the upper atmosphere especially for the discovery of the so called Appleton layer. The E and F regions are responsible for the long distance propagation, by reflection, of radio signals in the short wave and broadcast bands. Ionization in the ionospheric region is chiefly effected by solar radiation at X-ray and ultraviolet wavelengths. The particles ionized are mainly molecular nitrogen and molecular and atomic oxygen, but at a height of about 1000 km, there is a region in which hydrogen nuclei (protons) constitute the dominant ionic species. Kennelly noticed that Guglielmo Marconi's reception, in Newfoundland in 1901, of radio signals transmitted from England was received far better than was predicted by radio wave theory. The following year he postulated that the radio waves were being reflected back to Earth from an ionized layer in the upper atmosphere. Shortly thereafter the British physicist Oliver Heaviside independently propounded the same theory, and the layer thus became known as the Kennelly - Heaviside layer.
It is interesting to note that Arthur Edwin Kennelly was born at Colaba, India. He was a U.S. electrical engineer who made innovations in analytic methods in electronics, particularly the definitive application of complex number theory to alternating current (ac) circuits. After working as an office boy for a London engineering society, as an electrician, and on a cable engineering ship, in 1887 Kennelly joined Thomas Edison's staff at West Orange, N.J., where he was chief assistant until 1894. Then, with Edwin J. Houston, he formed the consulting firm of Houston and Kennelly in Philadelphia.
Sisir Kumar Mitra (1890 - 1963) the Indian Physicist carried out important studies on the ionosphere. He set up an ionosphere field station at Haringhata near Calcutta, which was the first of its kind in India. He proposed a theory of "active nitrogen" to explain the luminescence of the night sky. He was the pioneer of radio physics in India.
Figure 1 : Main types of radio paths
Depending primarily on its frequency, a radio wave may travel from the transmitting to the receiving antenna in a number of ways. Figure 1 illustrates the main radio paths. Radio waves are sub-divided into several frequency bands depending on their frequency range. Different frequency bands are utilized for typical applications as shown in the Table 1.
Enter commercial companies
The first commercial company to be incorporated for the manufacture of radio apparatus was the Wireless Telegraph and Signal Company Ltd. (England) in July 1897 (later changed to Marconi's Wireless Telegraph Company, Ltd.); other countries soon showed an interest in the commercial exploitation of radio.
Among the major developments of the first two decades of the 20th century was De Forest's discovery in 1912 of the oscillating properties of his Audion tube, a discovery that led to the replacement of the spark transmitter by an electronic tube oscillator that could generate much purer radio waves of relatively stable frequency. By 1910, radio messages between land stations and ships had become commonplace, and in that year the first air-to-ground radio contact was established from an aircraft. In 1918 a radiotelegraph message from the Marconi long-wave station at Caemarvon, in Wales, was received in Australia, over a distance of 17,700 kilometres.
Though early experiments had shown that speech could be transmitted by radio, the first significant demonstration was not made until 1915 when the American Telephone & Telegraph Company successfully transmitted speech signals from west to east across the Atlantic between Arlington, Virginia, and Paris. A year later, a radiotele-phone message was conveyed to an aircraft flying near Brooklands (England) airfield. In 1919 a Marconi engineer spoke across the Atlantic in the reverse direction from Ballybunion, Ireland, to the U .S.
William Bradford Shockley
From 1920 onward radio made phenomenal progress through research activities in Europe, America, and Asia. The invention of the electron tube and later the transistor (1948) made possible remarkable developments in fields such as control and computing as well as telecommunications.
Major breakthroughs in radio circuitry
An electron tube or transistor, designated an active element, functions basically as an amplifier, and its output is essentially an amplified copy of the original input signal. The simplest amplifying electron tube is the triode, consisting of a cathode coated with material that provides a copious supply of electrons when heated, an open-mesh grid allowing electrons to pass through but controlling their flow, and a plate (anode) to collect the electrons. The plate is maintained at a positive voltage with respect to the cathode in order to attract the electrons; the grid usually has a small negative voltage so that it does not collect electrons but does control their flow to the plate. The output voltage is usually many times greater than the input voltage to the grid. The tube must be pumped to a high degree of vacuum, or the plate current flow is very erratic.
