Федеральное агентство по образованию

Федеральное агентство по образованию

Федеральное агентство по образованию

Государственное образовательное учреждение высшего профессионального образования

Ульяновский государственный технический университет

Ч Т Е Н И Е

ОБЩЕНАУЧНОЙ

Л И Т Е Р А Т У Р Ы

Методическое пособие по английскому языку для студентов радиотехнического факультета

Составитель О. А. Кытманова

Ульяновск

2005

CONTENTS

1.

Radio engineering and electronics…………………………..………………4

2.

Means of telecommunications…………………………………………...…20

3.

Television…………………………………………………..……………....24

4.

Broadcasting……………………………………………………………..…29

5.

Radar…………………………………………………………………….…36

6.

Radio…………………………………………………………………….…39

7.

Radio astronomy………………………………………………………...…46

8.

Vocabulary………………………………………………….……………...

50

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RADIO ENGINEERING AND ELECTRONICS

Electron Emission

There is little doubt that wireless, radio, and television are among the greatest miracles of modern science. Traveling with the speed of light, code signals, the human voice and music can be heard around the world within the very second they are produced in the broadcasting studio. Through television, world events can be observed in full color at the same moment they occur hundreds of miles away.

The more we learn of the fundamental principles of radio and its operation the more amazing does their reality become. The heart of a tube is the source of electrons. There are several ways in which free electrons are obtainable.

Thermionic Emission. The velocity of electrons and atoms, as they move about within the confines of the material they comprise, is dependent on the temperature.

At a temperature of absolute zero all molecular activity is supposed to cease. As the temperature is increased, the activity of electrons and atoms increases until a point is reached where the electrons have sufficient velocity to enable them to break through the potential barrier of the material. This evaporation of electrons from the body of a solid at high temperature is known, as thermionic emission. The emission or evaporation of electrons takes place at lower temperature than does that of atoms. The mass of electrons "being smaller, it reaches the higher velocities necessary for evaporation at low temperatures than does the heavier atom. The temperature becoming high enough for the atoms to evaporate, the material or solid that they compose rapidly disintegrates.

Secondary Emission. One knows to a high degree of certainty*1 that being accelerated to a sufficiently high velocity an electron may have enough kinetic energy imparted to it to knock one or more electrons out of any material it comes in contact with, either a metal conductor or an insulator. A positively charged electrode situated near the source of these "secondary" electrons will collect them.

In actual tubes the secondary electrons may be attracted back to the electrode they come from, as from the plate, or they may be collected by another electrode which is positively charged. In many tubes these secondary electrons give rise to undesirable effects, design steps being taken to reduce their number and to control their movements. In a few tubes, such as electron multipliers, the desired operation is based on the principle of secondary emission.

Photoelectric Emission. When light of proper wavelength is allowed to fall upon certain metals, electrons are released from the surface of the metal as a result of the energy imparted by the light. Here, then, is another electron source. Such sources are used in phototubes and in certain types of television camera tubes.

Diodes

The modern diode consists of a glass envelope in which are erected a small metal plate and a fine wire called the filament. The filament in a modern tube is referred to as the cathode or emitter, its material being chosen so that it will heat when an electric current flows through it. The cathode may be a sleeve or a cylinder of

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metal slipped over a filament so that heat from the latter will cause the cathode to become; hot. The first type is called a filament cathode; the second is known as a heater cathode, the filament merely acting as a stove or heater. Modern manufacturing methods dictate that most tubes should be mounted on a base of some insulating material such as hard rubber, bakelite or porcelain, into which are molded metal pins through which electrical connection is-made to the electrodes.

Suitable sockets, into which these pins are inserted, permit easy and rapid replacement of tubes, when necessary. Figure 8 shows the essential construction of a diode tube. The cathode, when heated either directly or indirectly, emits electrons which form an indivisible cloud in the area about it. This negative cloud of electrons is called the space charge. If another electrode in the tube, such as the plate (anode), be positively charged by an external electric source (battery or generator), the electrons will be attracted to the plate, because, of their being negatively charged and because the unlike fields attract. Thus there will be a continuous flow of electricity from cathode to anode across the intervening space.

The amount of current flow will depend upon the ability of the cathode to emit electrons and the amount of positive attraction at the plate, i. e., the positive charge. The positive charge being increased, the rate of the flew and the speed of the electrons will increase until a point is reached, when the cathode cannot emit electrons at a faster rate for its present temperature. This is spoken of as the saturation point. The emission rate can be increased by raising the temperature or the cathode, but the life of the tube will be shortened should the temperature exceed a fixed point. The amount of positive, charge that can be applied to the plate depends upon the distance between the elements and the size of the plate. The collision of the electrons with the plate results in heat. The plate must have enough surface area to dissipate this heat by radiation. From the foregoing it is apparent that if an alternating current (changing polarity) is applied to the plate, a current will flow in a plate circuit whenever the plate is positive, or during every other half cycle. Further, the frequency of these d. c. pulsations will depend upon the frequency at which the alternating current reverses its polarity from positive to negative. This quality in a diode is used to convert, or rectify, alternating current to direct current.

Three-Element Electron Tube

The insertion of the so-called third electrode between the filament and the plate has been found to make the electron tube more versatile by enabling it to serve a number of functions, especially in telephone and radio circuits. The three element tube and its circuits then appear as in Figure 9. The effect of the grid is like that of a shutter which, opening and closing, controls the flow of electrons going through it from the filament to the plate. This control is accomplished by changing the

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potential of the grid. The grid being positively charged, it attracts electrons and increases their flow from the filament to the plate, for most of them pass through the relatively wide spaces between the grid wires. On the contrary the grid being negatively charged, it repels the electrons and they cannot go to the plate. Consequently, when the grid G is made alternately positive and negative by joining the input terminals to a source of alternating potential, the electron flow from F to P is increased and decreased accordingly, thereby varying the direct current in the plate circuit. The grid potential might change thousands or millions of times per second and the plate current would change accordingly. Actually, the grid is not made positive with respect to the filament, but only more or less negative. This is done by inserting a so-called С battery," as* shown, to "bias" the grid negatively. When so biased there will be no current in the grid circuit. Thus, the grid serves as a gate-valve to control the plate current while taking practically no power itself.

The cathode of the tube Is often a thin metal sleeve coated with thorium or other proper material having a low work function; a heating coil of tungsten wire is mounted within but separated from the sleeve. This construction makes it possible to heat the cathode with alternating current without introducing disturbing effects.

The Traveling-Wave Valve

In any amplifier or oscillator the function of the electron stream is to convert d. c. energy from h. f. supply to a. c. energy at the required frequency. If a valve could be made in which a large fraction of the electrons continuously gave up their d. c. energy, to the h. f. circuit over a large number of periods of the latter, a highfrequency amplifier or oscillator would result, this being to "a certain extent successfully accomplished in the traveling-wave valve. The latter- may be used either as an oscillator or as an amplifier. As an amplifier it is characterized by a high gain associated "with an extremely large bandwidth. Its noise-factor, though hot much if at all better than that of a silicon crystal is considerably less than that of a klystron. It has therefore a certain value in some special communication and radar circuits. Imagine a concentric line the inner conductor of which is a wire helix, as in Fig. 10.

If a signal is applied across the terminals AB, it travels along the inner conductor to the terminals CD at a speed being determined by the total length of the wire forming the helix. That is, if the helix consists of N turns of diameter d, the total length Nicd and, to a first approximation, the time taken by a signal to pass from A to С is N

π d/c, where с is the velocity of light.If the shortest distance between A and С

(along the axis of the helix) is I, the apparent speed of the signal along ВС is lc/

π dN. By giving N and d suitable values, this axial speed may be made small compared with c. Now imagine that an electron gun at E produces a beam of electrons traveling

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along AC with speed y. Then if v = ic/Nicdt the electrons in the beam travel at approximately the same speed as the signal and a continuous interchange of energy between the electron stream and the circuit is feasible. . ' The principles of the travelling -wave valve associate the slowly moving signal with the words

"travelling-wave" in contradistinction to the standing wave which exists m a klystron or a conventional valve amplifier. It should be noted, however, that the signal in a multiresonator magnetron may be regarded similarly as a travelling wave, rotating round the anode.

Hot-Cathode Gas-Filled Triodes: Thyratrons

The useful properties of the hot-cathode gas-filled diode immediately suggest the introduction of a control grid. The most common valve of this type is one which contains mercury vapour but the filling gas may be helium or argon or a mixture of inert gases. In fact, electrically, the valve acts as a switch and so has been given the name thyratron "from the Greek word meaning a door. It opens the way to the electric current.

The thyratron grid is known to control the flow of electrons from the cathode to the anode, but only in the sense of an on/off switch. When the grid is sufficiently negative no electrons can pass through it to the cathode, and so the current is cut off. When the grid is made less negative there comes a stage at which a certain number of electrons can pass through it. If the anode is then sufficiently positive with respect to the cathode, these electrons produce positive ions by collision with the gas or vapour molecules These positive ions partially destroy the space charge near the cathode, the current flowing through the valve freely. When the current has started, however, the grid can no longer control it, for making the grid more negative only increases the potential difference between it and the anode and draws to it a greater number of positive ions. These positive ions form a space charge which masks the potential of the grid wires and allows electrons to pass freely through from the cathode, since, therefore, the grid looses control of the current which it has initiated, to render the thyratron nonconducting, again it is necessary to cut off the positive potential applied to the anode. When- the current ceases, the grid regains control. The thyratron is not constructed in the same manner as the conventional high-vacuum valve. The usual structure is shown diagrammatically by the axial section drawn in Fig. 11, where the cathode is an indirectly-heated cylinder, K, coated with oxides. The grid, G, is a cylinder surrounding the cathode and containing an annual disk, D. Opposite the aperture in the disk, the cup-shaped anode is placed. This old construction is used for two reasons. First, as in the gas diode, it is unnecessary for the emitting surface of the cathode and the collecting surface of the anode to be placed opposite each other, and secondly, since an extremely small primary electron current is sufficient to start the valve conducting, the grid control must be made as strong as possible.

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In fact, no electrons should be able to reach the anode when the grid is at a reasonably. high negative voltage. The grid cylinder, G, therefore almost completely surrounds the cathode, leaving only a narrow aperture near the anode.

The critical grid voltage , Vgo, at which the valve begins to conduct (or" strikes" as it is called) varies with the anode voltage Va.

However, with the valve containing mercury vapour, the critical grid voltage also varies with the temperature, for the mercury-vapour pressure determines the onset of ionization to a certain extent. For example, if Vec = —6.5 at Va= 1,000 and at an effective temperature of 40° C, Vgc might be — 10 at a temperature of 70° С and —3 at 35° C. This variation of a striking voltage with temperature can be a grave disadvantage where large changes in temperature are experienced.

Rectifiers

Rectifiers are devices designed to convert alternating current into direct current. For this purpose devices with asymmetrical conductance such as vacuum and semiconductor diodes are used. Until the end of the twenties of this century vacuum diodes (kenotrones) were the main rectifying devices. In the thirties highpower kenotrones were replaced by more efficient mercury vapour rectifiers.

At present high-power gas-filled and gas-control tubes are used. Low-power rectifiers mostly used in low and medium power rectifying systems are replaced by highly efficient and reliable semiconductor rectifiers. Let us examine the simplest types of rectifiers. a) Half wave rectifiers (Fig. 12).

The load resistor R is connected in series with the transformer secondary and the semiconductor rectifying diode D. The wave-form in graph 1 shows the change in voltage across the transformer secondary. Due to inherent rectifying properties the diode conducts current only during the positive half-waves of the voltage cycles.

The current I and the output voltage VR are of a pulsating pattern (graph 2). To smooth these pulsations a capacitor С is connected across the load. When the transformer output voltage goes negative or drops lower than the voltage applied to the capacitor, the ion diode does not conduct current and the capacitor is discharged through load resistor R (sections ВС, DE, FG). b) Full-wave rectifiers can be considered as composed of two half-wave rectifiers connected to one common load. Shown in the figure 12 is a full-wave rectifier with two rectifying elements and a centre-tapped secondary winding on the transformer.

When the voltage on the upper terminal of the secondary is positive with respect to

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the centre tap, diode D, conducts current I, in the direction indicated by an arrow.

During the next half-wave the voltage polarity will be reversed and Diode D2 will conduct current I2 to the common load. During both half-waves the current through the load resistor R (It and I2) is of the same direction, the output current I

= Ij + Ia being of a pulsating wave-form.

Vacuum-Tube Oscillator

То broadcast the human voice by radio, a generator of alternating current of extremely high frequency and constant amplitude is required. In commercial broadcasting stations and amateur transmitters this function is performed by a vacuum tube and circuit of relatively simple design.

One type of oscillator circuit is shown in Fig. 13. When the switch S is closed connecting the B-battery to the plate of the tube an electron current from the cathode К to the plate P starts a current in the circuit

PRVL3K. This growing current in L3 creates an expanding magnetic field, which cutting across L2 induces a current in the grid circuit in such a direction that the grid becomes negative. A negative charge on the grid causes the plate current to decrease. This decreasing current causes the field about L3 to collapse, thus inducing a reversed current in the grid circuit and therefore a positive charge on the grid. Such a charge increases the plate current and the above process is repeated.

If the two circuits L2C2 and PRVL3 were properly tuned by adjusting C2 resonance would occur and energy from the B-battery would be continuously supplied to keep the oscillations going with constant amplitude. The graph of the continuous oscillations shown in Fig. 14 represents the voltage across L3 as it varies in time. The L3C2 circuit controls the frequency by controlling the grid potential while the large voltage and current

fluctuations take place in the L3 circuit.

The oscillatory circuit. An inductance and capacitance, connected as shown in simplest, schematic form in Fig. 15 form the necessary elements of all oscillating circuits. If initially the capacitance is charged as indicated, the surplus electrons on the plate below cause a surge of negative charge counterclockwise around the circuit to neutralize the positives and, in so doing, set up a magnetic field in and around the inductance. When the positives become neutralized and the electron current tends to cease, the magnetic flux linking the circuit decreases and keeps the

9

current flowing in the same direction. Once this field has vanished and the current has ceased, the capacitance is found to be in a charged condition, the upper plate being negative and the lower plate positive.

Having reversed the charge on the capacitance, the above process will repeat itself, this time the electron current surging clockwise around the circuit. Thus the current rushes first in one direction, then in the other, oscillating back and forth in an electrical way just as any spring pulled to one side and released vibrates in a mechanical way.

When a straight spring is pulled to one side and released, the kinetic energy it gains upon straightening keeps it moving and it bends to the other side. Just as the vibration amplitude of the spring slowly decreases because of friction, so also does the current in the electrical circuit decrease due to electrical resistance. A graph showing how current slowly dies out in an electric circuit is given in Fig. 16.

These are called damped vibrations, or damped oscillations. If the resistance of the circuit is high, the damping is high and the current quickly dies out after but few oscillations. The resistance being low, however, the damping is small, the amplitude decreases slowly, and there are many oscillations.

Radio-Frequency Amplifiers

The functions of a radio-frequency amplifier are to increase the voltage of the radio-frequency (r. f.) signal and to secure the required selectivity of the receiver.

The voltage applied to the input of a r. f. amplifier is from units to hundreds of microvolts depending on the sensitivity of the receiver. Before the signal reaches the detector it should be amplified a million times or more. Such voltage gain may be obtained only with the aid of several amplifier stages.

A r. f. amplifier stage contains a valve or a transistor and a load, which is a resonant circuit tuned to the frequency of the signal applied to the input of the stage. This resonant circuit may be a single tuned circuit or a band-pass filter.

R. f. amplifiers in which single-tuned circuits serve as a load are known as tuned amplifiers. In case r. f. amplifiers employ band-pass filters for load they are called band-pass or filter amplifiers.

Band-pass amplifiers have a nearly rectangular resonance curve. They are mostly fixed frequency amplifiers, i. e. their tuned circuits do not have to be retuned when the receiver is in operation. Band-pass amplifiers are widely used as i. f. amplifiers in superheterodyne receivers.In a band-pass amplifier the anode load is a band-pass filter which may have widely differing circuit configurations and may be connected to the anode of the amplifier valve in many ways.

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General Classification of Amplifiers

It is common knowledge that amplifiers are divided into three general classes: А,

В and C, depending on the type of service in which they are to be used.

A class A amplifier is one which operates so that the plate output wave shapes of current are practically the same as those of the exciting grid voltage. This is accomplished by operating the tube with sufficient negative grid bias so that some plate current flows at all times and by applying an alternating excitation voltage to the grid of such value that the dynamic operating characteristic is essentially linear.

The grid must not go positive on excitation peaks, and the plate current must not fall low enough at its minimum to cause distortion due to curvature of the characteristics. We know the characteristics of class A operation to be free from distortion and relatively low power output, practically all a-f amplifiers being operated in this manner.

Radio-frequency amplifiers of the type used in receiving sets to amplify the signal voltage prior to detection are also considered to be of this class. Class В amplifiers are operated with a negative bias approximately equal to cutoff so that the plate current is almost zero when the alternating grid excitation is removed. With a sinusoidal voltage applied to the grid, the plate current consists of a series of halfsine waves, similar to the output of a half-wave rectifier. The load impedance is adjusted so as to obtain an approximately linear dynamic characteristic. The grid swings positive on excitation peaks, causing grid current to flow. We know of class

В amplifiers being used in radio-telephone transmitters following the modulated

stage.

