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Journal of the Institution of Electrical Engineers - Part IIIA: Radiolocation

Volume 93, Issue 5, 1946

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Volume 93, Issue 5

1946

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  • Cathode-Ray Tubes

    • An examination of cathode-ray-tube screen characteristics
      • Author(s): R.G. Hopkinson
      • Source: Journal of the Institution of Electrical Engineers - Part IIIA: Radiolocation, Volume 93, Issue 5, p. 779 –794
      • DOI:  10.1049/ji-3a-1.1946.0171
      • Type: Article
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      • p. 779 –794 (16)
        The development of cathode-ray tubes for radar has demanded improved screen performance. A wider range of screen types was found to be necessary, and many of these types were of a complex character.This paper deals with the general problems of screen design, with the exception of problems of afterglow, which are treated elsewhere. The approach to the problem has been photometric, and less emphasis is placed upon the electrical characteristics of screens than upon their optical properties.A range of luminescent materials was examined, and their luminous efficiency measured under the conditions of screen voltage and beam-current density normally met with in practice. The effect of various common methods of screen preparation upon colour and efficiency was studied, and also the various causes of loss of luminous output.A study of the optical properties of screens was undertaken with reference to methods of measuring and specifying screen thickness, and to the effect of the texture of the screen upon its value as a radar indicator.
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      Visibility of cathode-ray-tube traces in radar displays
      The photography of cathode-ray-tube traces
      • Author(s): R.G. Hopkinson
      • Source: Journal of the Institution of Electrical Engineers - Part IIIA: Radiolocation, Volume 93, Issue 5, p. 808 –814
      • DOI:  10.1049/ji-3a-1.1946.0173
      • Type: Article
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      • p. 808 –814 (7)
        The investigations described in this paper were undertaken to meet u number of isolated requests from the Services for information on photographic oscillography. Although the circumstances were not favourable for a planned investigation, information was obtained of the properties of luminescent materials for use as c.r.t. screens for photographic cathode-ray oscillograph tubes, and parts of the problem of the photography of c.r.t. traces was studied.Blue zinc-sulphide screens were found to be the best for general photographic work, and to have about the same “speed” to fast panchromatic, orthochromatic, or non-colour-sensitized photographic materials. Light from this screen is about six times as actinic as white light with a panchromatic emulsion, and about twelve times as actinic with an orthochromatic emulsion.Green willemite screens approach the actinic value of the blue zinc sulphide when used at very low anode voltages (less than 1 000 volts) in conjunction with a fully green-sensitized emulsion. At higher voltages, however, the blue screen is greatly superior.The recording of fast transients was studied, and the results tabulated in the form of an exposure table. Writing speeds above 2 000 km/sec can be recorded under suitable conditions.The effect of the electrical arrangements of the cathode-ray tube was also studied, and some special photographic techniques have been investigated.It was found that, for general photographic oscillography, little is to be gained by the use of special emulsions, as the standard range of photographic recording materials is adequate for most purposes. Similarly, little improvement over standard screens can be achieved by special phosphors and screening methods. Only in extreme cases is a combination of a special screen and emulsion justified.
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      Cathode-tay-tube screens for radar
      • Author(s): G.F.J. Garlick ; S.T. Henderson ; R. Puleston
      • Source: Journal of the Institution of Electrical Engineers - Part IIIA: Radiolocation, Volume 93, Issue 5, p. 815 –821
      • DOI:  10.1049/ji-3a-1.1946.0174
      • Type: Article
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      • p. 815 –821 (7)
        In radar displays of the intensity-modulated type, luminescent screens with afterglow are used to produce traces which persist for a time comparable with the interval between successive frames of the picture, which in the Plan Position Indicator is the time of rotation of the directive aerial.Theoretical and experimental studies show that the afterglow is a complex function of the excitation conditions. Two types of screen-testing apparatus are described which simulate the operational behaviour to the extent of using cyclic excitation at the aerial-rotation frequency in the application being considered. One particular case, H2S, is an airborne equipment with fairly high aerial-rotation frequency and low screen-excitation density. Some results for conditions simulating this are given. When measured by the methods described in this paper, afterglow intensity has been found to correlate reasonably well with operators' assessment of the usefulness of the screen in the equipment.The quantitative relations between excitation density, afterglow, and derived functions are shown graphically and discussed. Some factors which may affect the operator's response are considered, and some details given of the various screen materials used.
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      The Skiatron or dark-trace tube
      • Author(s): P.G.R. King  and  J.F. Gittins
      • Source: Journal of the Institution of Electrical Engineers - Part IIIA: Radiolocation, Volume 93, Issue 5, p. 822 –831
      • DOI:  10.1049/ji-3a-1.1946.0175
      • Type: Article
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      • p. 822 –831 (10)
        The Skiatron, or dark-trace tube, was developed to meet a radar requirement for a large-screen “black on white” picture with a persistence of several seconds.The present Skiatron consists of a magnetically focussed and deflected cathode-ray tube, with a screen consisting of a translucent micro-crystalline layer of potassium chloride. This screen is obtained by evaporation of the material in vacuum on to the tube face. An intensity-modulated scanning electron beam produces a picture by causing darkening in the areas bombarded. The picture is episcopi-cally projected using external illumination from mercury-vapour lamps.Screen characteristics have been measured by a photo-electric cell and a recording galvanometer. It is shown that the intensity of coloration increases with excitation until a saturation value is reached. Coloration produced by a pulse of excitation decays in several seconds and is referred to as “transient,” while that produced by a series of pulse excitations, applied at intervals, may take several hours to decay and is therefore known as “persistent.” Increase of temperature reduces contrast but increases the decay rate. Decay rate also increases with illumination, from zero in the dark to a saturation value at about 5 000 ft candles. The overall efficiency of the screen increases with beam voltage. The decay rate of the persistent colour can be greatly increased by periodic electron bombardment at low intensity. The presence of impurities also increases the decay rate, but the effect is not permanent.Application of basic coloration theory to the Skiatron indicates that the persistent colour is due to ionic diffusion.The Skiatron is fairly successful for radar work but is unsuitable in its present form for television.
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      Discussion on “Cathode-ray tubes”