Other electrodes, also in the form of open-mesh grids, may be included in the tube to perform various special functions. An example is the four-electrode tube known as the tetrode, in which an open-mesh grid (screen grid) maintained at a positive voltage is placed between plate and control grid. This reduces the effect of plate voltage on electron flow and increases the amplifying property of the tube. Introduction of a third grid, known as a suppressor grid, produces the pentode (five-electrode tube), which can provide even greater amplification.
The transistor, which has largely replaced the electron tube as the active element in low-voltage electronic circuits, is made from semiconductor materials-that is, substances that are neither good conductors nor good insulators. Two common semiconductor materials are germanium and silicon, to which small amounts of impurities such as indium, gallium, arsenic, or phosphorus are added to impart electrical charges to them. Arsenic and phosphorus, for example, provide extra negative charges, giving n-type (signifying excess negative charges) material; indium or gallium yield a shortage of electrons or an excess of positive charges or holes, giving p-type (signifying excess positive charges) material. A transistor is a sandwich of semicondutor materials with the same impurity in the two outer layers and a different impurity in the centre layer providing current carriers of opposite charge to those produced by the outer layers.
A transistor is an amplifier of current; the vacuum tube, in contrast, is an amplifier of voltage. The transistor produces a very adequate supply of current carriers (electrons and holes) at room temperature and does not require a heated cathode like the vacuum tube. Thus the power required from the power supply is much reduced, much less heat is produced, and the transistors and their circuitry can be packed into a smaller space. Transistors are also physically much smaller than comparable electron tubes. Thus the transistorized portable radio can fit in a pocket in contrast to the cumbersome tube radio it has replaced.
Printed circuit boards and wiring, developed during 1940s, elminited much of the hand work and helped manufacturing of radio communication equipment at highly economic rates. The development of integrated circuits in recent years have made the design even more compact. A large scale integrated circuit, also called a chip, can perform a large number of functions. Incidentally, a chip is also a fundamental unit of a computer.
Broadcasting a programme
During a radio broadcast, a microphone picks up speech and other live sounds that make up the program. An electric current runs through the microphone. When sound waves enter the microphone, they disturb the current in the microphone, creating vibrations in it that match the sound waves. These electric waves are used to produce the radio waves that make up the broadcast. In a similar way, equipment in the radio station changes the prerecorded sounds of a program into electric waves.
The electric waves representing the sounds of a program travel over wires to the control board. From the control board, the electric waves go to the transmitter. In some stations, the transmitter is in the same room as the control board, and the electric waves that make up the program travel between the two instruments over wires. Other stations have their transmitter far from the radio station, at the site of the transmitting antenna (the device that sends radio waves through the air). In such cases, the electric waves are passed to the transmitters either by wire or by a special beam of radio waves.
The transmitter strengthens the incoming electric waves representing the broadcast. The transmitter also produces another kind of electric waves called carrier waves. It combines the carrier waves with the electric waves from the radio studio. This 'modulation' of waves becomes the radio signal that brings the program to radios.The transmitter sends the radio signal to the antenna. The antenna, in turn, sends the signal out into the air as radio waves.
The main parts of a radio receiver include: (1) antenna, (2) the tuner, (3) amplifiers, and (4) the speaker.