The power output obtainable from a given tube is much greater than with class A operation, the plate efficiency being much higher. As with a-f power amplifiers, tubes operating as class В r-f amplifiers may also be operated in push-pull.

A class С amplifier is one in which high output and plate efficiency are the primary considerations. The grid is negatively biased to a point considerably beyond cutoff, so that the plate current is zero with no grid excitation. The latter is quite large and is often sufficient to cause the plate current to reach saturation on positive swings. Plate efficiencies in the vicinity of 90 per cent may be obtained with the larger tubes, these high efficiencies being made possible by allowing the plate current to flow during less than 180 deg. of the cycle and only at a time when the plate potential is comparatively low. In radiotelegraph transmitters all stages are operated in class C, while with radio-telephony it is only the modulated amplifier and the stages preceding it that are so operated.

Either triodes or screen-grid tetrodes may be used as power amplifiers. The latter have the advantage of. not requiring neutralization. The screen-grid voltage in transmitting tubes is usually about 15 per cent of the plate supply voltage, which is proportionally much lower than in receiving tubes. Тhese tubes are difficult to construct for power outputs much greater than 500 watts, and, where larger outputs are required, triodes must be used.

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Transistor Radio Frequency Amplifiers

Like valve amplifiers a transistor r. f. amplifier may be of the tuned or of the band-pass variety. On long, medium and short waves, the transistor is usually connected into a common emitter circuit while in the

VHP and UHF bands use is sometimes made of the common base arrangement.Transistor amplifiers differ from valve amplifiers in interstage coupling.

Operation of a transistor amplifier is affected by the output resistance of the transistor, which is much lower than the output resistance of an amplifier valve.

This is why transformer and tapped-coil coupling is used extensively in r. f. transistor amplifiers.

The stability cf the operation of a transistor amplifier largely depends on the position of the quiescent operating point. To stabilize this point the circuit employs negative direct-current feedback provided by R8 (Fig. 17) connected in the emitter circuit. Such an arrangement is similar to the current feedback arrangement in valve circuits.

To eliminate a. c. feedback R3 ,is bypassed to earth by C6. Should the operating point shift due to temperature changes it will be restored by the feedback voltage built up across R3 and applied to the transistor base. It should be noted however that in both configurations the tuned circuit may be connected to the collector circuit directly, provided the output resistance of the transistor is sufficiently high.

Radio Transmitters

General Considerations. A radio transmitter is known to be essentially a device for producing radio-frequency energy that is controlled by the intelligence to be transmitted. A transmitter accordingly represents a combination of oscillator, power amplifiers, harmonic generators, modulator, power-supply systems, etc., which will best achieve the desired result.

Commercial transmitting equipment is ordinarily mounted on a framework of structural-steel members fronted by a vertical metal panel containing the controls and meters necessary for adjusting and monitoring the transmitter. All equipment appearing on the panel is at ground potential, instruments which must be observed during adjustment or operation and which are not at ground potential being located behind the panel and viewed through windows. The steel frame is normally enclosed with wire mesh of some sort and is provided with doors that cut off the transmitter power when opened. This type of construction requires a minimum of floor space in proportion to the amount of apparatus involved, makes the transmitter accessible for inspection and repairing, and eliminates all hazard to persons.

The design of most radio transmitters, particularly those intended for broadcast and short-wave transmission, is dominated by the need of maintaining the transmitted frequency as nearly constant as possible over long periods of time.

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In broadcast work two or more transmitters are commonly assigned the same carrier frequency, and in order to minimize the resulting interference it is essential that the carrier frequencies be as nearly as possible the same.

The Microphone Transmitter. The microphone transmitter may be one of the ordinary carbon granule type. Without going into details, it will suffice to state here that such a microphone consists simply of an elastic diaphragm bearing against a mass of carbon granules enclosed in a suitable chamber, the carbon granules forming part of an electrical circuit, When the microphone is not being spoken into the diaphragm remains stationary and exerts a constant pressure upon the carbon granules, the resistance of which remains, therefore, constant. On the other hand, when the diaphragm is set vibrating, as it is done by speaking into the microphone or through a noise or sound reaching it, the pressure exerted by the diaphragm against the carbon granules changes, and this change of pressure causes the resistance of the carbon granules to increase or decrease in accordance with the displacement of the diaphragm from its position of rest.

When the microphone is not being spoken into, the alternator produces a highfrequency current of constant amplitude, i. e., an undamped current; the amplitude of this current is adjusted to the maximum by adjusting the inductance so as to make the natural frequency of the circuit equal to the frequency of the alternator.

Now, assume, for the sake of simplicity, a vibrating tuning fork to be placed in front of the microphone. The harmonic vibrations of the tuning fork will bring about harmonic vibrations of the microphone diaphragm, and these will produce variations in the resistance of the microphone. Since no other part of the circuit is undergoing any change, it is plain that a variation of the microphone resistance will produce a corresponding variation in the amplitude of the high-frequency antenna current. Thus, when the diaphragm is displaced inwardly the resistance of the microphone and, therefore, of the entire alternator circuit, decreases, and the amplitude of the current supplied by the alternator must necessarily increase, the reverse taking place when the diaphragm is displaced outwardly.

Radio Transmitters

To use an oscillating tube circuit, as part of a radio transmitter the high frequency oscillations in the L2C2 circuit must be modified by sound waves and then applied to an antenna and ground system for broadcasting as electromagnetic waves.

A simplified circuit diagram showing one of the many ways of doing this is given in the figure below. There are three parts to this particular "hook up", (1) the microphone circuit containing a battery D and a transformer T, (2) the oscillator circuit in the middle, and (3) the antenna circuit C1L1G at the left.

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By talking or singing into the microphone the diaphragm inside moves back and forth with the sound vibrations, thus altering the steady current previously flowing around the circuit DMX. Current pulsations in X, the transformer primary, cause similar pulsations in Z, the secondary . circuit carrying the plate current. The effect of the relatively low frequency audio currents on the high frequency oscillations already there is to alter their amplitude.

The modulated oscillations are induced in the antenna circuit by resonance, and are radiated as electromagnetic waves of the same frequency and form. The continuous wave produced by the radio frequency oscillations alone is called the carrier wave, the alteration of its amplitude by audio frequencies being spoken of as modulation. Although radio transmitters with one vacuum tube have been used by radio amateurs, it is customary to find transmitters with half a dozen or more tubes. The principal function of additional tubes in receivers as well as transmitters is to amplify currents wherever they are needed and thereby give greater transmitting range and clearer reception.

Radio Receivers

Transmission of intelligence by radio is based on modulation, this being a process by which the message to be transmitted is superimposed at the sending end of a radio link as a modulating signal on a strong carrier wave, thereby changing the latter's amplitude, frequency or phase. The modulated carrier is radiated by a transmitting aerial as a wave of electromagnetic energy which propagates through space at the velocity of light. At the point of reception the modulated wave is picked up by a receiving aerial and is fed to the receiver input. In the receiver the signal is separated from the radio-frequency carrier and drives the receiver load, which may be a speaker, a recorder, a cathode-ray tube, etc. As an electromagnetic wave travels away from the transmitter it is weakened or attenuated. This is why radio receivers should be capable of picking up relatively weak signals. Radio serves a variety of purposes such as communication, broadcasting, navigation, radar and telecontrol.

Radio communication is the transmission and reception of messages without wires or wave guides. It includes communication by radio telegraph, radiotelephone, radio teletypewriter, radio facsimile and television. It is the only method of communication between stationary and mobile objects (e. g. from ship to shore, from ground to aircraft, from ground to spaceships, etc.). Radio broadcasting is radio transmission for general reception, including speech, music and commercial television.

Radio navigation is the use of radio facilities for determining the position and direction of ships and planes. Radar (which is an acronym for Radio Detection and

Ranging) is a technique for determining the range and bearings of objects (targets) by transmitting beamed high-power signals against reflective targets, the reception of the reflected signals and the presentation of the resultant data on a dial or a cathode ray display. Telecontrol is a technique for control of machinery by radio.

There exist two classes of receivers: communication and broad-cast receivers, the

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former being used in point-to-point radio telephone and telegraph service while the latter are designed for the reception of sound and visual programs.

Radio Receivers

The simplest possible receiver is exactly the same as used for spark telegraphy. The manipulations necessary for the operation of this receiver are the same as for any spark receiver; the antenna circuit and the closed circuit must be tuned to the incoming high frequency, and the coupling between the antenna circuit and the closed circuit should ordinarily be made loose. The e. m. f. impressed upon the receiving antenna, due to the electromagnetic waves emanating from the transmitter is known to produce a current in receiving antennas which will be a reproduction of the current in the transmitting antenna. Assume, for the sake of simplicity, the rectifier used in the receiving circuit to have a characteristic such that a negative e. m. f. impressed upon the circuit of the rectifier produces no current whatsoever, a positive e. m. f. producing a current varying directly with the e. m. f. As a matter of fact, the e. m. f. impressed upon the receiving antenna and transferred to the rectifier circuit by suitable coupling coils will produce a current in the rectifier circuit. The current, though unidirectional, is yet one changing at high frequency, and as such it cannot flow through the high impedance winding of the telephone receiver, the current in the receiver being the average current. It will be noted that the current in the telephone receiver which corresponds to a period of activity of the microphone at the distant transmitting station, is one which changes periodically, between a maximum and a minimum, at the "modulating frequency", on the other hand, the current corresponding to a period during which the microphone transmitter is idle, is constant.

Characteristics of Broadcast Receivers. The most important characteristics of a receiver for radio-telephone signals are the sensitivity, the selectivity, and the fidelity. The sensitivity represents the ability of the receiver to respond to small radio-signal voltages, and is measured quantitatively in terms of the voltage that must be induced in the antenna by the radio signal to develop a standard output from the power amplifier. This standard output has been arbitrarily chosen as 0.05 watt in a non-inductive load resistance having a value corresponding to the load resistance into which the power amplifier is designed to operate. The sensitivity is arbitrarily defined as the effective value of the carrier voltage that must be induced in the antenna to develop this standard output when the carrier is modulated 30 per cent at a frequency of 400 cycles. The sensitivity is measured with the radio receiver tuned to give maximum response at the carrier frequency involved and with the volume controls adjusted for maximum volume. j Selectivity is the property that enables a radio receiver to discriminate between radio signals of different carrier frequencies. Selectivity cannot be defined in a single term as can sensitivity but must be expressed in the form of curves, which shows the amount by which the signal input must be increased in order to maintain the standard output as the carrier frequency is varied from the frequency to which the receiver is tuned. These curves therefore indicate the extent to which interfering signals are

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discriminated against, and in general will depend somewhat on the carrier frequency.

Fidelity represents the extent to which the receiver reproduces the different modulation frequencies without frequency distortion. The fidelity of a radio receiver is expressed in curves, which give the variation in audio-frequency output voltage as the modulation frequency of the signal is varied. In order to facilitate comparison, the output in? expressed in terms of the ratio of actual output to the output obtained when the modulation frequency is 400 cycles.

Radiation of Electrical Energy

Every electrical circuit carrying alternating current radiates a certain amount of electrical energy in the form of electromagnetic waves, the amount of energy thus radiated being extremely small unless all the dimensions of the current approach the order of magnitude of a wavelength. Thus a power line carrying 60 cycles current with 20 ft. spacing between conductors will radiate practically no energy because of a wavelength at 60 cycles being more than 3000 miles and 20 ft. is negligible in comparison. On the other hand, a coil 20 ft. in diameter and carrying a 2000 kc current will radiate a considerable amount of energy because 20 ft. is comparable with the 150-meter wavelength of the radio wave. The common radio antenna consisting of a vertical wire with a flattop structure is essentially a condenser in which one plate is the ground, the other one being the flat top. Such an arrangement will be a good radiator of electrical energy when the ratio of height to wavelength is appreciable, i. e., at least 1 : 100, and preferably 1 ; 10. Similarly a coil will be a good radiator of electrical energy provided the size of the coil be sufficiently great. The usual loop antenna consists of a coil and will be an efficient radiator to the extent that the ratio of loop diameter to wavelength is appreciable.

It is apparent from above considerations that the size of radiator required is inversely proportional to frequency.

High-frequency waves can therefore be produced by a small radiator, lowfrequency waves requiring a high-antenna system for effective radiation. The practical result of this fact is that the antennas of low-frequency transmitting stations are sometimes suspended from towers over 500 ft. high and yet are less efficient radiators than an antenna of one-tenth, this height operating at a very high radio frequency.

Every radiator has directional characteristics as a result of which it sends out stronger waves in certain directions than in others. Thus, while a vertical wire radiates the same amount of energy in directions that are perpendicular to the wire, the radiation in vertical plane varies from a maximum in a horizontal direction to zero in a vertical direction. Directional characteristics of an antenna are taken advantage of " to concentrate the radiation toward the point to which it is desirable to transmit.

The amount of energy sent out from any radiating system is proportional to the square of the radio-frequency current that flows in the radiator. Due to all the common sources of radio-frequency energy being relatively low voltage high

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current sources, it is necessary that the radiating system offer a relatively low impedance to the radio-frequency energy to be transmitted. This is accomplished by tuning the antenna circuit to resonance with the frequency to be radiated, which makes the impedance of the antenna circuit low and enables' a relatively small applied voltage to produce a very large antenna current and hence a high radiated energy. This is the only reason for tuning the transmitting antenna, as the mere tuning of the radiating systems to the frequency being transmitted does not increase the radiated power per ampere of current. The tuning is effected by inserting an inductance or a condenser in series with the antenna, as circumstances require. Thus in the flat-top antenna the antenna has a capacity resistance and so is tuned by the use of the inductance coil.

Cathode-Ray Tubes

Cathode-ray tubes are widely used in various branches of radio engineering such as oscillography, radiolocation, television, etc. In the narrow part of the tube the cathode K, focusing system and beam-deflecting system are mounted. Deposited on the inner surface of the glass face-plate is a luminescent screen S. The cathode is of the indirectly heated oxide-coated type. It is fabricated in the form of a cylinder with the oxide coating on its end cap. The cathode is mounted inside a control electrode (modulator) CE in which an aperture is provided. The brightness of the spot on the tube screen can be varied by changing the negative potential on the control electrode with respect to the cathode thus changing the electron-beam current. Moving along the tube axis after passing the control electrode is the electron stream which encounters two anodes A! and A2, both of which are cylindrical in shape. The accelerating field provided by the two anodes ensures the motion of electrons towards the screen and simultaneously focusses the stream into a narrow beam. Electron beam focussing can be accomplished with the aid of either an electric or magnetic field. In the first case focussing is termed electrostatic and takes place in the electric field between A1 and A2. An electron E moving at some angle to the device axis is deflected by the electric field set up between the anodes. Proper selection of the voltage difference on these electrodes . ensures focussing of the beam on one . spot on the tube screen.

Magnetic beam focussing is achieved by a focussing coil mounted onto the tube neck. Deflection of the electron beam is accompanied in the same manner as focussing that is either by an electric field or by a magnetic field. The electrostatic system of beam deflection consists of two pairs of vertical and horizontal deflecting plates. An electron passing between two parallel plates to which a certain voltage is applied, it will be deflected towards the positively charged plate.

There being two pairs of mutually normal plates, the electron beam can be deflected in horizontal and vertical planes.

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Magnetic field deflection is accomplished by two pairs of deflecting coils mounted onto the tube neck at right angles to each other. The greater the magneticfield intensity H and the lower the voltage V which accelerates the electrons, the greater is the beam deflection. The tube screen is a semitransparent thin layer of a luminous substance. Most cathode-ray tubes are oscilloscopes used to display rapidly changing voltages and currents.

The Televisor

The most important element in any television transmitter is the televisor or "pickup-tube", an instrument used in the broadcasting studio-or in the field to convert light images into electric currents. Although many televising systems involving numerous principles- have been devised, nearly all of them are to be classified as either mechanical or electronic in nature. Because mechanical systems have proved to be more cumbersome, electronic systems are now used almost exclusively.

The Scanning Process in Television. For years the sending of pictures by wire or radio has been everyday occurrence. The fundamental principle, involved in this process, is known as scanning. Every picture to be transmitted is scanned by an exploring spot which, starting at the top, moves in straight lines over the entire picture. The exploring spot in any scanning device is so constructed that it generates an electric current proportional to the brightness of its instantaneous position. Such a pulsating current, referred to as the video signal, is transmitted over wires or radio waves to the receiving station. There in a specially designed instrument a reproducing spot, whose brightness is proportional to the video signal amplitude,- moves over a viewing screen in a path similar to that of the exploring spot. In this way the reproducing spot reconstructs the original picture. It will be realized that the smaller the scanning and reproducing spots and the greater the number of lines the better will be the details of the scanned picture being reproduced at the receiving end. If a single picture is to sent by wire, as is generally the case in the telephotographic newspaper service, the scanning process requires from 10to20min. In television, however, it is a matter of standard practice to scan and transmit thirty distinct and separate pictures every second of time. At the receiving station these pictures are rapidly flashed one after the other upon a viewing screen. All are still pictures differing progressively one from the next so that, due to persistence of vision, the motions seem to be smooth and continuous, just as with moving pictures. To avoid spurious shadows and images, the process of interlacing is employed. By this process each picture is scanned twice, first by running the exploring spot over the odd numbered lines I, 3, 5, 7 etc., and then over the even numbered lines 2, 4, 6, 8, etc.