  • Valves

    • Triodes for very short waves—oscillators
      • Author(s): J. Bell ; M.R. Gavin ; E.G. James ; G.W. Warren
      • Source: Journal of the Institution of Electrical Engineers - Part IIIA: Radiolocation, Volume 93, Issue 5, p. 833 –846
      • DOI:  10.1049/ji-3a-1.1946.0177
      • Type: Article
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      • p. 833 –846 (14)
        Section 1 of the paper deals with the excitation of triode oscillators. The limitations of conventional circuits are discussed and it is shown that the progress in the design of oscillators for very short waves has been mainly due to the reduction of electron transit time and the reduction in the inductance and h.f. resistance of the leads to the electrodes.The electron transit time has been reduced in two ways, (a) by the use of very small inter-electrode spacings—in some types the grid-cathode spacing is between 0.08 mm and 0.1 mm, and (b) by increasing the space current density—values as high as 5 amp/cm2 have been employed under pulse conditions.Electrode-lead inductance has been reduced considerably by the use of disc seals and other forms of glass-metal joints, whereby the valve becomes an integral part of the circuit.In general, h.f. oscillators and amplifiers have two adjustable circuit elements, one electrode being common to the two circuits, and another being earthed. Five types of circuit have been used, but the most important are(a) Common-anode earthed-anode using a CV52, VT90, NT99, CV55 or CV240 valve.(b) Common-grid earthed-anode using a CV90, CV153 or CV273 valve.(c) Common-grid earthed-grid using a CV16, CV53, CV88, CV257 or CV288 valve.(a) and (b) have been the most commonly used for oscillators, and (c) for amplifiers.Section 2 describes the evolution of transmitting triodes for radar, and of local oscillators for reception. Most of the valves are of metalglass construction, and several types are described to illustrate the new techniques employed. The anode usually forms the metal component of the glass-metal seal, giving low inductance and good cooling. The grid is mounted directly either on a disc seal or on a copper thimble. The electrode seals are designed so that the valve can be plugged directly into a coaxial-line circuit.Oxide-coated cathodes are used in all the later designs, including transmitting valves operating at a pulsed anode voltage of 15 kV and a peak input of 500 kW.Some typical valves are the common-anode transmitting valve NT99—a pair giving 200 kW at 600 Mc/s, the common-grid transmitting valve CV288—a pair giving 100 kW at 1000 Mc/s, the common-grid receiving valve CV273—giving 4 to 5 watts at 1 000 Mc/s and 0.5 watt at 3000 Mc/s.Section 3 summarizes some of the problems associated with testing and specifications.
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      Crystal valves
      Velocity-modulation valves
      Grounded-grid amplifier valves for very short waves
      • Author(s): J. Foster
      • Source: Journal of the Institution of Electrical Engineers - Part IIIA: Radiolocation, Volume 93, Issue 5, p. 868 –874
      • DOI:  10.1049/ji-3a-1.1946.0180
      • Type: Article
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      • p. 868 –874 (7)
        A theory of the common-grid earthed-grid amplifier, commonly called the grounded-grid amplifier, for medium transit angles is presented, showing the advantages this valve possesses over the earthed-cathode amplifier. A description of a range of valves using disc seals is given and their performance indicated, mention being made of the circuit advantages offered by the use of disc seals in valves. The results obtained on experimental valves are presented, and for signal/noise ratio improvement the probable upper limit of 2 000 Mc/s with present techniques is suggested, 5 000 Mc/s being the probable limit for oscillators.The two important problems awaiting theoretical attack are “total-emission damping” and high-frequency slope.
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      Principles of velocity modulation
      • Author(s): J.H. Fremlin ; A.W. Gent ; D.P.R. Petrie ; P.J. Wallis ; S.G. Tomlin
      • Source: Journal of the Institution of Electrical Engineers - Part IIIA: Radiolocation, Volume 93, Issue 5, p. 875 –917
      • DOI:  10.1049/ji-3a-1.1946.0181
      • Type: Article
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      • p. 875 –917 (43)
