Showing papers in "Journal of the Institution of Electrical Engineers - Part III: Radio and Communication Engineering in 1945"

Non-Linear Distortion in Transmission Systems

Abstract: The problem of the distortion noise generated by a signal in its transmission through one or more non-linear devices is analysed. This subject has become of increasing importance in connection with the development of the transmission of different types of signals on a multi-channel basis through a common network, e.g. modulator, amplifier or transmitter. The provision of such networks is now of frequent occurrence, and in order that economic and satisfactory designs may be obtained it is essential to be able to determine precisely the effect of non-linearity and also the limiting linearity requirements, preferably in a form which can be readily comprehended and applied by designers. The problem has been considered to be very complex, and existing literature is both scanty and involved. This paper presents a simple solution, which is capable, in general, of immediate application to any type of complex signal (e.g. speech, music, television, voice-frequency telegraphy or thermal noise) covering any frequency band or frequency allocation. The results are presented to the designer in the form of the limiting levels of the various harmonics of a single tone, which levels must not be exceeded if the noise power from the non-linear device in any specified frequency band is not to exceed a predetermined value. The differences in the linearity requirements of a single amplifier and of a repeatered system are demonstrated. Provided the input/output characteristic of the network can be expressed as a single-valued power series independent of the frequency in the working band, the results obtained should cover with adequate accuracy the design of all types of transmission systems.

Notes on the stability of LC oscillators

Abstract: The paper deals with the problem of the stability of LC oscillators, largely from the point of view of the development engineer, and is based on the author's experience within a limited field.Some familiarity with the art is presumed, though references are given to a number of the more important publications.Sections are devoted to the two fundamental oscillator components, namely the amplifier and the resonator.Methods of thermal compensation are discussed and the importance of humidity is emphasized, a diagram for the permittivity of moist air being included.The paper concludes with a description of the performance of the Franklin oscillator and with experimental oscillators which have been built.

Forced oscillations in oscillator circuits, and the synchronization of oscillators

Abstract: The behaviour of a feedback oscillator circuit under the influence of an injected tone having a frequency close to the natural frequency of oscillation is considered from the point of view of the steady-state equilibrium of the loop transmission circuit. The conditions for the absence or suppression of the free oscillation and for the stability of the forced oscillation are found. The application of the principles to certain common types of oscillator is discussed, and the effect of impurities in the injected tone is outlined.

The alignment of grids in thermionic valves

Abstract: The system formed by the cathode, control grid and screen grid of a tetrode valve is considered from an electron-optical point of view. It is shown that the ratio of the screen current to the anode current, and its variation with grid bias, can be accounted for over the whole of the characteristic. The formulae developed yield the data required to obtain optimum performance under any given conditions.Attention is also given to the application of electron-optical considerations to the problem of noise in the so-called silent screen-grid valves, and also to an interesting filamentary-beam tetrode in which the screen current is kept low by the application of electron-optical principles different from those used in other aligned valves.

The development of polythene as a high-frequency dielectric

Abstract: The paper is mainly concerned with the power factor of polythene (the high polymer of ethylene), which, being normally of the order of 0.00015-0.0003, renders the material highly suitable as a high-frequency dielectric. It has been found, however, that oxidation may occur during the processing of the material in the manufacture of cables and mouldings, and that this increases the power factor and may also lead to difficulties in extrusion. These effects may be virtually eliminated, however, by the use of small amounts of anti-oxidants. The measurable, albeit low, power factor of pure polythene is hardly concordant with the supposedly non-polar nature of the substance, and a number of possible explanations of the small basic power factor have been investigated. Measurements of power factor over wide frequency and temperature ranges show that its variation for pure polythene is extremely sluggish, but that oxidation causes the appearance of marked peaks; these observations are examined in the light of present theories of dipole loss. A brief account is given of the structure of polythene, and of its main physical and mechanical properties.

The permeability of ferromagnetic materials at frequencies between 105 and 1010 c/s

Abstract: Methods of measuring the permeability of ferromagnetic materials in high-frequency fields are described, and the main results obtained by various investigators are collated and discussed. It appears that hysteresis effects cease between 105 and 107 c/s, and that the permeability decreases with increase of frequency until it reaches unity in the region of 1011 c/s. Various explanatory hypotheses are discussed. The most probable explanation is that the phenomena are due to eddy currents which delay the magnetization processes. These eddy currents are caused by the extensions or rotations of the magnetic domains, and act so as to slow down and finally prevent these changes. The theoretical data evaluated on the basis of this hypothesis are in general agreement with the practical observations.