The antenna is a length of wire or a metal rod that picks up radio waves. It may be entirely inside the radio, or part of it may be outside the radio and connected to it, as is the case in automobile radios. When radio waves strike the antenna, they produce extremely weak electric waves in the antenna. However, an antenna receives radio waves from many stations at the same time. In order to hear a single programme, a listener must tune the radio to the desired station. The tuner is the part of the radio that makes it sensitive to particular frequencies, or channels of the stations that may be tuned in. The heart of a tuner is a radio part called a variable capacitor. This device consists of two sets of semicircular metal plates. The two sets mesh closely together. One set never moves. The other set shifts its position when a person twists the tuning control. This shifting produces changes in the radio's circuits that make the radio sensitive to various frequencies.
Amplifiers strenghten the program signal selected by the tuner. The amplifiers in a typical radio are parts of what is called a superheterodyne circuit. In most radios sold today, the main operating parts of this circuit are transistors. These transistors may all be part of a single integrated circuit. Most radios made before 1950 used vacuum tubes.
In most modern radio receivers, reception is based on what is known as the superheterodyning principle. The incoming radio frequency is mixed (heterodyned) with the output of a local oscillator (LO), the frequency of which is adjusted so that the difference between it and the incomimg signal is constant; the result is called the intermediate frequency (IF). Amplification is thereafter carried out at this intermediate frequency (IF). The signal next enters the detector. The detector removes the carrier wave from the signal and leaves only the audio frequency the part that represents the program. Finally, one or more AF amplifiers strengthen the audio frequency part of the signal and send it to the speaker. The speaker is the final link between the broadcasting studio and the listener. It changes the electric signal back into the original program sounds.
It was in 1916 that David Sarnoff, then contracts manager to the American Marconi Company, recommended that transmitting stations be built for the purpose of broadcasting speech and music and that "a radio music box" should be manufactured for the general sale. His proposals were not implemented at once - partly because America's entry into World War I in 1917, but they proved highly practical. Technically, the first entertainment broadcasting was done by the German Army in 1917, but, the first regular broadcasting station was KDKA in Pittsburgh, which started operations in November 1920. The service gained instant popularity, and the idea spread at once around the world. The medium-wave band of frequencies proved inadequate to provide sufficient broadcast channels. As a result, there was considrable development of the very high frequency (VHF) band after World War II. The greater availability of channel space in the VHF band allowed the use of frequency modulation (FM) techniques for broadcasting stereo programmes. Unless a satellite is employed, long distance broadcasting becomes possible only by means of short waves - also called high frequency (HF) waves - which are reflected back to earth from ionosphere as noted earlier.
Broadcasting from a geo-synchronous satellite, which maintains a fixed position over a particular location on Earth, also is possible, but, the power supply and antenna size limitations on the satellite require the carrier frequency to be in the ultra high frequency range - also called microwaves, which can pass through the ionosphere without getting absored. Since the satellite is situated at a height of nearly 36,000 kilometres, the signal at the Earth's surface becomes rather weak. A sensitive receiver and directional antenna system is, therefore, essential. Such services have become available in recent years.
Broadcasting service in India was set up in 1927. The first radio programme was broadcast in June 1923 by the Radio Club of Bombay (see box).
Radio had already attracted numerous amateur enthusiasts and experimenters, and at the end of World War I, they were granted permission to communicate by radio with each other at the upper end of the medium wave band, and in the short-wave band, frequencies then thought to be useless for long distance communication! To distinguish them from professionals, they were called "radio amateurs". Amateurs in Britain and USA could establish radio contact across the Atlantic at wavelengths of about 100 metres throughout most of February 1925. As remarked earlier, Marconi also could pick up signals from a distance of about 3700 kilometres at about 100 metres wavelength around this time. Radio amateurs thus helped estabish that short wavelengths could be used to provide a satisfactory service at great distances.
The need for economic operation, conservation of available frequency channels, and better rejection of noise led to the development of a mode of communication called the single side band (SSB) system. This important development by amateurs helped transmit speech at a power one third of the usual double side band (DSB) transmission. Modulating audio signals with a "carrier" radio frequency generates two side bands, one above the carrier frequency and one below it (hence the term double side band) along with the carrier. The speech signal information on such a system is present in either of the side bands. Elimination of one side band halves the frequency channel width as well as improving the strength of the signal with respect to the noise. This saves over two thirds of power. Eliminated side band frequency space can be used for another speech channel so that one transmission carries two different conversations simultaneously.