        Just before the war a number of papers were published on the subject of velocity modulation. Since then very many advances, both theoretical and practical, have taken place, but publication has been impossible. The present paper is a comprehensive treatment of the subject in which improved methods are given for dealing with one group of velocity-modulated devices.The theory presented has been developed by a team of workers in the valve laboratories of Messrs. Standard Telephones and Cables, namely J. H. Fremlin, A. W. Gent, D. P. R. Petrie, C. Strachey, S. G. Tomlin and P. J. Wallis. It is divided into five parts. The first, by Dr. Fremlin, is of an introductory character. Detailed discussion then follows on specific aspects of the problem, as under:Part 2 Small-amplitude theory and starting current, by A. W. Gent.3 Gap efficiency factors, by D. P. R. Petrie.4 Power output and efficiency, by P. J. Wallis.5 Electronic frequency control, by S. G. Tomlin.For the sake of consistency a uniform series of symbols has been adopted, A list of these symbols appears on page 916.
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      The CV35—a velocity-modulation reflection oscillator for wavelengths of about 10 cm
      • Author(s): A.F. Pearce  and  B.J. Mayo
      • Source: Journal of the Institution of Electrical Engineers - Part IIIA: Radiolocation, Volume 93, Issue 5, p. 918 –927
      • DOI:  10.1049/ji-3a-1.1946.0182
      • Type: Article
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      • p. 918 –927 (10)
        The paper describes a stable tunable oscillator, made in three models covering the waveband from about 8 to 11 cm, which was designed in 1940–41 as a beating oscillator for superheterodyne receivers, and has since been in manufacture and operational use. The construction is of the disc-seal type, tuning being achieved by the insertion of screw plungers into the resonator. Outputs of 300–500 mW are obtained at an input of 1300 V, 10 W (efficiency 5%). Dimensional details and characteristics are given, as well as a short account of the experimental development of the valve. The factors affecting the performance are mentioned and the efficiency and stability discussed on the basis of the first order theory of Barford and Manifold. Finally the theoretical efficiency is calculated and seen to be in good agreement with the observed value.
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      The cavity magnetron
      Magnet design for large air-gaps
      • Author(s): E.C.S. Megaw
      • Source: Journal of the Institution of Electrical Engineers - Part IIIA: Radiolocation, Volume 93, Issue 5, p. 939 –948
      • DOI:  10.1049/ji-3a-1.1946.0184
      • Type: Article
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      • p. 939 –948 (10)
        A simple basis for the design of magnets with large air-gaps is developed, in the first place for magnets of constant cross-section. This is applied to permanent magnets having gap lengths comparable with the pole diameter, and is shown to give results in satisfactory agreement with experiment. The treatment is then extended to magnets of tapered section and, with the aid of experimental results for such magnets, simple design relationships are obtained for these also. The special case of a pair of bar magnets is briefly considered.The relationship between gap dimensions and the degree of non-uniformity of the gap field is then discussed, with particular reference to requirements for magnetron magnets.A comparison is made between some alternative forms of permanent magnet construction with the conclusion that they differ negligibly in magnetic efficiency; the other factors which may determine the best form for a particular application are briefly reviewed. The approximate relationship between performance and minimum attainable weight of magnetron magnets in typical modern alloys is presented in simple form. The treatment given in the paper is not valid for designs in which the required gap field strength approaches the optimum flux density for the magnet material used, though it may provide a convenient basis for empirical extension to cover such cases.
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      The development of mercury-vapour thyratrons for radar modulator service
      • Author(s): H. de B. Knight  and  L. Herbert
      • Source: Journal of the Institution of Electrical Engineers - Part IIIA: Radiolocation, Volume 93, Issue 5, p. 949 –962
      • DOI:  10.1049/ji-3a-1.1946.0185
      • Type: Article
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      • p. 949 –962 (14)
        In 1939, the Admiralty successfully used the industrial type BT9 high-voltage mercury-vapour thyratron as a modulator switch. As the development of high-power radar progressed, special valves were needed to meet conditions very different from those of industry. The requirement of high peak-current, with very rapid build-up, set new problems in cathode design. These were met by new cathodes, one indirectly and two directly heated, in which the surface had maximum availability for the emission of electrons and the reception of positive ions. The gauze cathode prevents flaking off of the emissive coating.Cathode life is limited by the effects of bombardment by positive ions, the greatest effect being produced during the ionization period, when the current reaches a considerable value before the anode-cathode voltage has fallen to its steady arc-drop value. The