Some considerations in the design of wide-band radio-frequency amplifiers

Abstract: The object of this paper is to review some of the theoretical considerations and design requirements of wide-band radio-frequency amplifiers for television, radar and similar applications. In order to keep the paper within reasonable bounds, great detail in the various Sections has not been attempted. For instance, Section 4.3 on Bandwidth, including the design of intervalve couplings, could easily be amplified to the status of a complete paper. A number of papers on intervalve couplings have been published, and others will undoubtedly be published in the near future; a great deal has also been written about noise in radio receivers and about detection: only the features which have special significance in television amplifiers have therefore been surveyed.

The measured performance of horizontal dipole transmitting arrays

Abstract: Measurements of the performance of horizontal dipole arrays are described, and are compared with the theoretical performance under ideal conditions. The measurements include polar diagrams, the effect of ground slope and radiation through other arrays, and the performance over a frequency band of ?2% relative to the design frequency, which is typical of the width of existing short-wave broadcasting bands. One method of measurement consisted in elevating a calibrated frame receiving aerial using a captive balloon; by varying the height and position of the balloon, the field strengths in different directions from the array were measured. In the second method a frame aerial at ground level was used; this gave only relative values of field strength and was used mainly to determine the frequency characteristic. It was found that for an array radiating over a flat site free from obstacles there was good agreement between the theoretical and measured performance; the maximum field strength was of the order of 0.8?0.9 of the theoretical value. A sloping site or radiation through other near-by arrays, however, may cause appreciable departures from the theoretical characteristics. An array having no reflector curtain covers the frequency band of ?2% without appreciable loss in radiation efficiency. This is also true for an array with a parasitic reflector curtain which is tuned to the working frequency. If, however, the reflector is tuned always to the mid-band frequency, the radiation efficiency at other frequencies is reduced. The band-width can be increased by reducing the characteristic impedance of the dipoles, and measurements have been made on two types, one consisting of single wires and the other of two parallel wires spaced 6 in apart. For an array of four rows of single-wire dipoles, the band-width is ?1% for a 10% drop in field strength relative to the mid-band frequency; for a similar array of two-wire dipoles the corresponding band-width is ?2%. The band-width of arrays containing less than four rows is approximately the same.

Pulse Response: A New Approach to A.C. Electric-Network Theory and Measurement

Abstract: The a.c. theory of electric networks advocated in this paper provides a method parallel to the conventional steady-state method as based on the sine wave, with the difference that the variables of time and frequency are interchanged. Practical application of this theory is recommended, using a signal generator to develop pulse-type signals. The advantages are discussed, and details given, of the use of such a generator and of the resulting "pulse response" or "time characteristics" for the direct assessment of a network's behaviour and a determination of its transient response, without the need for Fourier analysis and synthesis. An interpretation is given of the transient response of networks with the most general types of steady-state characteristics, using the method of paired echoes 11, 12; this interpretation is made in terms of the response to pulse-type signals, and a proof is given showing that this method follows directly from the superposition theorem. A simple theorem is given relating the real and imaginary parts of the direct or transfer impedance of a physical network, which distinguishes all such networks from arbitrary "idealized" ones. The effects are discussed of relying on practical forms of pulse having a finite time of duration, rather than the ideal ones of infinitesimal time on which the theory depends; figures are quoted for assessment of the resulting errors.