Even today, large numbers of enthusiasts are attracted to radio communication as a hobby. Indeed, many youngsters build an early interest in amateur radio into a career. Later, they may run into ideas which they try out in ham radio. A good example in the OSCAR series of satellites, initially put together by amateurs who worked in the aerospace industry and launched as payloads with other space shots. Today, a few Indian radio amateurs are working upon an Indian Amateur Satellite. Radio amateurs - or hams as they are called - communicate through a variety of techniques like radioteletype, slow and fast scan TV, packet radio etc. A few hams even have computers hooked to their radio. See box on how to become a ham.
Radio Communication Today
Applications of radio and its importance to human life needs no over-emphasis. Radio often implies communication through speech or Morse code. In fact radio communication, in general, may imply broadcast or two way communication using electromagnetic waves in any part of the radio frequency spectrum, and may involve wavelengths ranging from very low frequency (VLF) at 10-30 kilometres for long distance point-to-point communication to super high frequencies (SHF) at 1-10 centimetres or even less for radar, radio relay or satellite communication.
We have already mentioned the application of radio in broadcast services, entertainment and during relief operations following a natural or a man-made disaster. Widespread use of radio for personal communication involves citizen's band (CB) radio. Channels are made available for voice communication and remote control, such as opening of car doors. Cordless phones and mobile telephones involve frequencies in the VHF or UHF range. In industries, radio-frequency energy is of value in industrial heating applications, radio frequency electric furnaces, microwave ovens and so on.
However, the progress in the space sciences has been possible only through the application through the technique called radio telemetry. What it involves is the remote reading of instruments taking measurements in conditions either dangerous to human life or in places inaccessible or not easily accessible. Information about solar radiation, solar wind, magnetic fields, and ionospheric soundings from probes in outer space is transmitted to the information centre on Earth through telemetry. Commands to satellites or space probes are also sent through telemetry. Indeed, radio has found applications in many fields besides communication and has accelerated developments in all branches of science; medicine, surgery, astronomy and mathematics all have benefited.
Radio astronomy, now an important branch of science, was born with the reception (1932) of appreciable 20 megahertz radio emissions from the Milky Way galaxy. In 1942 investigators measured radio emissions from the Sun. Subsequency work has shown that emissions reaching the Earth from outer space cover a range of at least 10 megahertz to 30 gigahertz. Well-defined sources of radiation have been discovered, and some have been associated with visible stars or have led to the discovery of hitherto unknown stars; but many of these quasars, as they are called, emanate from a part of space where no stars have yet been seen. Certain of the radio sources show a slow periodic variation in emission, and these have been named pulsars.
We have briefly outlined the development of radio communication and the discovery of ionosphere which took place during the golden decade. Since the days of Hertz, Bose and Marconi, newer means, methods and techniques have emerged. How easily we pick up mobile phone and talk anywhere in the world, or how fast we can send the information from one corner of the world to the other through the Internet. It is through the development in techniques of radio communication that applications of space science and technology in various spheres like remote sensing and satellite communication became possible. If it were not for the communication satellites, like the INSAT series of geosynchronous satellites, linking and networking of different parts of our huge country could have proved extremely difficult. Lately, there has been an ingenious development in radio communication, the digital radio communication which also enables hooking the radio directly to your computer and transfering the data files - the WorldSpace satellite radio. Such satellite radios would prove to be a boon for development especially in the rural areas. It may also prove to be a boon in disaster management. It is the faster communication through radio that has saved innumerable lives after a disaster strikes - be it the devastating earthquake in Gujarat or supercyclone in Orissa. Radio will continue to serve the humanity.