In many respects the apparatus used in television differs very little from that used in radio broadcasting. The varying current from the exploring element of a

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scanning device, called a televisor, takes the place of the voice currents from a microphone. In other words, instead of modulating the carrier wave of a radio transmitter with the voice currents due to sound waves it is modulated with the video current from the light of a picture image in a televisor. Except for the televisor tube used in the transmitter, and a similar device known as a kinescope used in the receiver, television equipment consists of numerous electrical circuits containing radio tubes similar to those in any radio receiving set.

The Kinescope. In many respects the construction of a television receiver and its operation is similar to an ordinary radio receiver. The carrier wave from a nearby transmitter after being tuned in, detected, and amplified with conventional radio tube circuits, is fed as a video signal into a kinescope in place of a loud-speaker. A kinescope is a large vacuum tube used for scanning and viewing the transmitted pictures.

A kinescope using electrostatic deflection plates for scanning is shown in Figure

20. Electrons from an electron gun at the left travel down the length of the tube to where, impinging upon a fluorescent screen, they produce a bright luminescent spot S. The purpose of the deflecting plates -V and H is to deflect the electron beam with the identical frequency and scanning motion of the transmitting station.

Two special oscillator tubes and circuits in the receiver supply saw-tooth potentials to these plates, the high-frequency potentials to the H-plates for horizontal scanning and the lower frequency potentials to the V-plates for vertical scanning.

The proper fluctuations in the intensity of the luminescent spot are brought about by applying the video signal to the grid of the electron gun. This grid controls the flow of electrons through to the anode in the same way that the grid controls the current to the plate in an ordinary three-element radio tube.

For a small fraction of a second, between successive pictures being scanned for transmission, current pulses of a certain type and frequency are sent out from the sending station as part of the video signal. These, picked up by the receiver, act as a trigger-like mechanism to bring the reproducing spot to the top left of the screen at the proper time to start the next picture. In other words, the transmitter sends out signals that enable the receiver to automatically keep "in step" with the pictures as they are sent.

Television in Full Color

To produce television pictures in full color the additive method of color mixing is employed. Several all-electronic color television receivers have been invented in recent years. Instead of the fluorescent screen being coated with one fluorescent pigment as in the black and white tubes, a separate flat glass plate just inside the large end of the tube becomes the screen and is coated with three fluorescent pigments. These three pigments R, G and B, under electron bombardment fluoresce with the additive primary colors: red, green, and blue, respectively. These fluors are painted on the glass in the form of hundreds of narrow vertical ribbons.

About one quarter of an inch beyond the fluorescent screen, and electrically insulated from it, are about 400 fine equally spaced wires mounted parallel to and

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with twice the spacing as the color fluor lines. With alternate wires at about +4200 and +4800 volts respectively, and the screen at about 18,000 volts, the narrow incoming beam is brought to a focus.

MEANS OF TELECOMMUNICATIONS

TELECOMMUNICATIONS is the name given to the field of communications that includes the transmission of information by such things as telegraph, telephone, telex, facsimile, radio, and television.

Telecommunications systems fall into two categories: broadcasting and personto-person communications. Broadcasting, which is carried out by radio and television companies, is the transmission of news and entertainment to the general public. Person-to-person communications is the sending of messages by such means as telephone or facsimile between particular people or organizations. This article deals with person-to-person communications; the other branch of telecommunications is dealt with in the article BROADCASTING.

The Telegraph

The telegraph was the earliest device for modern telecommunications, and because of this the former name for the whole field of distant communications was for a long time known as telegraphy. The electric telegraph was invented by the

American Samuel Morse during the 1830s. It was an early application of the link between electricity and magnetism. An electric current was sent along a wire from a battery in the form of coded bursts or "pulses" that represented the morse code.

The pulses were created by the sender who used a switch called a key to turn the current on and off. At the receiving end was a device for making a sound (the sounder) every time current passed through it. The sounder was easily adapted to wireless telegraphy when radio was invented by Guglielmo Marconi in the early

1900s. Unlike the electric telegraph, wireless telegraphy needed no wires and could be used to communicate with ships far out to sea.

For many years only one message in one direction could be sent or received at one time. The use of multiple electrical circuits introduced in the second half of the

19th century allowed several messages to pass back and forth simultaneously. In

1872 a Frenchman named Jean-Maurice-Emile Baudot invented a so-called timedivision multiplex circuit in which a number of messages could be sent over the same line at the same time. Baudot's method linked printing machines at the transmitting end to similar machines at the receiving end. Each sender typed his message on a keyboard similar to a typewriter and the corresponding keys on the receiver's machine were made to work by an electrical pulse. The reason why

Baudot's machine was such a revolution was that it sent each character (letter) of each message one at a time and in a strict order or rotation, so that the users literally shared time on the line. Baudot's equipment was so arranged that each pulse of electricity could be made to represent a unique code that matched a character on the operator's keyboard. Baudot's system of codes is still in use today,

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but has largely been superseded by the code system of the American Standard

Code for Information Interchange (ASCII).

Baudot's system was the forerunner of the teleprinter, or teletypewriter (also known as the teletype). Earlier telegraph machines fitted with keyboards and type wheels produced tape on which stock-market prices were transmitted. The machines used for this were known as tickers, and the tape was called tickertape.

Around the end of the 19th century and the beginning of the 20th, the teleprinte, was developed through the pioneering work of the Britons Donald Murray and

Frederic* Creed, and the American Charles Krumm Teleprinters continue to play a vital role » business. They have been improved and speeded up and are to be found in most offices , especially those of international companies-* Teleprinters combined in networks form what is called the telex service.

The Telephone

The possibility of sending speech along a wire was known as early as the 17th century. By the 1870s the idea of converting sound into an electric current that could be sent along 3 metal wire was well-known, and several inventors were rushing to make the first practical telephone. The Scots-born America "

Alexander Graham Bell succeeded in doing' this in 1875. Telephone lines and services spread remarkably quickly throughout the*-world. State-run and privately organized telephone networks now operate both nationally and internationally.

Though operator-j assisted calls are still made, most calls, even those over the longest distances, are dialed direct and are connected automatically.

The principles on which Bell's telephone worked are not so very different from the ones used by the telephone of today. The telephone consists of a transmitter in the mouthpiece of the instrument that is connected by wires through an electricity power supply to a receiver in the earpiece of the instrument at the other end. The transmitter is a carbon microphone .The sound waves of the caller's voice are carried to a diaphragm, which is thus made to vibrate in and out. The diaphragm, which is made of thin, flexible plastic, forms the lid of a little "pillbox" packed with grains of carbon, an element that is a good conductor of electricity. When nobody is speaking, current flows smoothly and evenly through the grains. Sound vibrations from the caller's voice, however, set the diaphragm moving in and out.

With each inward movement, the grains become more tightly packed together d their resistance to the passage of the electric current is reduced. Thus the power supply sends a larger current through the transmitter. With every outward movement of the diaphragm. the grains become less tightly packed and their resistance increases, reducing the strength of the current. Thus, when the caller 's speaking, a varying electric current is sent along the wire to the distant receiver.

The receiver, in the earpiece of the telephone, consists of an electromagnet and another thin, flexible diaphragm, this time made of soft iron. The ends of the electromagnet are connected to the incoming wire carrying the message transmitted from the caller Behind the electromagnet is a permanent steel magnet that puts a steady, unvarying pull on the soft-iron diaphragm. When the electric current passing through the coil of the electromagnet increases during a caller's transmission, the electromagnet strengthens the effect of the permanent magnet,

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attracting the diaphragm further inward. When the current is weakened, the attraction on the receiver's diaphragm is weakened too, and it moves outward. The vibrations of the receiver's diaphragm match those of the original sound The movements of the diaphragm set up waves in the air immediately surrounding it, and the sound of the caller's voice passes to the ear of the person being called.

Telephone Exchanges. When the telephone service began, all calls were connected manually by an operator at a local exchange who had in front of him or her a switchboard consisting of a bank of jacks. Plugs connecting the telephone of each subscriber to the exchange could be inserted into the jacks and coupled up so that one subscriber could talk to another.

An automatic process of call connection, known as automatic switching, was first introduced in 1889. The telephone user called up the number required by pushing each of a series of buttons a certain number of times. At the exchange the electrical contacts leading to other users' telephones were arranged in rows on a cylinder.

When a number was called, an arm on the central shaft of the cylinder was moved step by step in accordance with the digits of the number until it found the contact of the called subscriber. It then moved across and made the connection, causing the called subscriber's phone to ring.

Improvements to this system were made, including the introduction of a rotary dial for selecting the numbers, and the first fully automatic exchange using mechanical equipment was opened in Omaha, Nebraska, in 1921. A major development in automatic switching was brought about by the introduction of electronic tubes, which were later replaced by transistors. These pieces of equipment could be arranged in circuits that could be either "on" or "off' and could be set up to work in a similar way to the old mechanical selectors. Further advances were made in the second half of the 20th century, and by the late 1980s fully computerized automatic switching was commonplace. The telephone handset too underwent many changes in design. Rotary dials were improved and eventually replaced by "push-button" keypads. Telephones fitted with suitable silicon chips have an internal memory. They can remember frequently dialed calls, can display the number of somebody who is calling, and in some cases can be used as small computers. "Cordless" telephones, which are portable radio-telephones, arc also available. For more information about radio-telephones, see the next section of this article.

Lines of Communication

Up to the 1960s, telegraph and telephone messages were carried from transmitter to receiver along insulated copper wire cables. The great spread of telegraph services in the second half of the 19th century led to the need to communicate over longer and longer distances. Overhead wires supported on poles and requiring little protective covering could be used for connecting two places inland. However, it was more of a problem to connect two places separated by sea. Cables had to be laid under water, they had to be protected by insulation, and they had to be strong enough to withstand high tension (that is, a strong pulling force) and immense pressure from the seawater above them. The first undersea cables were laid in the

1850s and in 1858 the first transatlantic cable joined Ireland with Newfoundland.

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Another problem engineers faced was that a signal gets weaker the further it travels. Items of electrical equipment called repeaters , placed at suitable points along the course of a cable, can boost, or strengthen, the signal and send it on its way. Yet reception can still be poor because of interference, loss of current, or other factors.

These problems have been overcome in the second half of the 20th century by combining conventional cable links with radio and microwave links. Calls still go part of the way by cable, but most of their journey is through space. Satellites play a vital part in this area of telecommunications today. Using microwaves and satellites means that less of the signal is lost, and the quality of reception, even for a call between, say. New York and Tokyo, is generally excellent.

The problem of providing for a large number of users all at the same time has been tackled in several ways. Wrapping several wires within one cable has been an obvious approach. Each wire can carry several calls at once, each call being handled on a time-share basis. Modern methods of transmitting the signal digitally help in this way and also provide a better quality of reception. Perhaps the most revolutionary method of transmitting thousands of messages over one line all at the same time has come from the field of fiber optics .In this form of telecommunications technology, speech sounds are converted into pulses of laser light and directed along glass tubes as thin as a human hair. Because the laser beam is reflected many times off the inside of the tube, there is hardly any loss of signal. Fiber-optic cables can carry up to 80.000 telephone conversations at once.

Among the latest developments in telecommunications has been that of the mobile telephone. This system, in which short-wave radio transmitter-receivers can be linked to he ordinary telephone network, allows people access to the telephone service while on the move. In a ''cellular radio" network, a metropolitan area is split up into sections, or cells, -o that a person can keep in contact with his or her call, even at long range, because the link can be kept open as the caller moves from one "cell" to another. Radio-paging services also exist in which short pieces of text can be sent over the radio-telephone network.

It is now possible to send copies of documents over the telephone lines using a

"fax", or facsimile machine, which scans the page and converts its contents into electric signals that can be sent through the telephone system.

Computers can also be linked through the telephone system. Information stored on a central computer can be shown on the screen of a "terminal" anywhere in the world through the telephone system. In this way transactions can be made between banks, airline and theater tickets can be booked, library catalogs and indexes can be consulted, and information retrieved from agencies about such things as stock market prices. The computer is linked to the system through a device called a modem that converts the information to be sent into a form that can be transmitted by telephone.

Messages transmitted this way are sometimes known as electronic mail, They are entered at a computer terminal and can be sent to any other computer "mail box" in the system. Messages are picked up by computer through the modern and can then be printed out at the terminal.

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TELEVISION.

The word television comes from the Greek word tele, meaning "far", and the Latin word visio, meaning "sight". Thus television really means "seeing at a distance".

The first television pictures were publicly demonstrated in 1926 by John Logie

Baird, a Scots engineer. At first the images were tiny and flickering but Baird gradually improved them. Among rival systems was a Marconi-EMI system, which operated electronically instead of using the mechanically rotating disks of the

Baird apparatus. (Details of the development of television broadcasting in various countries , throughout the world can be found in the article BROADCASTING.)

Making the Pictures

The first problem in television is to change the light reflected from a scene in the studio or elsewhere into another form — one that will travel long distances, penetrate solid objects, and go round corners. Electric currents will do all these things and there are ways of converting light into electricity. The second problem is how to use the electric currents to control a light source at the receiver in such a way that an image of the original scene is recreated.

The article PHOTOELECTRIC DEVICE explains how certain substances alter when exposed to light. These substances can be made into* devices that will change light of varying intensity (brilliancy) into electric currents of corresponding strength.

If we merely place a photoelectric device in front of the scene, the current produced will represent no more than the average amount of light reflected. There will be no information as to the nature of the picture, nor as to which parts should be light and which dark.

This difficulty can be overcome by dividing the scene into small parts and presenting these to the photoelectric device one by one. The simplest (but not the best) way of doing this is to use a rotating shutter consisting of a disk with holes drilled through it in a spiral pattern. If this shutter is placed between the scene and the device, the holes as they rotate expose one small part of the picture after the other. For each exposure the device will produce a pulse of electricity proportional to the light received. This method of scanning, 3s the process is called, was used by Baird and had been invented by Paul Nipkow, a German engineer, in 1884. The modern method is to use a surface coated with light-sensitive material. The first man to make a workable device of this kind 'which we now call a camera tube) was

Vladimir Zworykin, a Russian-born physicist. He patented his iconoscope (as he called it) in 1923, hut it was so difficult to make that he was unable to demonstrate it until 1929. Modern systems operate on the principles used in the iconoscope, although great improvements have been made.

One form of camera tube is a glass cylinder, from which air has been removed, mounted inside the camera casing. The image of the scene being televised is focused by lenses on to the flat glass wall forming one end of the cylinder. Inside this wall is a transparent coating of electrically conductive material, called the

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signal plate. Inside this again is a coating of light-sensitive material which is photoconductive. This layer, called the target, consists of microscopic granules

(grains) which form millions of tiny photoelectric devices. The light from the image passes through the signal plate and falls upon the target. This causes each granule to become positively charged by an amount that depends on the intensity of the light falling upon it. Thus the target carries an image of the scene in terms of larger or smaller amounts of electric charge.

At the other end of the cylinder is an electron gun which can shoot a beam of electrons at the target. Electric currents are passed through coils mounted on the outside of the cylinder. These currents are produced by circuits called time bases.

They control the beam so that it starts at a top corner of the target and sweeps horizontally across it, then switches back to start another line and continues until the whole target has been scanned, or swept. Your eyes are scanning this page in much the same way. When the beam reaches the bottom of the target, the current alters to bring the beam back to the starting point again. The scanning process repeats itself indefinitely

As an electron is negatively charged, the beam sweeping across the mosaic neutralizes, or wipes out, the positive charges on the granules. This causes a current to flow. If the image of the scene is bright at one point, the charge will be large and the current will also be large. A dark part of the image produces only a small current. So, as the electron beam scans the target, it causes a changing signal.

This forms the electrical equivalents of light and shade in the picture.

As soon as the beam wipes out a charge on a granule and passes on, the charge builds up again until the next sweep of the beam. This storage effect (which is common to all camera tubes) gives a sensitivity which the old mechanical scanning, using the Nipkow disk, lacked.

Every time the camera beam comes to the end of a line or a field, special marker pulses are generated in equipment linked to the camera. The purpose of these will be described later. We therefore have two sets of signals coming from the studio, or three if we count the audio (sound) signals from the studio microphones.

Different standards exist in different parts of the world, particularly regarding the number of scanned lines that make up the television picture In Baird's system, in the early days, 30 lines were scanned to produce low-definition pictures, that is, pictures in which the amount of detail and clarity were relatively poor. Definition

(amount of detail) increases if the number of lines used in one complete scan of the system is increased. The European standard is 625 lines; each completed 625-line sequence is called a frame. Although 25 pictures, or frames, are formed every second, 50 half pictures, or fields, are scanned every second in a process called interlacing, which reduces flicker In the United States the pictures are scanned in

525 lines at 60 fields a second.

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Sending the Pictures

The audio (sound) signals from the microphone are carried on one carrier wave

(similar to the method in which radio waves travel, see RADIO), while the video

(picture) signals and the marker pulses are carried on a second carrier wave.

The signals radiate into space and are picked up at the home receiver which detects, amplifies and separates them The sound carrier wave, its job done, is discarded. The audio signals are channeled through a separate amplifier to operate the loudspeaker. The carrier wave, which brought the video signal and marker pulses, is also discarded.