Theory and performance of corner reflectors for aerials

Abstract: The paper opens by arguing that the problem of constructing a highly directive aerial system is dominated by the difficulty of providing the necessary feeding cables. Continuous reflecting sheets are used as a device for reducing the number of such cables. A construction which readily suggests itself is a pair of sheets inclined to one another to form a V, with a single aerial on the bisector, more especially because the field would be known everywhere if the sheets extended to infinity. For the field of an aerial in a V can be calculated by image treatment, and an algebraic formula for the diffraction pattern can be found when the angle of the V is a proper fraction of 180?. It is shown that such algebraic expression can be expanded in a Fourier series which has the same form for all angles of the V and has coefficients which are the Bessel functions JN, J3N, J5N, etc. It follows at once from this expansion that the ideal pattern must be indistinguishable from a simple sine curve unless the circumferential width across the V at the aerial exceeds ??, and will not differ appreciably from a sine curve unless this width is verging on 3?/2. Recognition of this general condition is very valuable in practice and saves much wasted effort in laborious computation. A numerical example illustrates the convenience of the Fourier series for evaluating the pattern when the aerial is sufficiently distant from the apex to make the main beam much sharper than a sinusoid, and concurrently to produce side lobes. The second part of the paper describes an experimental investigation, at a wavelength of about 1.25 m, of the equatorial pattern produced by a half-wave aerial on the bisector of a V formed by two sheets, 3?/2 high and about 2? wide, inclined at 90?, 60? or 45?. The purpose of the experiments was to compare the observed pattern with the ?ideal pattern? appropriate to infinite sheets: they are restricted to the range in which the ideal pattern differs insensibly from a simple sinusoid. Sheets 2? wide produce a beam narrower than the ideal when inclined at 90?, and wider than the ideal when inclined at 60?. If the sheets are 2? wide then the best angle between them is about 60?. Experiment shows the pattern is not modified appreciably if the apex of the V is amputated and the resulting hole closed by a flat sheet. Such an amputation affords a saving of space and also shows that the pattern is insensitive to the shape of the back of the reflector: therefore it is not necessary to construct V reflectors to close tolerances of manufacture. Moreover, the optical concept of the advantage of a concave mirror does not apply when the source is distant only some ?? from it. Experiment shows that the pattern is affected insensibly by replacing continuous sheets by wire netting whose mesh has a side of about ?/40. Experiment also shows that the continuous sheets can be replaced by a comb of open rods, about ?? long, without appreciable detriment to the pattern. A blunt resonance effect occurs when the frequency is such as to make the rods precisely ?? long, but such as it is this resonance is undesirable. The distribution of current in a flat reflecting sheet is solved analytically in Appendix 11.1.

Multipath interference in television transmission

Abstract: This paper deals with television interference caused by reflections from buildings. Expressions are derived for the loci on which buildings must lie to give rise to interference having a given delay on the main presentation. The field strength of reflections from idealized buildings is calculated for various wavelengths of transmision and various distances between the transmitter and receiver. It is concluded that, although the interference increases with frequency, most of it could be eliminated by a simple directive antenna.

Television broadcasting practice in America-1927 to 1944

Abstract: This paper reviews the history of television broadcasting in America from 1927 to the present, with particular emphasis on current practice. Section 1, the historical survey, traces the evolution of standards of transmission, frequency allocations, and broadcasting practice. Noteworthy programmes are recalled. Section 2, on present practice, gives a detailed account of the standards of transmission governing public broadcasting under the current regulations of the Federal Communications Commission. The stations currently operating are listed. Typical equipment used in these stations is described in four categories: studio equipment, transmitters, radiators, and mobile pick-up equipment. The design of current (i.e. immediately pre-war) receiving equipment is described. The paper concludes with a digest of post-war prospects.

A radio-frequency capacitance and conductance bridge

Abstract: A bridge suitable for the measurement of partial capacitances and conductances is described. It operates at a frequency of 1 Mc/s and is suitable for capacitances from about 0 to 28 μμF and parallel conductances from 0 to 11 micromhos (90000 ohms). The theory of the bridge and the method of calibration of the variable conductance are given. The design of the bridge lends itself to operation at much higher frequencies than 1 Mc/s.

The electrical properties of soil at wavelengths of 5 metres and 2 metres

Abstract: The paper gives the results of a year's observations on the electrical constants of a grass-covered sandy loam site at the National Physical Laboratory. Measurements of the reflection coefficient of the ground for radiation of wavelength 5 m indicated that the dielectric constant varied between 5 and 25, and that the conductivity varied from 0.7 × 108 to 4 × 108 e.s.u. Measurements of the attenuation in soil were also made at wavelengths of 5 m and 2 m, and the results indicated similar values for the electrical constants.