President of the Royal Swedish Academy of sciences, H. Hildebrand had the following to say during the presentation speech of the Nobel award ceremony when the prize was conferred on Marconi and Braun in 1909:
'Research workers and engineers toil unceasingly on the development of wireless telegraphy. Where this development can lead, we know not. However, with the results already achieved, telegraphy over wires has been extended by this invention in the most fortunate way. Independent of fixed conductor routes and independent of space, we can produce connections between far-distant places, over far-reaching waters and deserts. This is the magnificent practical invention which has flowed upon one of the most brilliant scientific discovery of our time.' How True!
1. Elements of Radio
2. Jagdis Chandra Bose and the Indian Response to Western Science
3. The Real Inventor of Wireless
4. Guglielmo Marconi
5. Encyclopaedia Britannica
6. World Book Encyclopaedia
7. Dictionary of Scientific Biography
9. Electrical oscillations and wireless telegraphy
10. Wireless telegraphic communication
12. Be a ham! Talk to the world!
Riding on Radio Waves
Important terms used in connection with radio waves are given below. The terms given do not necessarily appear in the present article
Amplifier : A device capable of increasing the magnitude or power level of a physical quantity, such as an electric current.
Amplitude modulation : Modulation in which the amplitude of a wave is the characteristic varied in accordance with the intelligence to be transmitted.
Antenna : A device used for radiating or receiving radio waves. Also known as aerial; radio antenna.
Audio frequency : A frequency that can be detected as a sound by the average young adult, approximately 15 to 20,000 hertz. Abbreviated af. Also known as sonic frequency; sound frequency.
Audio-frequency amplifier : An electronic circuit for amplification of signals within, and in some cases above, the audible range of frequencies in equipment used to record and reproduce sound. Also known as af amplifier; audio amplifier.
Audio-frequency oscillator : An oscillator circuit using an electron tube, transistor, or other nonrotating device to produce an audio-frequency alternating current. Also known as audio oscillator.
Audio signal : An electric signal having the frequency of a mechanical wave that can be detected as a sound by the human ear.
Broadcast : A television or radio transmission intended for public reception.
Broadcast station : A television or radio station used for transmitting programmes to the general public. Also known as station.
Broadcast transmitter : A transmitter designed for use in a commercial amplitude-modulation, frequency-modulation, or television broadcast channel.
Capacitor : A device which consists essentially of two conductors (such as parallel metal plates) insulated from each other by a dielectric and which introduces capacitance into a circuit, stores electrical energy, blocks the flow of direct current, and permits the flow of alternating current to a degree dependent on the capacitor's capacitance and the current frequency. Designated by C. Also known as condenser; electrical condenser.
Coherer : A cell containing a grannel conductor between two elctrodes. The cell becomes highly conducting when subjected to an electric field, and conduction can then be stopped by joining the grannel.
Continuous wave : A radio or radar wave whose successive sinusoidal oscillations are identical under steady-state conditions. Abbreviated CW. Also known as type A wave.
Crystal : A natural or synthetic piezoelectric or semiconductor material whose atoms are arranged with some degree of geometric regularity.
Crystal detector : A crystal diode, or an equivalent earlier crystal-catwhisker combination, used to rectify a modulated radio-frequency signal to obtain the audio or video signal directly.
Crystal diode : A two-electrode semiconductor device that utilizes the rectifying properties of a pn junction or a point contact. Also known as semiconductor diode.
Crystal set : A radio receiver having a crystal detector stage for demodulation of the received signals, but no amplifier stages.
Current : The net transfer of electric charge per unit time; a specialization of the physics definition. Also known as electric current.
Demodulate: T recover the modulating wave from a modulated carrier. Also known as decode; detect.
Detector : The stage in a receiver at which demodulation takes place; in a superheterodyne receiver this is called the second detector. Also known as demodulator; envelope detector.