When the television receiver is switched off' its screen is grayish-white. This color corn from a phosphor coating inside the glass fro t of a cathode ray tube The coating glows at an point where it is hit by a beam of electron; the greater the strength of the beam, the brighter the glow.

Behind the screen the tube narrows rapidly to a cylindrical neck. Inside this is an electron gun which can shoot a beam of electrons at the screen. As in the camera tube, coils around the neck are fed with currents from time bases to make the beam scan the screen. The marker pulses are applied to the time bases and control them so that the receiver scanning keeps in step with that of the camera. At rest, the beam produces a bright spot on the phosphor coating, but the rapid scanning movement tricks the eye into seeing the spots as a series of horizontal lines (called a raster) across the face of the tube.

The video signal controls the electron gun so as to vary the number of electrons in the beam and therefore the brilliance of the spot. For example, if the beam in the camera is scanning a white point on the scene, the signal produced is large. But a black spot scanned by the camera produces a signal too small to release any electrons from the receiver's electron gun, so no light appears on the screen at this point. Thus the picture is built up by a single spot of light, moving and varying in brilliance so rapidly that the eye is deceived into seeing a complete picture. The effect of movement is conveyed, as in the camera, by the rapid presentation of slightly different still pictures.

Color Television Systems

John Logie Baird gave the first practical demonstration of color television in

1928, although it was a further 25 years before commercial color television systems were developed. The first of these was the NTSC (National Television

Systems Committee) system which was introduced into the United States in 1954 and is still used there and is Canada, Mexico, and Japan. A modified form of

NTSC called PAL (phase alternation line) was developed in West Germany and is used Australia and most European countries. Another system. SECAM

(systeme electronique couleur avec memoire), is used in France, the countries of the former Soviet Union, eastern Germany, Hungary and Algeria.

Color Television

Almost any color, including white, can be firmed by adding together suitable amounts f red, green, and blue light. It is on this that color television depends.

Inside a color television camera are three camera tubes. One has a filter in front of it that permits only red light to pass; another has a tiller passing only green light

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and a third a filter passing only blue light Mirrors allow the image of the studio scene to fall on each tube. The tubes work exactly as described for black-and-white television and each tube produces a video signal and a stream of pulses. The signal from the red-filtered camera represents red parts of the scene and the signals from the other two represent the green and blue parts. Modern cameras, especially cheaper ones, use fewer tubes. The light-sensitive coating, similar to that found in black-and-white cameras, is covered by a colored filter mosaic to produce separate color signals. If the three sets of camera signals were amplified and fed to three cathode ray tubes, one with a red-glowing phosphor, one with a green, and one with a blue, and the resultant pictures were combined by mirrors, the screen would be in full color - not merely reds, greens and blues but all the colors of the original scene. This is how television projectors work.

The band of frequencies taken up by the transmission of three separate video color signals IP about three times that taken up by a black-and-white station, which is uneconomical The problem is solved by transmitting a high-definition blackand-white picture and filling it in with color, with very much less detail. This is perfectly acceptable to the human eye The color information is contained in a subcarrier wave which is added to the "black-and-white" signal so as to take up no extra band-width, and is hardly noticeable on Color television depends upon the fact that nearly alt colors can be produced by combining three primary colors of light in the correct proportions. The diagram shows what happens when three beams of light, red, green, and blue, are projected on to a white screen to overlap

.This form of color combination, called "additive mixing", works only with light, and not with pigments such as paints or inks, so the diagram can only give a rough idea of the effect Additive mixing of red, green, and blue light gives the following colors. red plus green forms yellow, red plus blue forms magenta, green plus blue forms cyan; red plus green plus blue forms white a black-and-white receiver, so that the system is compatible. In this way, all the three-color information is squeezed into a frequency band no wider than that occupied by a black-and-white station. At the receiver the three separate cathode ray tubes described for the simple system are combined in one tube. The viewing end is a screen carrying about 1.7 million phosphor dots arranged in tiny triangles. When hit by an electron beam one dot in the triangle glows red, another green, and the third blue. Three electron guns are mounted at the other end. A shadow mask, consisting of a metal plate pierced with tiny round holes, prevents a beam from hitting phosphor dots of another color; that is, the beam from the green gun can hit only the green-glowing dot in each triangle When each dot in a triangle is hit at In the shadow mask tube many electrons are blocked by the mask But in the Trinitron tube the metal grille allows more electrons through to the phosphors and so produces a brighter television picture the same time by its individual beam all three glow but as they are so close together the eye sees them as a dot of white light The Trinitron tube developed in Japan uses a single electron gun to produce three in line beams .The mask is slotted and the three color phosphors instead of being in the form of dots consist of large numbers of very narrow strips repeated across the face of the tube.

This gives exceptionally sharp pictures Other types of in-line" tubes using slots and

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stripes are fast replacing the older shadow mask tube Flat panel screens using solid-state techniques instead of cathode ray tubes have been developed

Miniature television sets use liquid-crystal displays like those used in calculators and digital watches.

A color is made up of three elements luminance or brightness, hue which is redness yellowness and so on and saturation or intensity .From the signal that the television receives these elements can be reconstructed to give a faithful picture

Cable Television

Television is increasingly used for observing processes that cannot be approached closely For instance the furnaces and pressure gauges in a power plant may be

''watched" by a television camera which sends its pictures to a central control room. Television can also be used to watch and control the pouring of large castingsinfoundries and the movement of radioactive substances .It is also used to watch the behavior of rocket and jet engines undergoing tests, and for salvage operations on sunken ships. It can be used to give medical students in a lecture room a close up view of surgical operations taking place in another part of a hospital and for many tasks in scientific research .Television cameras connected to sets in a central security office can act as electronic "guards".

This kind of television is called "closed circuit' and is useful over short distances

Cables connect the camera to the receiver This is the kind of television system you sometimes see in large stores.

Cable television can also be used to bring programs into the home. By combining a cable network on the ground with satellite relays in space, a viewer can pick up programs from all over the world paying a fee for whatever cable channel is chosen for viewing. To send programs over long distances, the television signals normally have to be "boosted" a network of relay stations. If you live more than 80 kilometers (50 miles) or so from a transmitter your broadcast television picture may be rather poor .This is because the signals travel in straight lines and, owing to the curvature of the Earth, travel away from the surface and out into space

Worldwide Television Networks.

Increasingly powerful communications satellites arc being sent into space making it possible for viewers to see events as they are happening in distant countries. There are regular transatlantic broadcasts between the United States and Europe, and

Eurovision is the name for the network of microwave radio and cable links throughout Europe. The DBS (Direct Broadcasting by Satellite) system, which enables direct broadcasting of television programs from a satellite into the home is available in several countries.

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BROADCASTING.

Apart from the widespread use of electricity as a source of energy and the improved transportation provided by motor vehicles and aircraft, broadcasting has probably had the greatest impact on our lives and has helped to shape our society more than any other development. Television and radio provide us with entertainment, information, and education. It is hard to imagine a world without them, for if there were no television or radio, news would be less immediate, there would be fewer opportunities to enjoy music, drama, and other entertainment, and chances to learn interesting things would be much more limited. Some people think that television and radio invade our privacy too much and kill the art of conversation, but few would doubt their value as means of telecommunication and their power to inform and amuse.

This article deals with the history of broadcasting and the modern methods and technology that are employed in it today. For a discussion of the technical aspects of radio and television transmissions.

Radio

Radio communication was born on 12 December 1901, when the Italian inventor

Guglielmo Marconi succeeded in receiving at St. John's, Newfoundland, radio signals that had been transmitted from Poldhu, in the English county of Cornwall.

Those signals were in Morse code .Although the first demonstration of the transmission of speech by radio had taken place in the United States in 1892, little had come of it. The first advertised broadcast in the world was not made until 24

December 1906. This famous transmission was made by Reginald Aubrey

Fessenden from the 128-meter (420-foot) transmitter of the National Electric

Signaling Company at Brant Rock Massachusetts This event included the first music broadcast a performance of Handel s Largo.

In the next dozen years or so the potential of radio broadcasting slowly emerged as technical improvements were made to transmit ting and receiving equipment.

Military controls were placed on radio during World War I. In 1916 Marconi was experimenting with shortwave communications and methods of preventing enemy interception of military transmissions. After the war experimental radio stations were set up Regular broadcasting in Britain began on 14 February 1920 from the

Marconi transmitter 2MT at Writtle, in the county of Essex .The radio broadcasting industry expanded rapidly because people liked the idea of hearing music and other bounds coming "out of the air' and wanted to buy radio receiving sets. In the

United States and in Britain , the industry developed in different directions.

In the United States, regular broadcasting started on 2 November 1920 from the radio station KDKA in Pittsburgh Pennsylvania. Its success encouraged the

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opening of other stations. Right from the beginning, radio broadcasting in the

United States was dominated by private commercial companies who financed their programs by selling advertising. United States radio stations still obtain most of their financial support from advertising Individual firms can sponsor (pay for) whole programs or buy brief "spots" of time on the air to talk about their products or services.

Another early development was the connection of certain radio stations across the

United States by means of special telephone wires. The idea was introduced in

1922 In 1926 the National Broadcasting Company (NBC) opened the first permanent network of radio stations linked by telephone lines, distributing daily programs to them from its chief station in New York.

The rapid growth of the broadcasting industry in the United States was uncontrolled in its early stages but in 1927 the government passed the Radio \ct to regulate its future

Development. Under this act radio frequencies or wavelengths were allocated to the various types of radio, stations including maritime services , aircraft navigation, and radio beacons as well as commercial broadcasting organizations

The act also set up an agency called the Federal Communications Commission to enforce the new regulations. The commission listens in to licensed radio stations to make sure they stay within the law and also uses its equipment to detect illegal, unlicensed stations.

The 1927 act also led to the establishment of the United States four national networks : NBC, the Columbia Broadcasting System (CBS), the Mutual

Broadcasting System iMB8 and the American Broadcasting Company (ABC)

Although commercial organizations have dominated United States broadcasting since the 1920s non-commercial broadcasting organizations have also emerged.

For example, the National Association of Educational Broadcasters provides programs to educational stations both in and outside the United States Many stations are supported by donations from the public from foundations and from other sources. Official broadcasting services to countries outside the United States are operated by the Board for International Broadcasting, which is known as the

Voice of America. It broadcasts in more than 30 languages, including English all over the world Radio Free Europe and Radio Liberty are two foreign-based

American organizations There are many other external broadcasting services, including American Forces Radio and Television, which transmits programs to

American Armed Forces units wherever they are based.

In Britain, commercial radio broadcasting stations quickly came under the control of the government. The Post Office banned the first radio stations because the armed forces were afraid that they would interfere with important military communications .But 150 amateur radio broadcasters obtained licenses from the

Po«t Office and so did about 4,000 owners of radio receivers or wirelesses as they were commonly known. The British government used some of its license revenues to finance the British Broadcasting Company which made its first transmission on

18 October 1922. The company was set up as a private corporation providing both national and regional programs that included news, information concerts lectures

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educational material speeches, weather reports, and drama and theatrical entertainment. In 1925, the British Broadcasting Company was wound up and replaced by a public corporation the British Broadcasting Corporation (BBC), responsible to the British parliament but with daily control in the hands of an impartial board of governors. The BBC is now financed out of license fees, the bale of programs abroad, the sale of books and records, and other resources.

The licensing of private radio broadcasting organizations was prevented for decades after the BBC was formed. Guided by its first Director-General, John

Reith it developed a unique idea of public service a kind of mission to inform, educate, and even improve the tastes of it listeners.

Commercial radio broadcasters set up stations in other countries beaming programs to Britain - the best-known of them being Radio Luxembourg - but it was not until the 1970s that the BBC’s monopoly w as broken. ‘Pirate’ radio stations playing pop music - some of them actually transmitting from ships at sea created a demand for a commercial alternative to the BBC s rather more conservative programming. By the 1980s there were a large number of independent local radio stations throughout the United Kingdom regulated by the Independent

Broadcasting Authority (IBA). In competition the BBC set up its own network of local stations serving regional interests.

The BBC s national radio broadcasting operation is divided into five channels:

Radio 1 transmits pop music; Radio 2 broadcasts light entertainment (comedy shows, light music, and so on); Radio 3 serves listeners who prefer serious music and culture; Radio 4 specializes in plays, talks, and news and current affairs programs; and Radio 5 carries sport educational programs, and programs designed to appeal to young people. The 1970s saw the emergence of community radio services, dealing with local issues and encouraging participation in programming by ordinary members of the public The BBC b external services broadcast in many languages to all parts of the world.

Other countries also developed their radio broadcasting services about the same time as those of the United States and Britain. In The Netherlands, a radio station in the Hague began regular broadcasts in November 1919.

Australian broadcasting began in 1923 and now operates on three levels. The national service is provided by the Australian Broadcasting Commission a statutory body w which started its work in 1932. Alongside this national service, which is supported by direct grants from Australia’s federal government commercial stations broadcast programs financed by advertising. The third level is pub lie broadcasting which provides a service of programs of local interest on a commercial basis Public broadcasting emerged during the 1970s.

In Canada, the first regular programs were transmitted from Montreal in 1920.

Canada has a mixed broadcasting system, with private commercial companies operating alongside the Canadian Broadcasting Corporation (CBC), a statesponsored independent corporation similar to the BBC. The CBC was established by the (Canadian) Broadcasting Act of 1958. The CBC broadcasts in both English and French. The BBC has provided a model for the official broadcasting organizations of France and Japan Several European countries including Britain

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belong to the European Broadcasting Union (EBU) an organization created in

1950. Its membership has been extended outside western Europe to take in the

United States the Commonwealth and other countries as far apart as Iceland and

Israel. For an account of the use of radio as a means of communication for example in citizens' band radio.

Television.

The research carried out by the Scots inventor John Logie Baird at Hastings,

England, led to the first demonstration of television in 1924. After this event developments in television broadcasting continued swiftly, not only in Britain, but also in Russia, the United States, and in several countries in Europe. In 1931, a

British research group was set up under Isaac Shoenberg, an inventor who had worked on television development in Russia. The technical standards that

Schoenberg's group established were put into use by the BBC when it launched the world's first regular television service on 2 November 1936, from Alexandra

Palace. London.

Britain. The BBC began its television service using the rival Baird and Marconi-

EMI systems but settled in 1937 for the Marconi-EMI system, which operated electronically instead of using the mechanically rotating disks employed by the

Baird apparatus The BBC was the only organization allowed to broadcast television programs in Britain for the next 18 years. During that time it operated continually except for the period from 1939 to 1946, when it was off the air because of World War II.

In 1954 the BBC's monopoly of television broadcasting was broken by the formation of the Independent Television Authority (ITA). This organization was set up to regulate the activities of commercial television companies, which were allowed to operate throughout Britain on a regional basis. In 1973 the authority's name was changed to the Independent Broadcasting Authority (IBA) and it became responsible for regulating commercial radio as well. The commercial television companies serve local areas, but programs made by the larger companies are often shown nationally. Two companies provide news for the independent broadcasting companies on a national level: these are Independent Television News Limited

(ITN) and its subsidiary company, Independent Radio News Limited (IRN).

Independent Television (ITV) is financed by revenue from "commercials", brief

"spot" advertisements made for private firms. Programs are not sponsored on ITV.

Both the BBC and the ITV broadcast programs on two channels each, BBC 1 and

BBC2, and ITV and Channel Four. Television programs in the Welsh language are transmitted on Channel Four in Wales (Sianel Pedwar Cymru, or S4C). Both the

BBC and Independent Television transmit daytime programs for schools and colleges.

The United States. Television developed more slowly in the United States than it did in Britain. The first public demonstration of television was made by NBC as late as 30 April 1939, at the New York World Fair. Both NBC and CBS began their regular television services that year. By the middle of the next year, 1940, there were 23 television stations. World War II interrupted development in the

United States as it did in Britain, but from 1946 the television broadcasting

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industry grew rapidly. In 1949 there were one million television receivers in use in the United States; over the next ten years that figure grew to 50 million.

Commercial television was first authorized by the Federal Communications

Commission in 1941. Since World War II, a vast number of television stations and companies have come into being. The stations arc all supported by the sate of

"spot" advertising and the sponsorship of programs. The number of stations operating in the United States is now so vast and the type of programs they transmit is so varied that it is not easy to give a clear picture of television broadcasting in that country. At the beginning Of the 1980s the Federal

Communications Commission reported that there were more than "50 commercial television stations, 269 educational television stations, and more than 4,000 television relay stations, Commercial television in the United States is dominated by three major networks, CBS, JJBC, and ABC. A fourth network, the Public

Broadcasting Service (PBS), is funded by government subsidy, grants from corporations and foundations, and contributions from viewers. A. valuable contribution to television in the United States comes from community antenna television (CATV), better known as "cable television" or "cablevision". In this system, television signals are received at a single station and sent through wires or cables to television receivers whose owners pay a regular subscription for the service. Cable television originally grew up in areas where reception was poor or limited to one or two stations. But now companies offering cable services make and broadcast their own programs. Many of these programs are of local interest and offer, by means of computer-assisted telecommunications, the chance for viewers to express their opinions and participate in the programs.