higher the voltage prior to conduction, the more severe the cathode bombardment and the shorter the life.The paper discusses ionization, de-ionization, the effects of cathode temperature and of vapour pressure, anode sputter, and other features experimentally investigated in the course of the development. A short account is given also of mercury-vapour thyratrons used as auxiliaries in radar service.The use of mercury vapour necessitates a time delay for cathode heating before current can be drawn, and also temperature control to keep the vapour pressure within prescribed limits. These are recognized disadvantages, but with their acceptance, the mercury-vapour thyratron assisted the development of high-power radar and its application at an early stage and throughout the war.
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      The development of triggered spark-gaps for high-power modulators
      The high-power pulsed magnetron: a review of early developments
      • Author(s): E.C.S. Megaw
      • Source: Journal of the Institution of Electrical Engineers - Part IIIA: Radiolocation, Volume 93, Issue 5, p. 977 –984
      • DOI:  10.1049/ji-3a-1.1946.0187
      • Type: Article
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      • p. 977 –984 (8)
        After outlining the main trend of magnetron development before the war, with particular reference to the factors which played a part in its rapid progress during the war, the paper describes the events leading to the development, in June 1940, of the first high-power pulsed magnetron for a wavelength of 10 cm.The multi-resonator system developed by Randall and Boot at Birmingham University and the large oxide cathode developed by Gutton in Paris for a different type of magnetron were combined in a construction, designed for use with a small permanent magnet, which met the requirements for airborne service and was suitable for quantity production. The result of these steps was an immediate increase in pulse power and life by a factor of at least 10, with a similar reduction in magnet weight.The systematic development of design procedures, based on pre-war work, played a major part in the 100-fold increase in pulse output power at 10 cm, which was achieved between June and December 1940. The fundamental difficulties of multi-segment magnetron design were, however, only beginning to be appreciated at the end of this initial period of rapid expansion and, by comparison with modern knowledge, the ideas of that period and the technique available for experimental study were very incomplete. A companion paper provides a review of some of the vast amount of work which still remained to be done after the close of the phase with which the present paper deals.
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      The high-power pulsed magnetron: development and design for radar applications
      • Author(s): W.E. Willshaw ; L. Rushforth ; A.G. Stainsby ; R. Latham ; A.W. Balls ; A.H. King
      • Source: Journal of the Institution of Electrical Engineers - Part IIIA: Radiolocation, Volume 93, Issue 5, p. 985 –1005
      • DOI:  10.1049/ji-3a-1.1946.0188
      • Type: Article
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      • p. 985 –1005 (21)
        The paper gives a general picture of the development of the multiple-circuit magnetron from the time of the first manufactured design up to the present day. The principles underlying magnetron design and operation are given as fully as space permits and detailed constructional features are discussed where they are of particular interest.The manufacture and performance of the earliest types is dealt with first. The use of the technique of “strapping” is then discussed, the effect on the resonance characteristics of the magnetron block being described. Further Sections deal with the mechanism of oscillation, and the calculation of operating performance.After summarizing the performance of the established types operating in the 10-cm wavelength range, the difficulties experienced in the design of valves for shorter wavelengths are outlined. Various constructional techniques for these valves are mentioned, together with the apparatus necessary to ensure that a finished valve shall operate with correct electrical characteristics.Further Sections deal with valves capable of generating the highest powers at wavelengths of 10 and 3 cm, and examples of finished designs are given. The fundamental difficulty of “mode change” is described, and a later Section discusses the method which has been evolved for estimating the mode-change performance of a given magnetron.The design of cathodes for magnetrons presents some unusual problems and these are discussed in detail.Finally, an appendix deals with some aspects of magnetron-circuit design using equivalent circuits.
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      Discussion on “Valves”

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