Diode : A two-electrode semiconductor device that utilizes the rectifying properties of a pn junction or a point contact. Also known as crystal diode; crystal rectifier; semiconductor diode.
Dipole antenna : An antenna approximately one-half wavelength long, split at its electrical centre for connection to a transmission line whose radiation pattern has a maximum at right angles to the antenna. Also known as doublet antenna; half-wave dipole.
Double-side band : The two-side bands produced about the carrier frequency on modulating with the audio frequency containing identical information, one above the carrier frequency, and one below it. Both are mirror images of each other.
E layer : A layer of ionized air occurring at altitudes between 100 and 120 kilometers in the E region of the ionosphere, capable of bending radio waves back to earth. Also known as Heaviside layer; Kennelly-Heaviside layer.
Electromagnetic wave : A disturbance which propagates outward from any electric charge which oscillates or is accelerated; far from the charge it consists of vibrating electric and magnetic fields which move at the speed of light and are at right angles to each other and to the direction of motion.
Feedback : The return of a portion of the output of a circuit or device to its input.
F layer : An ionized layer in the F region of the ionosphere which consists of F1 and F2 layers in the day hemisphere, and the F2 layer alone in the night hemisphere; it is capable of reflecting radio waves to earth at frequencies up to about 50 megahertz.
Ground wave : A radio wave that is propagated along the earth and is ordinarily affected by the presence of the ground and the troposphere; includes all components of a radio wave over the earth except ionospheric and tropospheric waves. Also known as surface wave.
Half-wave antenna : An antenna whose electrical length is half the wavelength being transmitted or received.
Half-wave dipole : An antenna approximately one-half wavelength long, split at its electrical centre for connection to a transmission line whose radiation pattern has a maximum at right angles to the antenna. Also known as doublet antenna; dipole antenna.
Induction coil : A device for producing high-voltage alternating current or high-voltage pulses from low-voltage direct current, in which interruption of direct current in a primary coil, containing relatively few turns of wire, induces a high voltage in a secondary coil, containing may turns of wire wound over the primary.
Kennelly-Heaviside layer : A layer of ionized air occurring at altitudes between 100 and 120 kilometers in the E region of the ionosphere, capable of bending radio waves back to earth. Also known as Heaviside layer; E layer.
Marconi antenna : Antenna system of which the ground is an essential part, as distinguished from a Hertz antenna. Modulated amplifier : Amplifier stage in a transmitter in which the modulating signal is introduced and modulates the carrier.
Modulated carrier : Radio-frequency carrier wave whose amplitude phase or frequency has been varied according to the intelligence to be conveyed. Modulated continuous wave : Wave in which the carrier is modulated by a constant audio-frequency tone.
Modulation : The process or the result of the process by which some parameter of one wave is varied in accordance with some parameter of another wave.
Morse code : A telegraph code for manual operating, consisting of short (dot) and long (dash) signals and various-length spaces; now used only for wire telegraphy.
npn junction : A junction transitor having a p-type base between an n-type emitter and an n-type collector.
pn junction : The interface between two regions in a semiconductor crystal which have been treated so that one is a p-type semiconductor and the other is an n-type semiconductor; it contains a permanent dipole charge layer.
pnp-transistor : A junction transistor having an n-type base between a p-type emitter and a p-type collector.
Radio : The transmission of signals through space by means of electromagnetic waves; usually applied to the transmission of sound and code signals, although television and radar also depend on electromagnetic waves.
Radio frequency : A frequency at which coherent electromagnetic radiation of energy is useful for communication purposes; roughly the range from 10 kilohertz to 100 gigahertz. Abbreviated rf.
Radio-frequency amplifier : An amplifier that amplifies the high-frequency signals commonly used in radio communications.
Radiotelegraphy : Telegraphy involving the use of radio waves in place of wire lines.
Radio telemetry : The presentation of data at a location remote from the source of the data, using radio-frequency electromagnetic radiation as the means of transmission.