Other Countries. In Australia, the development of television broadcasting began as early as 1934. But World War II arrested progress on a national service, and a change of government in 1949 stopped it altogether for a time. Despite economic problems, Australian people demanded a television service. Regular television broadcasting began in Sydney on 16 September 1956. Television spread slowly throughout the country in stages and had reached about 98 per cent of the

Australian population by the 1970s. Commercial stations operate alongside those of the Australian Broadcasting Commission. Color television transmissions began in 1975, and in 1980 the first foreign language station went on the air to serve

Australia's ethnic minorities.

In Canada, television broadcasts began under the control of the Canadian

Broadcasting Corporation (CBC) in 1952. The 1958 Broadcasting Act placed the regulation of television stations under the control of the Board of Broadcasting

Governors. Under this board television broadcasting expanded rapidly, and a second network, the Canadian Television Network (CTV), opened in 1961. The

CBC continued to distribute television programs in both French and English.

Under new legislation passed in 1968, a new body, now called the Canadian

Radio-Television and Telecommunications Commission (CRTC) regulates broadcasting in Canada, including cable television, which has become popular since the 1960s. In English-speaking party of Canada, about 70 per cent of the programs watched on television are bought in from the United States. In French-

33

speaking areas, most of the programs originate in Canada.

Technological Advances

You can read about how television and radio signals are sent and received in the articles RADIO and TELEVISION. However, it is worth noting some important developments in this brief section. In television, the greatest advance came with the development of color transmissions. The television research pioneer John

Logie Baird provided the first successful practical demonstration of color television in 1928, using mechanical scanning. Ten years later, the French inventor

Georges Valensi patented the first system that allowed color transmissions to be received both by TV sets appropriate for the purpose and by sets only able to receive programs in black and white. Some pioneering color broadcasts began before World War II, but serious work on color broadcasting did not take place until the 1950s The method of color transmission accepted by the National

Television Systems Committee (NTSC) was introduced in 1954 and has been adopted in many countries as well as the United States. It is a "compatible" system that can be received by both color sets receiving signals on the ultra high frequency

(UHF) channels and also by the black-and-white receivers taking very high frequency (VHF) signals. Britain and Germany use a modified form of this NTSC color system called PAL (phase alternation line). France and Russia use a more complex system called SECAM (systeme electronique couleur avec memoire).

These systems are explained under TELEVISION. Regular color broadcasts began in Britain on the BBC2 channel in 1967.

Satellite broadcasting became a reality on 10 July 1962, when the American

Telephone and Telegraph Company, using its artificial satellite "Telstar", beamed television pictures in black and white from its transmitter in And-over, Maine, to receiving stations at Goonhilly .

Downs, in the English county of Cornwall, and Pleumeur-Boudou, in Brittany,

France. The first color transmission followed on 16 July. These transmissions were limited to short periods lasting only as long as the satellite was above the horizon for both the transmitting and the receiving stations. Now broadcasts across the world can be much longer because modern communications satellites are in what scientists call "geostationary" or "geosynchronous" orbits. This means that they always stay in the part of the sky relative to the transmitter and receiving station.

Transatlantic broadcasts between the United States and Europe are now a regular occurrence. Space satellites now provide a world-wide television and telephone network that links nearly every country on the globe.

During the 1980s the British government relaxed controls on broadcasting to allow operations by cable companies similar to those of the United States. It also opened the way for the spread of Direct Broadcasting by Satellite (DBS). This system, already operating throughout Europe, uses satellites to beam programs of high technical quality to television sets fitted with special large dish antennas to collect the signal. DBS makes possible such advances as television with stereophonic sound.

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Programs

The job of every television and radio service around the world is to transmit programs. In the case of television programs, some are made by the broadcasting organization itself, while the rest arc bought, either from independent producers or from foreign stations. This is also true of radio programs, though the larger national broadcasting organizations buy far fewer programs from outside sources.

Programs on both radio and television are classified in several different categories, covering news and current affairs, light entertainment (comedies, game shows, and so on>, drama, music, sport, and outside broadcasts.

Programs can be of three different types: "live" programs (including sports events and state occasions that are broadcast at the time when they happen); radio programs recorded on tape (including music programs and concerts recorded

"digitally" in stereophonic sound and television recordings made on film or video

(including plays or series). In addition, television companies broadcast films made for theaters and also films made especially for television.

The person who decides which programs should be broadcast is generally called the director or controller of programs. This person organizes a number of departments to produce the specific categories of programs. Each department has its own head, who oversees the work of several producers and directors. The departments share certain central services, such as make-up, filming, transport, and even casting. Recorded programs take many weeks or even months to prepare, and both the actors or announcers and the camera personnel have to rehearse. Some

"live" programs also require rehearsal, so that camera angles can be worked out in advance in order to obtain the best shots. Even news bulletins and weather reports require rehearsal, although the time available for such preparation is very short.

Television Programs. Each department makes its studio programs in the same way. The producer and director sit in the gallery or control room. In front, a row of television screens, called monitors, show the pictures obtained by each of the cameras on the studio floor Each cameraman has a "camera card" which is his script and shows his moves and the shots required—for instance, long shot, medium shot, or close-up. The director decides which pictures to use, and asks the vision mixer to cut, blend or fade from one picture to another. It takes about four days to record a full-length play.

The production secretary watches the timing of the program and the technical crew control the picture and sound quality. The technical crew and floor manager wear headphones so that they can receive instructions from the director, who is linked to them through his small "talk-back" microphone.

If the production contains film inserts or tape recordings of music or sounds, these are fed into the transmission from the studio control room. For example, in a musical production a song may have been tape-recorded beforehand by a singer.

The director "cues in" this tape while the actor in the studio mimes the song.

("Miming" means acting in silence The actor looks as if he were singing but is simply moving his lips.)

Filming "on location", that is, outside the studio, takes much longer. The director and film crew (consisting of a cameraman, a sound recordist, a lighting man, and

35

others) have to travel to find the right spot and then set up the cameras, microphones, and other equipment needed to make the film. A crew might spend a whole day getting only a few minutes of film. In the case of a "live" outside broadcast, the director and his crew are linked to the studio control room by special

"land lines".

Computers now play a large part in presenting and "processing" the images that we see on our television screens. They are particularly valuable in generating, or producing, "graphics", or pictures.

Radio Programs. Studio broadcasts are prepared, produced, and directed in much the same way as television programs, except that no pictures are transmitted. In radio programs, the microphone is the camera, as it were, but apart from this distinction, many of the same procedures apply. For example, a production assistant keeps a check on running times, tape-recorded material has to be inserted at the appropriate spot, tape recording needs to be edited to fit the correct time period, and so on.

Modern radio broadcasting has come to accept the principle of having no silences in transmissions. On pop-music stations where programs consist mainly of "disk jockeys'* playing records and talking in between them, any gaps can easily be filled. On channels specializing in speech, drama, or news, a continuity announcer provides links between programs and is on hand to fill up any awkward gaps caused by technical problems.

The world of broadcasting is more than ever a world of fascinating and varied sounds and visual images. It is a world that could not have been imagined in 1901. when Marconi made his first transatlantic transmission in Morse code.

RADAR

RADAR is an electronic system for locating distant objects. It works by sending out radio waves from an antenna in a thin beam of very short pulses and listening for echoes to return to the antenna. (For an explanation of radio waves, see under

RADIO.) By measuring the time a single pulse of energy takes to return to the antenna (the speed of radio waves is 300,000 kilometers or 186,000 miles a second) it is possible to work out the distance the pulse traveled. If the direction of the pulse is known, then the combination of distance ("range") and direction

("bearing") gives the position of the object (known as a "target") which caused the reflection of the pulse. A rotating radar-reflecting antenna sweeps out a circle with its microwave radar beam. Clearly, a process like this which works just as well at night or in fog is very useful for detecting aircraft and ships. If the radar is fitted to an aircraft, it can be used to detect high ground. If it is on a ship, it can locate land and other ships. These are the most common uses for radar, but an enormous number of other uses have been found.

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Pioneers of Radar

From the early days of radio, scientists knew that very short radio waves could be reflected from solid objects. Heinrich Hertz, who discovered how to generate radio waves, was aware of this. Guglielmo Marconi, the pioneer of radio on whom there is a separate article, outlined a system like modern radar in 1922 but did no practical work on it. In 1925, the American scientists Gregory Breit and Merle

Tuve began a series of experiments to find the height of the reflective part of the atmosphere, called the ionosphere, and used a technique later used widely in radar.

However, it was the threat of war in the 1930s that gave a big boost to the development of radar.

In 1935 the British government asked the scientist R. A. (later Sir Robert)

Watson-Watt to look into the possibility of a "death ray" using radio waves. He told them such a ray was impossible but instead suggested that radio waves might be used to detect aircraft long before they could be seen or heard. A practical demonstration followed. On 26 February 1935, Watson-Watt succeeded in detecting a bomber aircraft using radio waves.

The ability to give early warning of enemy air raids was so important that

Watson-Watt's ideas were quickly adopted. By the time World War II started in

1939, some 40 secret radar stations had been built around the south and east coasts of England.

They were called "RDF" (standing for "radio direction finding") stations. While the British were in the lead, both the United States and Germany were also developing radar systems. In 1940, the public were told something of the secret, and newspapers called the new system "radiolocation". However, in 1942, Britain officially adopted the American word "radar" which stands for "Radio Detection

And flanging". By this time the radar principle had been.

The radar screen on a vessel carrying the radar antenna shown in the top diagram indicates the location and distance of the shore, an island, and a ship The indicator, or trace, sweeps the screen as the radar antenna rotates extended to a number of other purposes besides aircraft and ship detection. Guns were being armed by radar and radar was also fitted to night fighters to help them find and attack enemy bombers. Radar-aimed bombing de-\ ices were also developed.

So radar was first developed for wartime use. Since the 1940s, however, radar hay come to play an important part in navigation, both at sea and in the air. How do these different radars work?

How a Simple Radar Works

A simple radar system, as found on many merchant ships, has three main parts.

These are the antenna unit (scanner), the transmitter/receiver or "transceiver" and the visual display unit.

The antenna is about 2 or 3 meters (7 or 10 feet) wide and focuses pulses of microwave radio energy into a narrow vertical beam. The frequency of the radio waves is usually about 10,000 MHz (that is 10,000 million, or 10 billion, cycles per second! - equal to a wavelength of 3 centimeters (just over 1 inch). The antenna is rotated (turned round) at a speed of from 10 to 25 revolutions per minute

37

so that the radar beam sweeps through 360° round the ship to a range of about 90 kilometers (56 miles).

In all radars it is vital that the transmission (sending out) and reception of radio waves in the transceiver are in close harmony. Everything depends on accurate measurement of the time between the transmission of the pulse and the return of the echo. About 1,000 pulses per second are transmitted. Short pulses are best for short-range work, longer pulses are better for long-range work.

An important part of the transceiver is the modulator circuit. This "keys" the transmitter so that it oscillates, or pulses, for exactly the right length of time. The transmission power is generated in a device called a magnetron, which can handle these very high frequency oscillations and very short pulses.

Between each pulse the transmitter is switched off and isolated. The very weak echoes from the target are picked up by the antenna and fed into the receiver, amplified (made stronger), and then passed to the display unit.

The display unit usually carries all the controls necessary for the operation of the whole radar. It has a cathode ray tube similar to that used in a television set, but specially adapted for radar work. In the neck of the tube is an "electron gun" which shoots a beam of electrons at a chemically coated screen at the far end. The chemicals on the screen glow when they are hit by the electrons and. although the screen is on the inside wall of the tube, the resulting spot of light can be seen through the glass face exactly as you view a television picture. Unlike a television tube, however, a radar tube has a circular screen marked off ("calibrated") in degrees around its edge The electron beam travels out from the center to the edge.

This radial motion of the electron beam, known as the "trace", is matched with the rotation of the antenna. So when the trace is at 0° on the tube calibration, the antenna is pointing straight ahead The beginning of each trace corresponds exactly with the moment at which a pulse of radar energy is transmitted. When an echo is received it brightens up the trace for a moment. This is a "blip", and its distance from the center of the tube corresponds exactly with the time taken for the radar pulse to travel to the target and return. So the "blip" on the screen gives the range and bearing of the target As the trace rotates, a complete picture is built up from the "afterglow" of the chemical coating of the tube. This type of display is called a

Plan Position Indicator (PPI) and is the most common form of presenting radar information.

Computers and Radar

From this simple description, you can see that timing is important to the way radar works. By using computers, various "tricks" can make the display easier to read and also get rid of unwanted echoes. For example, in an air traffic control radar (controlling the movement of civil and military aircraft) it is important to be able to see only the aircraft. Other echoes, such as rain or high ground, only confuse the display A computer can be programmed to display only those targets which are moving fast enough to be aircraft. Also, the computer can "draw" the radar picture and show targets as crosses, squares, or triangles, more easily read than the old-fashioned blip. Whether the air traffic controller sees blips or computer-generated symbols, he must be able to identify the targets. He also needs

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to know the height of aircraft, so that those on the same course but flying at different levels can be kept apart . To give the controller this information, a second radar, called a "secondary surveillance radar", is used. This works differently and needs the "help" of the aircraft. It sends out a sequence of pulses to an electronic

"black box", called a "transponder", fitted on the aircraft. The transponder is linked to the aircraft altimeter (the device that measures the plane's altitude or height) to transmit back to the radar a coded message identifying the aircraft and its altitude.

Military aircraft use a similar radar system with secret codes to identify "friend" from "foe". A "hostile" aircraft does not know what code to transmit.

To work out the height of unidentified aircraft, special height-finding radars are used. There are two methods of height-finding: one called "nodding" radar and the other "three-dimensional" radar A nodding height-finder is like an ordinary radar standing on its side. When the controller wants to find the height of a target displayed on the screen, the height-finder is rotated to the bearing of the aircraft, whereupon the radar nods up and down. By measuring the angle of the antenna at the point when the beam strikes the target, the height of the aircraft can then be worked out, There are also what are called "3-D" radars. Instead of using two radars, the two functions are combined in a single antenna system Here the "nod" is an electronic signal instead of a mechanical movement of the antenna.

There are many other uses for these radars. Most missiles are guided by radar.

Quite often the radar is mounted in the nose of the weapon Radars are also fitted on board some aircraft to warn the pilot of air turbulence and thunderstorms Radars now play an important part in weather forecasting and are also found on hoard spacecraft, mapping the surface of the Earth below. Radars using continuous wave transmission (rather than pulses) are fitted in devices such as the proximity fuse, which causes a missile or shell to explode when close to its target. Also, as some car drivers discover to their cost, radar can be an effective way to catch the motorist who exceeds the speed limit.

RADIO

In nature there exists a vast range or spectrum of electromagnetic radiation. At the top end of the scale fin terms of the number of oscillations or vibrations per second] are cosmic rays, followed by gamma rays. X-rays, ultraviolet rays, visible light, infra-red rays and, at the bottom of the scale, radio waves.

Radio communication makes use of the "act that electromagnetic waves can carry electrical copies of sounds when the sounds have been transformed into electrical oscillations of the same frequencies. An electrical oscillation is an electric current which instead of flowing steadily in one direction reverses its direction of flow at regular intervals. A cycle is one complete sequence and the number of times it repeats itself every second is called the ''frequency''.

Radio waves cover a range of frequencies from about 30 million kHz to 10 kHz

(kHz=a kiloHertz, an oscillation of 1.000 cycles a second). These waves travel through space at a uniform speed of 300,000 kilometers 1186,000 miles) a second.

This article outlines the history of radio communications. The article

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ROADCASTING explains how radio programs are planned and produced;

RADAR explains the use of radio echoes for finding the position of distant objects, and TELEVISION describes how radio waves are used to bring pictures into the home.

Early Days of Radio Communication

Until the 19th century nobody knew anything about radio waves. Then in 1864

James Clerk Maxwell, a brilliant Scottish mathematician, on whom there is a separate article, showed that in theory radio waves, which are lower in frequency than infra-red waves, must form part of the electromagnetic spectrum. However,

22 years passed before a German scientist, Heinrich Hertz, became the first to generate these mysterious invisible waves and show that they obeyed the optical laws governing reflection, refraction, and interference, But neither he nor any other scientist of the day could find a practical use for them.

Hertzian waves, as they were called, remained merely a puzzling subject for laboratory research until 1894-5, when a way was found (.0 use them to transmit

Morse code. There is some doubt as to who first used a Morse key (see MORSE,

SAMUEL) and an antenna system in conjunction with a Hertzian transmitter. It was either a Russian, A. S. Popov, or Guglielmo Marconi, at that time an unknown young Italian. Popov worked for the Imperial Russian Navy and his work was secret, and Marconi too worked in secrecy, so no firm date can be given. However, there is no doubt that Marconi went on to become the greatest influence on the development of radio. In 1896 Marconi arrived in England and demonstrated his

"wireless apparatus". The transmitter was similar to that used by Hertz. It consisted of an induction coil which, when connected to a battery, developed a high voltage across two metal spheres placed close to one another. The air gap between the spheres broke down under the electrical strain and a stream of sparks jumped across the gap whenever the Morse key (which Marconi had added to Hertz's apparatus) was pressed down .

An elevated wire (antenna) was connected to one side of the spark gap and the other side was grounded, that is, connected to the earth. The stream of sparks produced a series of oscillations, which radiated into space from the antenna. By pressing the Morse key for differing periods, the radiations could be made to surge from the antenna is long or short bursts to form letters in Morse code.