Radiotelephony : Two-way transmission of sounds by means of modulated radio waves, without interconnecting wires.
Radio telescope : An astronomical instrument used to measure the amount of radio energy coming from various directions in the sky, consisting of a highly directional antenna and associated electronic equipment.
Radio transmission : The transmission of signals through space at radio frequencies by means of radiated electromagnetic waves.
Radio transmitter : The equipment used for generating and amplifying a radio-frequency carrier signal, modulating the carrier signal and intelligence, and feeding the modulated carrier to an antenna and radiation into space as electromagnetic waves. Also known as radio set; transmitter.
Radio wave : An electromagnetic wave produced by reversal of current in a conductor at a frequency in the range from about 10 kilohertz to about 300,000 megahertz.
Single-side band : Either of the two side bands produced on modulation of carrier frequency by the audio signal containing information, one above the carrier frequency or one below it.
Sky wave : A radio wave that travels upward into space and may or may not be returned to earth by reflection from the ionosphere. Also known as ionospheric wave.
Spark coil : An induction coil for producing spark discharges, as to initiate combustion in an internal combustion engine.
Spark transmitter : A radio transmitter that utilizes the oscillatory discharge of a capacitor through an inductor and a spark gap as the source of radio-frequency power.
Superheterodyne receiver : A receiver in which all incoming modulated radio-frequency carrier signals are converted to a common intermediate-frequency carrier value for additional amplification and selectivity prior to demodulation, using heterodyne action; the output of the intermediate-frequency amplifier is then demodulated in the second detector to give the desired audio-frequency signal. Also known as superhet.
Telecommunications : Communication over long distances. Telegraph carrier : The single-frequency wave which is modulated by transmitting apparatus in carrier telegraphy.
Telegraph transmitter : A device that controls an electric power source in order to form telegraph signals.
Telegraphy : Communication at a distance by means of code signals consisting of current pulses sent over wires or by radio.
Telephony : The transmission of speech to a distant point by means of electric signals.
Transistor : An active component of an electronic circuit consisting of a small block of semiconducting material to which at least three electrical contacts are made, usually two closely spaced rectifying contacts and one ohmic (non-rectifying) contact; it may be sued as an amplifier, detector, or switch.
Transmitter : The equipment used for generating and amplifying a radio-frequency carrier signal, modulating the carrier signal and intelligence, and feeding the modulated carrier to an antenna and radiation into space as electromagnetic waves. Also known as radio set; radio transmitter.
Triode : A three-electrode electron tube containing an anode, a cathode, and a control electrode.
Tuned amplifier : An amplifier in which the load is a tuned circuit; load impedance and amplifier gain then vary with frequency.
Tuning : The process of adjusting the inductance or the capacitance or both in a tuned circuit, for example, in a radio, television, or radar receiver or transmitter, so as to obtain optimum performance at a selected frequency.
Tuning capacitor : A variable capacitor used for tuning purposes.
Vacuum tube : An electron tube evacuated to such a degree that its electrical characteristics are essentially unaffected by the presence of residual gas or vapor.
Valve : An electron device in which conduction of electricity is provided by electrons moving through a vacuum or gaseous medium within a gastight envelope. Also known as radio tube; tube; valve.
Wavelength : The distance between two points having the same phase in two consecutive cycles of a periodic wave, along a line in the direction of propagation.
Wave propagation : The process by which a disturbance at one point is propagated to another point more remote from the source with no net transport of the material of the medium itself; examples include the motion of electromagnetic waves, sound waves, hydrodynamic waves in liquids, and vibration waves in solids. Also known as propagation; wave motion.
Yagi antenna : An end-ire antenna array having maximum radiation in the direction of the array line; it has one dipole connected to the transmission line and a number of equally spaced unconnected dipoles mounted parallel to the first in the same horizontal plane to serve as directors and reflectors. Also known as Uda antenna; Yagi-Uda antenna.