To receive the signals a device called a coherer was used. This was a small glass tube containing loosely-packed metal filings and was connected between an antenna and the ground. In the absence of any signals on the receiving antenna, the electrical resistance of the filings remained high But as soon as a signal arrived, the metal filings "cohered", or came together, and the resistance dropped considerably.

Whenever the resistance dropped in this way it caused electromechanical relays to come into operation and work a Morse printer. One disadvantage of the coherer was that every time a signal caused the filings to cohere, they had to be tapped to restore them to high-resistance condition. This was done automatically using the hammer of an electric bell mechanism.

When an operator at the transmitter tapped out a message in Morse code, it was picked up by the receiver and printed in Morse characters on a paper tape. The

40

range of the new wireless apparatus was only a few kilometers and sending messages was slow work There was no way of tuning, and so two transmitters within range of each other could not work at the same time If they did, the receivers picked up both messages simultaneously, with chaotic results.

But between 1896 and 1901 Marconi made great progress. Transmitters grew from a simple, battery-operated, table-top version to a high-power station on the cliffs at Poldhu in the English county of Cornwall The tuned circuit was developed, so that stations could operate without interfering with one another, and receivers were improved. In December 1901 Marconi announced that signals from

Poldhu had crossed the Atlantic Ocean and been received at St John's in

Newfoundland.

The Ionosphere

Yet, in the light of the knowledge of the time this was theoretically impossible.

Hert had shown that radio waves traveled in optical paths It was thought that to cross the Atlantic antennae over 150 kilometers (93 miles t high would he needed on each side because of the curvature of the Earth. Marconi had no independent witnesses and his claim was not believed at first. What was causing the waves to travel like this when in theory the> should have left the Earth's surface just beyond the horizon to vanish into space? No one knew. But in 1902 an American electrical engineer, Arthur E. Kennelly, and an English physicist, Oliver Heaviside, suggested that high above the Earth was a layer ionization (electrified particles) which acted as a mirror, reflecting the radio waves down to Earth again. Few people accepted this ,it first However in l924 the Kennelly-Heaviside layer was detected experimentally by the British scientist Edward Appleton.

In fact several regions (D, E, F1, and F2) of ionization surround the Earth, at different levels and in two main groups. They are caused by the Sun's action on gases in the upper atmosphere, where ultraviolet radiation frees some of the electrons from the gas atoms. These free electrons reflect part of the radio wave back to Earth while the remainder is absorbed by the ionosphere. The amount reflected depends on various factors, including the density of the layer, its height, the frequency of the radio wave and the angle at which it encounters the layer.

One of the two main layers (from 80 to 140 kilometers, or 50 to 90 miles, above the Earth) is called the Kennelly—Heaviside layer (region E). The other, known as the Appleton layer (regions FI and F>). is about 240 kilometers 1150 miles) up.

These layers made longdistance radio transmissions possible before the first satellite launches.

The Radio Wave Spectrum

The spectrum of radio waves is divided into seven main bands. The divisions are artificial ones, made on the basis of different transmission characteristics. In each case, there is a gradual shift from one band to the next, rather than a sudden break.

The bands are described in terms of wavelength or frequency. These values are directly related: wavelength is the velocity of electromagnetic radiation (300,000 kilometers or 186,000 miles per second) divided by frequency. A high frequency corresponds to a short wavelength and vice versa

In ascending order, the first division is the very low frequency (VLF) band Only a

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few, specialized, stations operate in this hand Their transmissions are almost wholly reflected from the ionosphere and are thus guided round the Earth in a

"channel"' formed by the ground and the atmosphere. This gives them reliable worldwide coverage.

In the low frequency (LF) band the ranges are not &o large because of absorption by the ionosphere Nevertheless LF stations can cover considerable areas by day and night Many radio sets have a long wave or LF band But few stations broadcast on this although their signals are fairly good

Medium wave or medium frequency (MF) stations behave quite different. After dark MF stations which cannot be heard by day are received at great strength often causing serious interference with other transmissions Also their signals tend to fade

I hose effects are caused by the ionosphere.

All radio transmissions have two components or parts the ground wave, which travels along the ground and the sky nave, which travels up to the ionosphere. In day light the sky wave of an MF station is almost completely absorbed by the ionosphere and the station's receiving area receives only the ground wave which gives a steady signal over reasonable distances. But because the ionized layers are produced by the Sun’s action, they are affected in density and height both by seasonal changes and by the change from day to night. When darkness falls there is much less absorption allowing the sky wave to be reflected back to Earth. The reflected waves which return to Earth within the ground wave area add to or subtract from the strength of the signal causing fading. They are also

42

reflected again from the Earths surface back to the ionosphere only to be reflected once more I his bouncing process may be repeated several times, and in this way signals can reach areas alter dark reach they cannot reach during the day.

The high frequency (HF) or short waveband is most important in long distance communications although for many years it was regarded as useless for anything other than short-range broadcasts. This was because the ground wav e extends only a short distance and nobody knew anything about the sky wave until the 1920s

Then it was discovered that the sky wave penetrates the lower layers of the ionosphere but is reflected by the Appleton layer higher up. The first reflection returns to Earth at a point far beyond the limit of the ground wave and, by making several ‘’bounces’’ can reach immense distances -even around the world. Between the limit of the ground wave and the first bounce is a zone where no signals can be heard. I his is because t what is known as the skip distance effect.

The atmospheric effects known as aurorae are caused by high-energy particles from the Sun entering the atmosphere. These particles also interact with the Faith's magnetic field and the ionosphere to produce sudden disturbances or radio fade outs. The sky waves of very high frequency (VHF) lira high frequency (UHF), and super high frequency (SHF) radio stations penetrate all the layers of ionization and escape into space. Their ground waves become shorter in range with increasing frequency until at the upper end of the SHF band the range does not extend much beyond the horizon. Engineers try to increase this distance as much as possible by siting the stations on hills or mountains.

In spite of their short ranges these bands ire very useful VHF and UHF signals can carry a lot of information in a short time which is necessary for example for television.

Such TV stations and TV receivers are said to have a wide_ bandwidth. The short range of VHF and UHF stations 35-60 kilometers 20-40 miles) means that many television stations are needed to cover even an area as small as the British

Isles!

The large amount of information that can be earned by high-frequency radio waves makes hem useful for carrying telephone and facsimile messages. The higher the frequency the greater the amount of information that can he carried. For example , an SHF wave can deal with up to 900 telephone conversations at the same time. The short-range limitation can be overcome by placing a chain of stations at intervals between two cities and repeating the messages along the line from one station to the next. This can be done automatically and distances of up to

1 500 kilometers (900 miles 1 can be crossed linking cities separated by deserts or forests.

If a VHF wave is directed skyward at a low angle most of the energy escapes into space. But a small fraction of it is scattered by the ionosphere and some of this returns to Earth at a distant point where it can be picked up by a highly sensitive receiver. This approach called ‘’ionospheric scatter’’ provides reliable reception over distances of about 1 500 kilo meters (900 miles).

The scatter principle is also used for medium range communication using the ultra-high frequencies. Here the scattering is caused by pockets of turbulence in the

43

troposphere about 8 kilometers (5 miles) above the Earth The ranges are around

300 to 500 kilo-meters (190 to 310 miles) suitable for communication between mainland stations and islands or oil rigs.

The Thermionic Tube

Until the 1920s nearly all the world s radio stations sent messages in Morse code

This is known as wireless telegraphy. The transmitters generated radio waves from sparks produced by a Poulsen arc or an Alexanderson high-frequency alternator.

None of these devices was suitable for radiotelephony, the electrical copying of sounds, including speech.

In 1904 the British scientist, Sir Ambrose Fleming invented the thermionic diode tube, to which the American physicist, Lee de

Forest, added an extra element, called the grid, in 1906 . The triode, as this device came to he known, could amplify (strengthen) weak signals. But not until 1913 was it discovered that the triode could also generate oscillations of a very pure character, which were ideal for radiotelephony.

During World War I there was great progress in the efficiency and powerhandling of the triode tube. By 1918 the armed forces were using large numbers of low-power radio-telephony sets. When the war ended, radio manufacturers looked around for new markets. Public radio broadcasting began in 1920 and its development throughout the world is described in the article BROADCASTING.

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Today, the thermionic tube has been largely replaced by the transistor although it is still used where large output powers are required.

How Radio Works

When a radio broadcast is being made, sound waves, vibrating at different frequencies are picked up in the studio by microphones and changed into electrical oscillations of the same frequencies. These signals are then passed through amplifiers to magnify them. Higher-frequency radio waves are generated in the transmitter. Their frequency or amplitude is then varied, or modulated, in accordance with the audio frequency signals: they therefore "carry" the information of the audio frequencies and are known as "carrier waves". The modulated signal is then fed into the antenna from which they radiate in the form of electromagnetic waves. In amplitude modulation, or AM, systems, the electrical oscillations change the amplitude of the high-frequency carrier waves. In frequency modulation, FM systems, the amplitude of the carrier wave remains the same but its frequency varies with the audio-frequencies, Television transmitters use AM to transmit video signals and FM, usually, to transmit the sound frequency modulated (FM) wave (voice & carrier wave) How a carrier wave is modulated to broadcast speech.

Inside the radio receiver is an arrangement of inductors (coils) and capacitors

These provide the "tuning", so that the circuit accepts the correct signal and rejects others. Transistors (or, less often, valves) amplify the signals, which are weak when they arrive. The audio-frequencies are separated and operate a loudspeaker, which changes the electrical oscillations back into sound vibrations.

Satellite Transmissions

By far the most important advance in recent years has been the widespread use of satellites in Earth orbit . In "along-the-ground" broadcasting, the ranges that VHP and other high frequency bands can achieve is roughly limited to just beyond the horizon. Bouncing the signals off a satellite in space extends the "horizon' very considerably.

Suppose that a UHF station in Britain wishes to contact a similar station in the

United States. Normally, this cannot be done bt cause of the curvature of the Earth.

If, however, a satellite is placed in orbit above the Atlantic there is a clear line of sight between it and both ground stations. Signals are transmitted from the first station to the satellite, v here a receiver picks them up and amplifies them. They are then passed to a small transmitter in the satellite, which beams them towards the receiving station, where they arc again amplified. One such amplifier is the maser, a word obtained from the initials of Microwave Amplification by

Stimulated Emission of Radiation .

A communications satellite of this type contains all the necessary transmitting and receiving equipment, usually in duplicate. It is placed in orbit by a rocket with great precision, so that it will remain at a height of about 36,000 kilometers

(22,000 miles) over a chosen spot on the Earth's surface. This is known as a

"geostationary orbit". In this way the satellite is always in the correct position relative to the transmitter and receiving stations on the Earth. Satellites are powered by batteries or by solar cells (which change the Sun's rays into

45

electricity). In some cases nuclear power is used to give the satellites almost endless working lives. For interplanetary travel lasers might possibly be used.

The laser's very intense beam of light can be modulated by signals to serve as a carrier wave for speech.

RADIO ASTRONOMY

Until the middle of the 20th century, astronomers could explore the universe only by using optical telescopes to study the light from planets, stars, glowing clouds of gas, and galaxies However, many objects in the universe are brightest at wave-lengths of electromagnetic radiation outside the visible spectrum. In order to "see" the stars and galaxies that emit radio waves infra-red radiation, or X-rays, special telescopes and detectors are needed. The world's best infra-red telescopes have been set up on high mountain-tops with pollution-free atmospheres, such as Mauna Kea, a dormant volcano on Hawaii. X-ray detectors have to be sent above the Earth's atmosphere into space. The longest-established kind of "invisible" astronomy is radio astronomy, and radio astronomers have found many objects whose existence had hitherto never been suspected. For information on other branches of invisible astronomy .

TELESCOPE

The view of the Universe through a radio telescope is very different from the picture obtained by optical astronomers. Using optical instruments, we can observe directly the light coming from stars, galaxies, and other objects. Radio astronomers, however, detect the radio waves coming through space and use complex equipment, such as computers, to interpret them. Radio waves can be detected coming from our Sun, the strength of which varies with the sunspot cycle

(see SUN). The planet Jupiter is also a powerful radio source. This planet is surrounded by an extensive and strong magnetic field. Electrically charged particles passing through this field emit radio waves. Beyond the Solar System, radio telescopes pick up a glow of emission from the Milky Way as well as from several of the large clouds of gas, called nebulae. Within our Galaxy there are rapidly-flashing radio sources called pulsars, while beyond the Milky Way lie remote but extremely powerful radio galaxies and quasars. From the far depths of space there is a background of radio emission left over from the birth of the

Universe itself—the echo of the "Big Bang" .

This article explains a little of the history of radio astronomy, the equipment used by radio astronomers, and some of the great discoveries to which their work has led.

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The Start of Radio Astronomy in 1932 an American electrical engineer, Karl

Jansky, constructed the first radio telescope capable of picking up waves from the

Milky Way. The invention arose because in the previous year Jansky had found that when he pointed an ordinary radio antenna towards the Milky Way he picked up a faint hiss on his radio set. In the 1940s an English scientist, J. S. Hey, realized that strange interference on radar equipment was in fact caused by radio waves from the Sun. About this time, an American amateur astronomer, Grote Reber, built the first true radio telescope with which he detected a strong source of radio waves at ' he center of the Milky Way. From these small beginnings the giant radio telescopes of the present day have evolved.

Radio Telescopes

A radio telescope has three essential components: something to reflect the cosmic radio waves, a detector to pick them up, and a receiver for amplifying and recording the signals, which are extremely feeble. The reflecting "mirror" can be either an array, or set. of many antennas connected in a special way, or a metal sheet or wire netting. It is not necessary for the reflector to be solid, and wire netmg reflects radio waves very well Nor does the "mirror"' need to be particularly smooth, unless it is going to be used to receive signals it wavelengths smaller than

10 centimeters (4 inches).

Radio telescopes need to be far bigger than optical ones. This is mainly because radio waves are about a million times longer in wavelength than those of visible light, and for that reason alone large reflectors and arrays are needed in order to make detailed maps of cosmic radio emission. A second reason for the size is that the radio signals are extremely weak, and therefore undetectable unless a large collecting area is used.

The detector is generally a simple dipole antenna or rod with a connecting wire fixed to its middle . This is positioned at the focus of the dish-shaped telescope; its protective housing includes an amplifier to boost the incoming signal, which is too weak to be sent to the receiver. The radio receiver is complex and normally part of a computer system The information collected by the telescope can he processed by computer to make maps of the radio sky or to produce a "photograph"' of the radio image. The receiving system may include a radio spectrometer which is capable of recording the strength of the radio signal over a narrow range of wavelengths. This particular technique can be used to identify, b} means of" characteristic radio signals, the atoms and molecules in gas clouds in the Milky Way.

The largest single-dish radio telescope in the world is in the United States, at

Arecibo, in Puerto Rico. Here engineers have constructed a 300-meter (1,000-foot) radio telescope by lining a natural crater with wire netting. The telescope cannot he pointed just anywhere in the sky, but observers using it can move the detecting dipole instead. In addition, the Earth spinning on its axis allows astronomers to view different parts of the sky at different times of day.

More efficient than a fixed single-dish radio telescope like this is a steer able instrument such as the one at Effelsberg. near Bonn, Germany. The dish of the

Effelsberg telescope is 100 meters (328 feet) across, making it the largest steer able single-dish instrument in the world. Other steer able single-dish telescopes include

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the 76.2-meter (250-foot) paraboloid dish at Jodrell Bank, in Cheshire. England, completed in 1957, and the instruments at Parkes, New South Wales (Australia), and Goldstone, California (United States). The huge size of these single-dish radio telescopes makes them very expensive to construct and maintain, and they take years to build. For these reasons radio astronomers developed a different type of telescope consisting of several smaller dishes which are linked electronically.

These act together just like one enormous instrument. By joining two or more dishes together, thus forming what is called an interferometer, some of the qualities of a very large telescope can be obtained, in particular the ability to "'see" fine detail in a radio source. Each dish in the array, or linked group, is pointed at the same radio source. The Earth spinning on its axis causes the array to rotate beneath the sky. Computer analysis of the signals obtained over many days of observing the same object can then mimic the effect of a huge optical telescope.

A telescope of this type, 4.6 kilometers (2.8 miles) long, at Cambridge in England stands on the site of a former railroad track. There are others in The Netherlands and Australia. At Socorro in New Mexico is the world's largest interferometer, the

Very Large Array (VIA), completed in 1979. It has 27 antennas, each of which is movable along the three arms of a Y-shaped array; each arm is 20 kilometers (12.4 miles) long. The VLA can map small radio sources with the same precision as the best optical telescopes.

Radio telescopes in different countries a great distance apart can be linked electronically. This arrangement, called very-long-baseline interferometry (VLBI), allows fine structure to be mapped in very distant radio sources in greater detail than any optical telescope has achieved.

Radio Sources

The unfolding picture of the radio universe, as briefly described in the first section of this article, has provided some surprise discoveries. Here in more detail are just some of them.

Pulsars are tiny, heavily condensed neutron stars that rotate at very high speeds.

The first pulsars were detected by radio astronomers in Cambridge, England, in

1968. Their radio emission consisted of a series of extremely regular pulses separated by a second or less. For a time scientists thought the pulses might

Ionized hydrogen in the Horsehead Nebula in Orion is set aglow by nearby hot stars. It emits radio waves as well as visible and ultraviolet light be interference, but they soon realized that they were really linked with neutron stars many lightyears beyond the Solar System.

Hundreds of these pulsars are now known. Some send very rapid signals: 30 times a second for the one located in the Crab Nebula, and 642 times a second for one found in 1982 in the constellation Vulpecula. An explanation of pulsars and why they generate regular bursts of radiation can be found under STAR.

Supernova Remnants. The gas thrown into space by a supernova explosion (a huge explosion in which some massive stars meet their ends) may itself be a detectable radio source. One of the strongest of these supernova remnant sources is the Crab Nebula in the constellation Taurus. This glowing cloud is the remains of a star seen to explode in 1054. Some supernova remnants are almost invisible

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optically but show up strongly in radio maps of the sky.

Radio Galaxies. The year 1951 was a crucial one for the new science of radio astronomy, One of the strongest sources was found to be a galaxy 500 million light years away, then considered a fantastic distance. Astronomers realized that this object, called Cygnus A, had to be making the radio waves in some extremely powerful way in order for them to be detectable at such a distance. In fact the radiation from Cygnus A is about a million times more powerful than the waves from our own Galaxy. Hundreds of similar galaxies are now known to be radio galaxies.

The radio waves are produced by electrons accelerated to near the speed of light through magnetic fields that surround the galaxies. This form of radio emission is known as synchrotron radiation. Some sources are double, some have jets, and yet others have long tails as if leaving a wake caused by motion through the Universe.

The investigation of these energetic and remote galaxies led to an even more intriguing discovery—quasars. Quasars, Discovered in 1963, quasars are objects with starlike images and very large redshifts (see UNIVERSE.) If the redshifts can be interpreted as equivalent to vast distances, then quasars are extremely remote objects, perhaps about the size of the Solar System, putting out as much energy

(both light and radio waves) as 1,000 normal galaxies. It is possible that radio galaxies and quasars have at their centers enormously massive black holes .

The Nature of the Universe

Radio astronomy has made two major contributions to our understanding of the

Universe. The first is connected with radio galaxies and quasars. The work of radio astronomers suggests that there were once a lot more radio galaxies than there are now, because there seem to be many more of them at great distances (and therefore earlier times) than there are nearer to us. This suggests that the ''steady state" theory of the Universe once held by some astronomers is wrong, for according to it, the Universe never changes its basic appearance.

The second discovery was that of the "microwave background radiation" in 1965.

Many theorists consider this the most important finding in astronomy in the last half century. US radio astronomers found that surprisingly strong radiation was reaching the Earth, which did not seem to have any particular source. It was equally strong in all parts of the sky. Most astronomers accept that it is left over from a much earlier phase of the Universe. It may even be the remains of the fireball that started the expansion of the Universe.

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VOCABULARY

A abacus n абак, счеты accelerate v ускорять acceptable а приемлемый accept v принимать, допускать acceptor n акцептор access n доступ accessible а доступный, достижимый accompany v сопровождать, сопутствовать account v объяснять, пояснять to take into account принимать во внимание; on account of из-за, вследствие; on no account ни в коем случае accurate а точный acquire о приобретать act v действовать на (upon) activate v активировать actuate v воздействовать, воз-

; буждать add v прибавлять, суммировать added n слагаемое adder n сумматор, суммирующее устройство additive n добавка; а аддитивный additive method аддитивный метод address selector адресный селектор adjacent а смежный, примыкающий, соседний adjust v регулировать, приспособлять adjustment n регулирование adopt v принимать advantage n преимущество, выгода advantageous а выгодный, полезный affect v влиять, действовать age v стареть; я возраст agency n действие, средство, фактор agitation n колебание, перемешивание air core воздушный сердечник (без сердечника) align v выравнивать, регулировать alignment n выравнивание, выверка, настройка, регулировка alkali n щелочь alloy n сплав, примесь; v сплавлять

(металлы) alternating переменный, синусоидальный (о токе) alter v изменять (-ся), менять (-ся) altimeter n альтиметр, высотомер amber n янтарь amperage n сила тока (в амперах) amper turn ампервиток ample а достаточный amplification n усиление amplifier n усилитель analogue n аналог, модель analogue-to-digital converter аналогоцифровой преобразователь angle n угол angular а угловой antimony n сурьма apart adv на расстоянии aperture n отверстие, апертура apparent а очевидный, видимый apparently adv очевидно, по-видимому apply о применять, прилагать, прикладывать appreciable а значительный, заметный approximately adv приблизительно arbitrarily adv произвольно arc я дуга; v изгибаться дугой area n площадь, область, пространство arise (arose, arisen) v возникать, являться результатом (from) armature я якорь (магнита или машины) arrange v устраивать, располагать, классифицировать arrangement я устройство, расположение, классификация arsenic я мышьяк artificial а искусственный assembly я блок, узел assume v допускать, предполагать, принимать за attach v прикреплять, приспосабливать attachment я приспособление attain v достигать attenuation я затухание, уменьшение attract v притягивать attribute v относить к чему-л.; приписывать audio frequency output voltage выходное напряжение звуковой частоты augend n первое слагаемое automate v автоматизировать

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В automaton я киб. автомат available а наличный, доступный axis (pi. axes) я ось baffle я перегородка, экран, щит, отражательная доска balance v уравновешивать band n полоса частот, полоса пропускания bandwidth я ширина полосы bar magnet полосовой магнит bead я бусинка beam n луч bearing я подшипник behave v вести себя; behaviour n поведение, режим bias v смещать; п смещение bilateral а двусторонний binary system бинарная система bit л двоичный знак, двоичный разряд bit storage запоминающее устройство двоичного разряда bottom я дно, низ boundary л граница, край brain л мозг branch instruction л команда ответвления brake л тормоз break-down л эл. пробой break loose v отрываться break through прорываться сквозь bridge л мост; v перекрывать, шунтировать broadcast v передавать по радио, вести радиопередачу; л вещание brush я щетка bulb л колба buffer л буферная система, буфер build -up v нарастать, наращивать bulk storage массивное запоминающее устройство button л кнопка by-pass л шунт, байпас; v шунтировать, обходить

С calculus n исчисление

51 cam л кулачок, палец camera tube передающая телевизионная трубка cancel v стирать изображение, вычеркивать, аннулировать capacitance я емкость, емкостное сопротивление capacitor я конденсатор, емкость capacity л емкость, номинальная мощность, способность, производительность, выход capacity reactance емкостное сопротивление carbon-granule type тип угольного порошка care я забота, уход; о заботиться (for, about) carrier л несущий (о токе, частоте), носитель carrier frequency несущая частота carrier voltage напряжение несущей carry v нести, носить, проводить cathode follower катодный повторитель cathode ray tube (с. г. t.) катод-нолучевая лампа cause v вызывать, заставлять, причинять; я причина, основание, дело cavity л резонатор, объемный резонатор, полость cease v прекращает(ся), переставать, приостанавливать cell я элемент centigrade scale стоградусная шкала character л знак, символ, цифра charge v заряжать; л заряд . check л проверка, контроль; v проверять, контролировать circuit я цепь, контур circular а круглый, круговой circuit breaker автоматический выключатель, прерыватель цепи clicking n незначительное потрескивание cling v цепляться, прилипать coarse-mesh grid сетка с крупными ячейками coat я покрытие; v покрывать clock pulse синхронизирующий импульс code л код; v кодировать coherence л когерентность; взаимосвязанность coherent а когерентный, последовательный

coil л катушка collision л соударение, столкновение command portion узел управления communicate v сообщать, общаться communication я связь, сообщение commutator л эл. коллектор, коммутатор, преобразователь тока compound n соединение concave а вогнутый conduct v проводить conductivity л удельная проводимость; электропроводимость conductor л проводник, провод conserve v сохранять constant а постоянный; л физ.-мат. постоянная величина constituent л составная часть constitute v составлять continual а беспрерывный continuous а постоянный (о токе), непрерывный contract v сокращать, -ся, сжимать, -ся control v управлять, регулировать; л регулирование, управление, контроль control grid управляющая сетка, управляющий электрод control unit блок управления conventional а обычный, стандартный, условный; общепринятый converge v сходиться, сливаться conversion л преобразование, превращение convert v преобразовывать, превращать convey v передавать; эл. проводить core л сердечник, память, запоминающее устройство counter n счетчик, пересчетное устройство, схема counter-clockwise adv против (движения) часовой стрелки couple n пара coupling л связь cover v охватывать, покрывать cross section поперечное сечение crucible л тигель cumulative а совокупный curve л кривая (линия) curvafure n кривизна cut v резать, отключать cut-off n отсечка

52

D damage v повреждать, портить; n повреждение, вред damp а сырой, влажный; v заглушать (звук), демпфировать data n данные, информация date n дата, период, продолжительность deficiency n нехватка, недостаток deflect v отклонять density n плотность deplete v уменьшать, истощать depletion n обеднение; истощение

(электронами) depress v подавлять; ослаблять derange v выпадать из синхронизма, расстраивать design v проектировать, конструировать; п проект, конструкция, расчет desk calculator настольная (малогабаритная) счетная машина detect v обнаруживать, детектировать detection n обнаруживание, детектирование device n устройство, схема, прибор develop v проявлять, разрабатывать, развивать dial n циферблат, шкала, лимб differ v различаться, отличаться difference n разница, различие diffuse v дифундировать, рассеивать diffused base диффузионная база difraction n дифракция digit n разряд (числа), цифра, знак, символ digital computer цифровая счетная машина dim а тусклый, неясный dimension n размер, измерение direct current (d. с.) постоянный ток directly adv прямо, непосредственно disadvantage n недостаток, ущерб, неудобство, невыгодное положение disc cathode n дисковый катод discharge л разряд; v разряжать discrete а дискретный discriminate v различать, распознавать, выделять disintegrate v распадаться, разрушаться disintegration n распад, разрушение

.

dislodge v выбивать, смещать displace v смещать, перемещать dissimillar а разнородный, несходный dissipate v рассеиваться dissociate v распадаться distant а отдаленный, дальний, далекий distinction а различие, отличие distinct а отчетливый, ясный, определенный distinguish v различать, отмечать distort v искажать, искривлять distortion n искажение distribute v распределять disturb v нарушать, мешать divide v делить(ся), разделяться) divider n делитель, делительное устройство domain n домен, сфера, область donate v служить донором (в полупроводниках) donor level донорный уровень dot п точка dotted line пунктирная линия double v удваивать drift v отклоняться, сдвигаться; п сдвиг, уход drive v двигать, приводить в движение; п передача, привод driver stage задающее устройство, задающий каскад . drop n капля; перепад; v капать, падать, спадать dual а двойной, двойственный due а надлежащий, должный dull а тусклый duration n длительность dyne n дина (ед. длины)

E eddy n завихрение eddy currents токи Фуко, вихревые токи effect n действие, влияние; v совершать, осуществлять, выполнять effective value эффективное значение effort n усилие elapse v проходить (о времени) electrify v электризовать, электрифицировать

53 е. m. f.— electro-motive force электродвижущая сила electron-pair covalent bond ко-валентная связь электронной пары electro-plating n гальванопокрытие electrostatic электростатический elements n стихия elevation n возвышение, поднятие eliminate v устранять, уничтожать elongate v растягивать (-ся), удлинять (ся) emergency n авария, непредвиденный случай emission n выделение, излучение, эмиссия электронов emit v испускать, излучать, выделять enclose v окружать, охватывать encode v кодировать, шифровать energize v возбуждать, пропускать ток energy band энергетическая зона engage v нанимать; тех. зацеплять enhanced emission повышенная эмиссия катода environment n окружающая среда equidistant а эквидистантный, равноудаленный equilibrium v равновесие erasability n способность стирать erasable а стираемый escape v выделяться, уходить (о газах, паре); п утечка, выпуск essential а существенный, необходимый evacuate v выкачивать, высасывать, разрежать воздух even а четный, ровный; adv даже, ровно evidence n очевидность, доказательство, данные evolve v развивать (-ся), выделять (-ся) exceed v превышать, превосходить excess n избыток, излишек excessive а чрезмерный excitation n возбуждение excite v возбуждать, возбудить (ток); execute v выполнять exertion n усилие, напряжение exhaust и разрежать, выкачивать, исчерпывать exhibit v проявлять, обнаруживать expand v расширять (-ся), увеличивать (ся) expose n подвергать действию

F exposure n выдержка, экспозиция external а наружный, внешний facilitate v облегчать, продвигать fading n затухание, фединг fail v неудаваться, отказаться действовать failure n авария, провал feed (fed, fed) v подавать, питать; п подача, питание feedback /с обратная связь, регенерация fibrous а волокнистый fidelity n точность воспроизведения field-effect transistor транзнс-тор с управлением поля, канальный триод file я массив, картотека, комплект field intensity напряженность поля field winding обмотка возбуждения filament я нить накала, волосок film я плёнка, оболочка fin я ребро finite а конечный fit v пригонять, снабжать f i x v прикреплять fixed а постоянный, неподвижный, прикрепленный fixed storage закрепленное (постоянное) запоминающее устройство flashlight я электрический фонарь flashover я короткое замыкание между щетками, перекрытие изолятора дугой flash tube лампа, вспышка;импульсная лампа flat-top antenna плоская антенна flexibility я гибкость flip v перебрасывать(ся) flip-flop circuit цепь мультивибратора, цепь ждущего мультивибратора fluctuation я флуктуация, случайное колебание или отклонение flux я поток flux density магнитная индукция foil я фольга forbidden band запрещенная энергетическая зона forbidden gap запретная зона force я сила; и заставлять, принуждать, форсировать

54 forward adv вперед frame я рама, станина, кадр, поле framework я корпус, рама, каркас frequency я частота frequantly adv часто friction я трение full-wave type rectifier двухполупериодный выпрямитель functional unit функциональное устройство fundamental а основной, существенный fission я деление, расщепление fusion я плавление; яд. физ. синтез, слияние

G gain v усиливать, приобретать gap я зазор, промежуток, разрядник gate valve стробирующая лампа gear я зубчатая передача; v зацеплять(ся) general-purpose computer универсальная

(общего назначения) вычислительная машина generate v генерировать, вырабатывать, производить germanium n германий give off v выделять give rise v вызывать, приводить (к ч.либо) glow v сверкать, накаляться докрасна, добела;светиться glow-lamp я лампа накаливания, лампа тлеющего разряда grid n сетка grid anode capacitance емкость сетка — анод grid bias сеточное смещение grill n решетка ground я земля, заземление; v заземлять grounded-cathode circuit схема с общим

(заземленным) катодом grounded-grid circuit схема с общей

(заземленной) сеткой grown-junction я вырощенный переход

Н hairspring я волосковая пружина half-sine wave я полусинусоидальная кривая half-wave rectifier полупериод-ный выпрямитель handle у управлять, обращаться с, доставлять, осуществлять hardware n оснастка, аппаратура, схемная часть headset я головной телефон heat я теплота, накал, нагрев; v нагревать (-ся) heavily doped сильно легированный helium n гелий hitherto adv до сих пор, прежде hole я дыра, отверстие, Дырка horsepower я лошадиная сила, мощность

(в лошадиных силах) hydrogen я водород

I hysteresis я зл. гистерезис, отставание фаз identical а равный, тождественный ignite v зажигать (ся), загораться

Ignition я зажигание, вспышка; воспламенение

Image я изображение, у изображать, давать изображение image orthicon ортикон с переносом изображения impact я удар, импульс, толчок impact strength сопротивление удару impedance я полное сопротивление, импеданс impede v препятствовать, задерживать, мешать imperfection я дефект кристалла

(кристаллических решеток) impinge v ударяться, падать impregnate v пронизывать, насыщать impress v приложить, прикладывать impurity n примесь, засорение incandescence я накал, каление incandescent а раскаленная, накаленная добела

55 incandescent lamp лампочка накаливания

Inch n дюйм induce v индуктировать, наводить

Induction n индукция, наведение infer v заключать, выводить; предполагать, подразумевать ingot я слиток inherent а присущий, свойственный inject v вводить, впускать, вбрызгивать input а вход input unit я входной блок (устройство) insert v вставлять; эл. включать instruction word команда, командное слово insulate v изолировать integer я нечто целое; мат. целое число integrator я интегрирующее устройство, интегратор

Intelligence я информация, сведения, программа intensity я напряженность inferchangeability n взаимозаменяемость, заменяемость

Interdependence взаимосвязь, взаимозависимость interface я граница, контакт-.ная поверхность interfere v вмешиваться, мешать, служить препятствием interference n помехи interlacing я сплетение, переплетение intermediate а промежуточный interpose v вставлять, вводить между intrinsic а внутренний, присущий, естественный intrinsic semiconductor беспримесный,

J чистый полупроводник involved а данный, сложный irradiation я иррадиация, излучение irrespective adv независимо, безотносительно iteration я итерация, повторение jar v вибрировать, дребезжать; п конденсатор, банка jewel n драгоценный камень jumble v смешивать, спутывать

junction transistor контактный транзистор, пластинчатый транзистор

К keyboard n коммутационная модель, клавиатура knock loose v выбивать

L lack v нехватать, недоставать; я недостаток, нехватка laminated а слоистый, пластинчатый lattice n решетка layer n слой, пласт; heaviside layer слой хевисайда lead n провод, ввод; свинец, свинцовый аккумулятор leak v течь, просачиваться leakage n утечка, рассеяние leg n ножка, подпорка, стойка lengthwise adv в длину, вдоль lens n линза, объектив level n уровень level logic system потенциальная логическая система lightweight n спорт, легкий вес likely adv вероятно likewise ado точно так же, подобно limb n лимб, деталь; сердечник электромагнита linear а линейный link у соединять; п связь, соединение, звено linkage n потокосцепление, полный поток индукции lint n корпия liquid а жидкий; п жидкость load resistance нагрузочное сопротивление locate v располагать logic gate n логический вентиль longitudinal а продольный loop n петля, отверстие, контур, виток, пучность (волны) loop antenna рамочная антенна lose (lost) v терять, ослаблять loose а свободный, просторный,неточный

56 loss n потеря loudspeaker n громкоговоритель,репродуктор luminous а светящийся, светлый

М magnetic flux магнитная индукция magneto-motive магнитодвижу-щийся; m. m. f.— magnetomotive force majority carrier основной носитель заряда

make use of использовать malfunction n неправильная работа, аварийный режим man-made а искусственный manual а ручной, с ручным управлением mark v отмечать, обозначать match v согласовывать, приравнивать matter n вещество, материал medium n среда; а средний,умеренный melt n плавление; v плавиться melting point n точка плавления memory n память, запоминающее устройство memory unit блок памяти, запоминающее устройство mercury n ртуть

merge v поглощать, сливаться mesh n сетка, отверстие meter n счетчик, измерительный прибор mica п слюда microwaves n микроволны, сантиметровые волны mil n одна тысячная дюйма minority carrier неосновной носитель заряда misalighnment n ошибочное направление, несовпадение missile n снаряд, управляемый снаряд mixture n смесь mobility n подвижность, мобильность, возбудимость moisture n влага, влажность mold v отливать в форму, формовать monitor о контролировать, проверять; анализировать; п монитор,

контролирующий прибор, прибор для управления moreover adv сверх того, кроме того mosaic n светочувствительная мозаика, мозаичный фотокатод motion n движение, ход movable а передвижной, подвижный moving coil instrument магнитоэлектрический прибор moving picture кинофильм, движущаяся картина multiply v умножать multiplier а умножитель, множительное устройство muon-mu meson мю мезон mutual а взаимный mutual conductance взаимоиндукция mutual inductance крутизна характеристики электронной лампы

N natural frequency собственная частота negative а отрицательный neglect v пренебрегать negligible а незначительный necessitate v делать необходимым needle n игла, стрелка (прибора) net а чистый net charge полный, общий, результирующий заряд network n сеть, четырехполюсник

(радио) neutralize v нейтрализовать neutrino n нейтрино nitrogen n азот noise factor коэффициент шума,шумовой фактор non-uniform а неравномерный notice v замечать, отмечать noticeable о заметный, значительный nuclear engineering ядерная техника nucleus (pi, nuclei) n ядро

57

О obliteration n уничтожение, стирание obtain v получать, добывать obvious а очевидный, явный odd а нечетный offer resistance оказывать сопротивление offset n смещение; v компенсировать once adv раз opaque а непрозрачный, непроницаемый operand п. операнд, компонент операции oppose v оказывать сопротивление, сопротивляться opposite а обратный, противоположный opposition n сдвиг фаз, противодействие order n команда, порядок, разряд

(числа) origin n возникновение, начало, источник происхождения oscillator а генератор, гитеро-дин, осциллятор oscillate v колебаться, вибрировать otherwise adv иначе, в противном случае outer а внешний, наружный output n выход, производство, производительность output auxiliaries выходное устройство outset n начало oven n печь, термостат overlap v перекрывать, заходить один на другой override n перерегулирование oxide n окись, окисел oxygen л кислород

P paper reader считывающее устройство paramagnetic а парамагнитный partial а частичный, неполный particular а особый, данный pass о пропускать, передавать (ток через цепь) passage n прохождение, проход, канал pattern n растр, рисунок, модель, образец, характеристика, кристаллическая решетка pendulum n маятник penetrate v проникать, проходить сквозь.

penetration radiation проникающая радиация pentavalent а пятиатомный, пятивалентный pentode n пентод perform v выполнять, совершать performance n характеристика, работа peripheral equipment внешнее устройство, внешнее оборудование permeability я проницаемость persistence of vision инерционность зрительного восприятия phenomenon (pi. phenomena) n явление photocell n фотоэлемент pick up v улавливать; п микрофон, адаптер pick-up tube телевизионная передающая лампа picture frame кадр изображения piece n кусок, штука, часть, участок pile n куча, груда; эл. батарея pin Л штырь, вывод, палец pitchblende мин. уранит, урановая смолка pivot v вращаться, вертеться; п точка опоры, стержень plate n анод, пластина, полоса; v осаждать, выделять, откладывать plug-in package сменный блок, штепсель, контактный штырек plunge v погружать point n точка, пункт, вопрос, дело; острие, наконечник; v показывать, указывать, заострить, наточить pointer n стрелка прибора polarity п полярность pole n полюс, столб pole face лицевая поверхность полюса pole piece полюсный наконечник poor о плохой, бедный positive а положительный potential difference разность потенциала power n мощность, энергия,степень pnur out v изливать, выделять power supply system система питания мощностью preaging n искусственное старение precaution n предосторожность predict v предсказывать primary а первичный, основной print n шрифт; v печатать printer n печатающее устройство

58 problem set-up n схема (макет), проблема, задача, задание procedure n методика, техника, прием proceed v приступать, переходить (к ч.-либо) process n процесс; v обрабатывать processor n вычислительная машина для обработки данных produce v вырабатывать, производить, вызывать product n продукт, результат;мат. произведение programme counter счетчик команд prohibit v запрещать project v выдаваться, выступать, проектировать proof n доказательство propagation n распространение proper а надлежащий, должный, правильный, подходящий; собственно proposition n теорема, предложение prove n доказывать, оказываться provide v обеспечивать, снабжать (with); предусматривать provided с/, при условии, если только provision n обеспечение proximity n близость pull v тащить, тянуть, выдергивать pulse train серия импульсов punch v перфорировать, пробивать отверстия punch-card reader устройство для считывания перфокарт punched-card-to - magnetic - tape converter n устройство, перепечатывающее с

"перфокарты на магнитную ленту punched-paper-tape n перфорированная бумажная лента purify v очищать push-pull amplifier двухтактный усилитель

Q quantity n количество, величина quantitative а количественный quantize v квантовать quench v гасить, подавлять quotient n частное, коэффициент

R radar n радар, радиолокатор radio frequency радиочастота range n дальность передачи range-finder n дальномер rate n скорость, темп, номинал rating n характеристика, номинальное значение, номинальная мощность ratio n мат. отношение, коэффициент, пропорция, коэффициент трансформации ray n луч read v считывать read circuit схема считывания, цепь считывания read in v записывать read out v считывание данных, выбор

(информации) reader n считывающее устройство readily adv легко reason n причина for this reason по этой причине reasonable а приемлемый, умеренный, разумный, целесообразный receive v принимать, получать receiver n приемник receptacle n гнездо, штепсельная розетка, приемник reception я прием reciprocal а взаимный, эквивалентный; п мат. обратная дробь record v записывать; п запись recorder n записывающее устройство; регистрирующее устройство rectifier n выпрямитель, детектор rectify v выпрямлять recurrence n возвращение, повторение reduce v понижать, ослаблять, уменьшать, приводить (к), восстанавливать reduction n снижение, уменьшение, сокращение refer v иметь отношение to be referred

(to) ссылаться на, называться reference n ссылка, эталон reference generator опорный или эталонный генератор reference voltage опорное, эталонное напряжение reflect v отражать regard v смотреть на что-л.; считаться с чём-л.; рассматривать, считать

59 refract v преломлять . refrigeration n охлаждение, замораживание register n регистр, регистрировать relationship n отношение, соотношение relay n реле; v передавать release v освобождать, отпускать reliability n надежность reluctance n магнитное сопротивление reluctance motor синхронный двигатель с выступающими полюсами на роторе без возбуждения постоянным током remote control дистанционное управление remove v удалять, снимать render v превращать, делать, оказывать repair v ремонтировать, исправлять; п ремонт, починка repel v отталкивать (-ся) replica n копия, модель repulsion n отталкивание repulsion start motor репульсионный двигатель repulsive v отталкивающий reset v повернуть в исходное положение, устанавливать на ноль (повторно) residual а остаточный, оставшийся residue n остаток resilient а упругий, эластичный resist v сопротивляться, противопоставлять resistance n электрическое сопротивление, активное сопротивление resistive load активная (омическая) нагрузка resistor n прибор омического сопротивления (в эл. цепи), сопротивление respond v реагировать, отзываться response n реакция, отклик responsive а легко реагирующий, чувствительный restrain n сдерживать, удерживать result n результат, исход; v следовать;

(from) проистекать; (in) приводить к, иметь результатом retain v удерживать, сохранять, поддерживать retard v задерживать, замедлять retentivity n остаточный магнетизм reverse v реверсировать, переменить направление движения, переменить направление тока

revolve v вращаться revolution n оборот, вращение revolutions per second — г. p. s. обороты в минуту rigid а жесткий, неподвижный rigidity n жесткость rocketry n ракетная техника rod n стержень, прут rugged а прочный ruggedness n стойкость против износа, прочность

S sag v оседать, провисать; п оседание, осадка salt n соль sample n образец, проба sand-blasted base пескоструйная база saturate v насыщать, пронизывать saturation n насыщение, насыщенность saw-tooth а пилообразный scale n шкала scan v развертывать, сканировать; тел. разлагать изображение; п развертка, сканирование scatter v разбрасывать, рассеивать scintillation n вспышка, сверкание scope n масштаб, охват, сфера screen n экран, экранирующая сетка screen-grid tetrode тетрод с экранированной сеткой seal запаивать, закупоривать secondary а вторичная (обмотка) seed n затравка, зерно, затравочный кристалл segregation n сегрегация, расслоение self-sustaining а самоподдерживающийся select v избирать, выбирать selectivity n избирательность, селективность self-induction n самоиндукция semiconductor га полупроводник semi-conducting а полупроводящий sensitivity n чувствительность sequence n последовательность sequencer n устройство, устанавливающее последовательность sequential scanning последовательная развертка series n ряд, серия in series последовательно

60 set v устанавливать; п ряд, комплект set up v создавать, установить set-up n установка severely adv резко, сильно, тяжело shaded-pole motor двигатель с экранированными полюсами shape n форма sheath n оболочка sheet n лист (бумаги, металла); эл. пластина коллектора shell n оболочка, электронная оболочка, трубка shield n щит, экран; v заслонять, экранировать shockproof а вибростойкий, стойкий при ударах short-circuit n короткое замыкание; v закорачивать shutter n заслонка, засов signal-to-noise ratio отношение сигнал — шум silicon n кремний silver n серебро simulate v моделировать skin effect скин-эффект slab n плита, слой, пластина slant я склон, уклон; а косой; v искажать sleeve n гильза (гнездо штепселя), корпус штепселя slider n контакт, движок s l i p ring контактное кольцо slope n наклон slot n паз, щель slow down v замедлять soft iron instrument электромагнитный прибор solid а твердый, сплошной; фаз. твердое тело solidify v затвердевать solution n раствор, разрешение (вопроса, проблемы) solvent n растворитель space n пространство, промежуток, расстояние space charge пространственный заряд spacing n расстояние, пространство spacistor n спейсистор (полупроводниковый прибор) span n перекрывать spark gap разрядник, искровой промежуток

spark telegraphy искровая радиотелеграфия special-purpose computer вычислительная машина особого назначения special-purpose analog computer аналоговая вычислительная машина особого назначения specific resistance удельное сопротивление speed n скорость, быстрота split v расщеплять, раскалывать split phase motor двигатель с расщепленной фазой spool n шпуля для катушки возбуждения; катушка обмотки возбуждения spot n пятно; место; v узнавать, опознавать spray распылять spring v разбрызгивать spurious а искусственный, ложный square n квадрат, прямоугольник, квадрат числа; а квадратный- square root квадратный корень; получение квадратного корня stage n каскад, стадия, степень stainless steel нержавеющая сталь standing wave стоячая волна starting torque пусковой момент state n состояние stationary а неподвижный, стационарный, постоянный steady а устойчивый, постоянный steam n nap step down v понижать step up v повышать stiff а жесткий storage n запоминающее устройство; элемент, память, хранение storage battery аккумуляторная батарея storage tube запоминающая трубка store v запасать-, хранить strain v напрягать (-ся), скручивать, деформировать; п напряжение, деформация stray а блуждающий, паразитный stray capacitance паразитная емкость stream n поток, течение strength n напряженность, сила, прочность, сопротивление strip off v очищать (провод) stroke n ход

61 structure n конструкция, устройство, строение sturdy а прочный, крепкий subdivision n подразделение subject v подвергать действию

(влиянию); п предмет submiiltiple n субгармоника substitute v заменять, замещать, подстанавливать subtract v вычитать subtracter n вычитающее устройство sulphuric acid серная кислота supercooled а переохлажденный super-speed computer сверхскоростная вычислительная машина suppress v подавлять suppressor n защитная (пентодная, антидинатронная) сетка surface barrier поверхностный барьер surge n волна, импульс напряжения, всплеск surplus n излишек, остаток susceptible а чувствительный, восприимчивый susceptibility n чувствительность, восприимчивость swamping resistance поглоти тельное напряжение, добавочное сопротивление вольтметра sweep n развертка, качание, колебание

T switch v переключать, включать, выключать; п выключатель, переключатель, коммутатор, ключ tangent n тангенс; а касательная tap n отвод, ответвление, зажим target n цель, антикатод technology n технология, техника ten-column desk пульт с десятью колонками; магазин перфокарт с десятью колонками tension n напряжение tend v стремиться, иметь склонность term n срок, термин; v называть,- выражать terminal n зажим, конец terminate v кончаться, заканчиваться, ограничиваться

terrain n местность, территория test v испытывать therein adv здесь, там thermionic а термоэлектрический thermocouple я термопара, термоэлемент thin out v редеть thread v пронизывать, нарезать резьбу; п резьба, нарезка thus adv так, таким образом tight а плотный, непроницаемый time v синхронизировать, хронизировать tip л кончик, наконечник tolerance n допуск, допустимое отступление torsion n кручение, закрутка tower п башня, вышка, опора trace n след, незначительное количество; v проследить tracer n прибор для записи

(характеристики, кривой, повреждения) tracer atom n меченый атом train n серия, последовательный ряд transcend v превышать, выходить за пределы transfer v переносить, перемещать, передавать; п передача, перенос transfer instruction n команда передачи управления, команда перехода; команда переноса transistor n транзистор transition п. переход translate v преобразовывать, переводить, объяснять transmit v передавать, транслировать transmission n передача transmitter n радиопередатчик, тел. микрофон transmute v превращать transmutation я превращение transparent а прозрачный, просвечивающийся transverse v пересекать, скрещиваться transverter n трансвертер, преобразователь переменного тока в постоянный traverse v пересекать, скрещиваться trigger n триггер, пусковая схема; v запускать, отпирать trouble-free без неполадок, безаварийный tube n трубка, лампа (электронная) tune v настраивать, звучать

62 tuning-fork n камертон tungsten n вольфрам tunnel v проникать сквозь потенциальный барьер turn v повернуть (-ся), навивать

(обмотку) turn into v преобразовывать, превращать; п виток, оборот turn on включать turn off выключать turns ratio отношение витков обмоток трансформатора; коэффициент трансформации two-dimensional а двумерный, плоский typewriter n печатное устройство; пишущая машинка

U underlie (underlay, underlain) v лежать в основе unidirectional а однонаправленный unit n единица, целое; агрегат, секция, узел, блок, элемент unit bead бусинка unit magnetic pole единичный магнитный полюс; единица магнитной массы unit of digit единица числового разряда unlike а непохожий на, разнородный; ргр в отличие от unlike poles физ. разноименные полюса unlike charges ел. разнозначные заряды

V vacancy n пустота, промежуток valence n валентность valence band валентная зона valence shell валентная оболочка value n значение, величина valve n электронная лампа valve amplifier ламповый усилитель vaporize v испаряться variable resistance переменное сопротивление varistor n реостат, величина которого меняется от приложенного напряжения varnish п лак

velocity n скорость, быстрота; частота versatile а многосторонний, гибкий, универсальный vibrate v колебаться, вибрировать, звучать vibration n колебание, вибрация vice versa лат. adv наоборот view v осматривать, рассматривать; п осмотр, вид visible а видимый, явный vision n зрение virtually adv фактически, в сущности voltaic cell гальванический элемент volume n объем, громкость, тон volume control регулировка громкости

W wafer n пластина, плата, диск waste n отходы, потеря; v терять, бесполезно затрачивать wave n волна wave length длина волны wedge n клин wedge shaped клинообразный weigh v весить, взвешивать (-ся) weight n вес wind v наматывать, обматывать (-ся) winding n обмотка wire n проволока, провод wireless а беспроволочный; п радио withdraw v брать назад, удалять (-ся) word n код, число, группа символов, кодовая группа work function работа выхода wrap v завертывать write n запись, записывание write circuit схема записи writer n записывающее устройство

Y yield v давать, производить yoke п ярмо

63

Z zero n нуль

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